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From the Rocky Mountains in the US to the Sea of Cortez in Mexico, the Colorado River is one of the most managed and monitored rivers in the world. As decreasing river flows from drought and climate change continue to impact the Colorado River Basin, the US federal government, seven US states, and Mexico have signed agreements that would reduce deliveries, including a Drought Contingency Plan (DCP), an agreement among the US states and the Bureau of Reclamation, and Minute 323 (a treaty agreement between the US and Mexico). As part of the DCP, the US Bureau of Reclamation (Reclamation) committed to increasing conservation efforts to augment storage in Lake Mead. This paper presents an analysis of Reclamation's Decree Accounting Reports that record water deliveries and conservation savings from 1964 to 2019. Overall, this analysis illustrates that (1) the agency has implemented significant conservation actions over the last three decades, (2) the agency has responded to increased accounting needs as dictated by national and international management agreements, (3) some actions have resulted in environmental costs, and (4) future actions could be designed to avoid environmental harms while also augmenting Lake Mead levels. As climate change continues to constrain Colorado River water supply, detailed accounting may help reveal areas for potential efficiencies or demonstrate where the greatest levels of savings have been reached while ensuring that environmental and social benefits are preserved. Over the last 100 years, humans have changed the path and personality of the Colorado River by distributing water to cities, communities, and agricultural fields to support over 40 million people. Throughout the twentieth century, the US federal government, seven US, and Mexico signed agreements enabling development of Colorado River water. By the early 21st century, as it became evident that water demands and climate change were reducing available supplies, these same governments signed agreements to reduce pressure on the river. All these agreements have, in part, been tracked by the US federal agency that manages the river, the Bureau of Reclamation (Reclamation), through Decree Accounting Reports. Since 1964, these reports have recorded how water is or is not accounted for that might be important to the future of the river. This paper examines those reports and reveals four main findings: (1) Reclamation has implemented numerous water conservation projects, (2) they have responded to new management agreements, (3) some of their actions have impacted the environment, and (4) there are proactive ways they can reduce environmental impacts and bolster water supplies in the future. [ABSTRACT FROM AUTHOR]

Partitioning methods such as cluster analysis are advantageous in pooling catchments into hydrometric similar regions. They help overcome data shortage in ungauged catchments, which is a common problem in Sud Mediterranean zones. Without accurate forecasts, it is difficult to assess and manage water resources efficiently this situation won't be of any assistance to hydrology decision-makers. This paper illustrates a Tunisian application case, that aims to pool catchments with a hierarchical clustering algorithm (HCA) based on distances calculated in multidimensional physiographical and hydrometric space. The homogeneity of generated clusters is checked by the silhouette index. Then the distances efficiencies are compared. Nineteen semi-arid Tunisian catchments monitored since 1992 are studied. Twelve physiographical attributes, nine rainfall and streamflow signatures are considered in the HCA with two clusters. Correlation distance provides the most homogeneous clusters. Statistically the: percentage of area affected by anti-erosive practices, percentage of forest cover and catchment area are the most discriminating attributes. However, hydrometrical signatures appear to be irrelevant. These partitions highlight two different hydrological behaviors that must support forecasting. Results are promising in the Sud-Mediterranean case, where the shortage of hydrometrical data is an ongoing problem. They have the advantage of enabling hydrologic forecasting without requiring heavy information. [ABSTRACT FROM AUTHOR]

Groundwater exploitations in semi‐arid regions result in significant declines of groundwater levels and streamflow and even trigger seasonal streamflow drying up, which may bring adverse impacts on riparian ecosystems. It is critical to explore future streamflow changes caused by variable climate scenarios and groundwater exploitation intensities; however, a comprehensive assessment of the streamflow drying up frequency remains lacking. Furthermore, the thresholds of controlling factors beyond which streamflow drying up occurs have rarely been explored. Here we develop an integrated surface water‐groundwater model for the Taoer River Basin (TRB), a typical semi‐arid watershed in northeast China. We apply the model to multiple future scenarios to reveal the frequency of potential streamflow drying up and explore the thresholds of controlling factors of streamflow drying up based on a decision tree method. We find that the groundwater level in the lower region of the TRB (LTRB) is hard to recover to the riverbed elevation and the streamflow drying up frequency is still high unless the groundwater exploitation is reduced by 50%. However, this leads to a competition between the ecological and economic objectives. Our results also show that the streamflow drying up frequency in the LTRB is very sensitive to the upstream water inflow that can be regulated through the reservoir operation or inter‐basin water diversion, indicating the importance of a basin scale strategic management. The findings of this paper can help policy makers sustainably exploit water resources in semi‐arid regions. [ABSTRACT FROM AUTHOR]

Besides existing challenges to water management under scarcity in Africa, climate change has the potential to impose additional pressures on water resources. This study aims at reviewing and identifying the climate change signals presented in 61 peer reviewed papers with respect to seven largest African river basins, viz. Nile, Senegal, Volta, Niger, Congo, Zambezi and Limpopo. Thirty percent of the reviewed papers focussed on the Nile basin while only 13% reported on the Congo basin. The climate change signals were segregated into decreasing, increasing and uncertain trends. Results stated the overwhelming tendency for using unmitigated future emission pathways. The use of ensemble models superiors the individual and synthetic scenarios. While projected temperature showed robust increasing trends, rainfall dominated by uncertain trends. The hydrological impacts were assessed through 19 hydrological models with accepted considerations of land uses at 52% of the cases. Modellers managed to understand and build robust rainfall-runoff relationships in Zambezi, Limpopo, Senegal and partially in Niger but failed in Congo and Nile. Climate change impacts on runoff, stream flows and evapotranspiration should be interpreted cautiously. Further researches are recommended to incorporate the effects of human interventions (e.g. dams) and land use changes in climate change-hydrological modelling. [ABSTRACT FROM AUTHOR]

Although hydraulic groundwater (GW) theory has been recognized as a promising tool for understanding the role of the aquifer(s) in the surface‐subsurface hydrologic cycle, the integrated modeling community still lacks a proper hydrologic structure to apply the well‐studied theory to large‐scale hydrologic predictions. This study aims to present a novel hydrologic structure that enables the Boussinesq equation‐based depiction of the bidirectional stream‐hillslope processes for applying hydraulic GW theory to large‐scale model configurations. We integrated the BE3S's (Hong et al., 2020, https://doi.org/10.1029/2020wr027571) representation scheme of the catchment‐scale stream‐hillslope continuum into the National Water Model (NWM) and applied the modified NWM (i.e., the NWM‐BE3S) to three major basins in Texas (i.e., the Trinity, Brazos, and Colorado River basins). Since the NWM currently relies on a single reservoir model for baseflow simulation, we used the Boussinesq aquifer as an alternative subsurface hydrology routine and evaluated its predictive skill and efficacy. We identified that the implemented Boussinesq aquifer(s) in the NWM‐BE3S yielded noticeable improvements in predicting streamflow for aquifers that exhibited higher nonlinearities in the observed recessions. The varying degree of improvements in streamflow predictions per the recession nonlinearities demonstrated not only (a) the algorithmic enhancement of subsurface hydrology (physics) but also (b) the applicability of the Boussinesq theory‐based depiction of the stream‐hillslope two‐way continuum. We diagnosed each stream's state based on the bidirectional stream‐hillslope exchanges and identified the dominant processes (i.e., river infiltration or baseflow) that were represented spatially in the NWM‐BE3S. Plain Language Summary: Streamflow is a critical land hydrologic component to manage water resources and the health of the associated ecosystem. While the water cycle between the stream and hillslope is the key process to accurately simulating streamflow, most currently used hydrologic/land surface models lack a theoretical basis to characterize the catchment‐scale groundwater to depict the stream‐hillslope water cycle. In this paper, we presented a new structure NWM‐BE3S that enables the Boussinesq approximation‐based characterization of the catchment(s) for improving the predictability of streamflow. The NWM‐BE3S was developed by integrating a recent numerical scheme BE3S (Hong et al., 2020, https://doi.org/10.1029/2020wr027571) into the WRF‐Hydro National Water Model (NWM) configuration as an alternative subsurface routing routine. The NWM‐BE3S was tested against streamflow observations from three major basins in Texas (i.e., the Trinity, Brazos, and Colorado River basins) to ensure the applicability of the Boussinesq‐based stream‐hillslope continuum scheme. We identified theory‐consistent improvements in the streamflow predictions from the NWM‐BE3S framework compared to the original NWM. Based on the two‐way fluxes modeled from the hydraulically continuous catchment(s) in the NWM‐BE3S, moreover, about 10% of the river reaches in the three basins were identified as losing streams during the evaluation period. Key Points: The Boussinesq theory‐based stream‐hillslope two‐way hydrologic interactions were newly introduced in the National Water Model (NWM)The Boussinesq aquifer yielded improved streamflow predictions than the single bucket model as the nonlinearity of recession increasesGaining or losing reaches in three Texas major basins showed improved predictions using the modified NWM [ABSTRACT FROM AUTHOR]

Multiyear climate predictions provide climate outlooks from years to a decade in advance. As multiyear temperature predictions become more mainstream and skillful, guidance is needed to assist practitioners who wish to explore this maturing field. This paper demonstrates the process and considerations of incorporating multiyear temperature predictions into water resources planning. Multiyear temperature predictions from the Community Earth System Model Decadal Prediction Large Ensemble are presented as discrete and probabilistic products and are used to force two common hydrologic modeling approaches: conceptual and empirical. The approaches are demonstrated to simulate streamflow in the upper Colorado River basin watershed in Colorado, where diagnostics show that increasing temperatures are associated with decreasing streamflows. Using temperature information for lead years 2-6, two analyses are performed: (i) a retrospective hindcast for the climatological period (1981-2010) and (ii) a blind forecast for 2011-15. For the retrospective hindcast, including temperature information improved the percent error as compared with climatology. For the blind forecast, the multiyear temperature prediction for warming was skillful, but the corresponding multiyear average streamflow predictions from both approaches were counterintuitive: with the predicted warming, the multiyear average streamflow was predicted to be lower than the climatological mean; however, the observed multiyear average streamflow was higher than the climatological mean. This was due to above-average precipitation during the prediction time frame, particularly for one of the years. With that year removed, the multiyear streamflow average became lower than the climatological mean. Temperature provides a marginal source of streamflow predictability, but there will be substantial uncertainty until prediction skill for year-to-year climate variability, especially for precipitation, increases. [ABSTRACT FROM AUTHOR]

Climate change impacts on water resources and increasing demand for renewable electricity will have significant impacts on the management of hydroelectric power catchments globally. In New Zealand, where 55% of electricity currently comes from hydroelectricity generation, rainfall over the major South Island hydroelectric power catchments is expected to increase in coming decades, and the seasonality and volatility of that rainfall is expected to change. This paper uses a change factor methodology linked to a model cascade to estimate changes to hydroelectric power catchment inflows in 12 regions of New Zealand. These future river flows are then put through an all of New Zealand electricity system model to examine impacts on flows downstream of the hydroelectric power stations, lake storage, and spill in the large South Island hydroelectric power catchments. An overall 2% increase in annual hydroelectric power catchment inflows over the whole of New Zealand is projected between 2020 and 2050. Seasonal impacts are projected to be larger, with total New Zealand inflows projected to be 10% higher in winter and 6% lower in summer by 2050. Projected changes to inflows are equivalent to between -3% of current total summer New Zealand electricity generation and +5% of winter generation. The four main snow-fed hydroelectric power catchments in the Southern Alps (Pukaki, Tekapo, Clutha, and Manapouri) are predicted to get higher inflows than currently in winter, and lower inflows in summer, with a small increase in annual flows. Modelled changes to inflows and catchment management results in, on average, higher modelled lake storage levels in the snow-fed hydroelectric power catchments in the South Island, which are predicted to spend more time at full lake levels and less time at low lake levels in 2050 than in 2020. The changing inflows, combined with higher storage levels and changes in the electricity system, results in a potential doubling of modelled spill from the large South Island hydroelectric power scheme lakes by 2050. These significant seasonal changes to inflows will better match electricity demand, but the increase to annual spill levels is projected to more than offset any increases in annual inflows over the study period. [ABSTRACT FROM AUTHOR]

Regionalization for prediction in ungauged basins at hourly resolution is important for water resources management (e.g., floods and hydropeaking). In the research reported in this paper, calibration of 26 catchments (39-3,090 km²) in mid-Norway was performed using hourly records and three spatially distributed (1 × 1 km²) precipitation-runoff models, as follows: (1) first-order nonlinear system model, (2) Hydrologiska Byråns Vattenballansavdelning (HBV) model, and (3) basic grid model. Four regionalization methods for each model [(1) parameter set yielding maximum regional weighted average performance measures (PMs), (2) regional median of optimal parameters, (3) nearest neighbor (NN), and (4) physical similarity (PS)] were evaluated and compared with three benchmarks. Parameter transfer from best regional donor and from an ideal best arbitrary single-donor, and local calibration (LC) were as benchmarks. The PS attributes include hypsometric curves, land use, drainage density, catchment area, terrain slope, bedrock geology, soil type, and combination of all. Comprehensive evaluation of single-donors and multidonors, simple benchmarks, and more advanced regionalization methods using multimodels, two PMs, and their statistical evaluation indicate that the identification of regionalization methods is dependent on the models, the PM, and their statistical evaluation. In general, the hypsometric curves, land use, and best regional donor methods performed better for the Nash-Sutcliffe efficiency based on boxplots and regional median values of both the Nash-Sutcliffe efficiency and relative deterioration or improvement of the Nash-Sutcliffe efficiency from the LC due to the regionalization. The methods also performed better for the individual catchments. The terrain slope, regional median of optimal parameters, maximum regional weighted average, and best regional donor methods performed better for the natural-logarithm-transformed streamflow (i.e., regarding the Nash-Sutcliffe efficiency) based on the same evaluation criteria. Similar performance to the more advanced regionalization methods of transfer of homogeneous parameter sets across the whole region from the best regional donor for both Nash-Sutcliffe efficiency and natural-logarithm-transformed streamflow (i.e., regarding the Nash-Sutcliffe efficiency) indicate the potential of the simple regionalization approach for predicting runoff response in the region. [ABSTRACT FROM AUTHOR]

The paper considers mountain rivers, which usually feature high dynamics of hydrological phenomena. The variability of the Sola river flows in the Polish Carpathians is presented based on water-gauge measurements from the hydrological long-period of 1956-2015. The flows for the upper section of the river, where no storage reservoirs are found, were analysed. A broad range of flows was shown - from 0.59 to 1250 m3/s at a mean flow of 15.5 m3/s. It was also shown that within a year, the flows grow in the spring (thaw freshets) and summer periods (rainwater freshets). Mainly the summer freshets are dangerous, consisting in a sudden rise of the flow due to rains. This is shown by the high values of maximum flows in June and July. In the case of mountain rivers, such as the Sola river, it is necessary to undertake the actions stabilising the flows, which on the one hand have flood-prevention importance, and on the other hand protect the water resources in long dry periods. The actions should be concentrated on the proper arrangement of the catchment area, slowing down the rainwater outflow. Appropriate land use is especially important (distribution of forests, of arable and developed areas) depending on the land elevation above the sea level and inclination, as well as creation of small storage facilities. [ABSTRACT FROM AUTHOR]

The exemption for groundwater wells for residential uses from the prior appropriations system, common in the western United States, has eroded in Washington State since about 2000 due to a series of legal cases. Water markets can allow the transfer of an existing water right, typically from an agricultural use, to compensate for the effect of a new residential well. But water must be legally and physically available in a way suitable to satisfy mitigation requirements. A recent court case in the Skagit basin in Northwestern Washington State has effectively halted residential development in rural areas of the basin because no suitable water rights are available to purchase for mitigation. This paper presents and examines the cost‐effectiveness of various water supply mitigation strategies. We find a small‐scale, distributed stream‐side storage system for augmenting instream flow purchased from downstream sources is relatively cost‐effective to mitigate against the effects of domestic groundwater use compared to more common alternatives. We consider transporting water to storage sites by both small‐gauge pipe and by truck. Overall, trucking water to stream‐side storage and release points tends to be more cost‐effective to mitigate against indoor‐use only given current subbasin housing densities, whereas piping for direct streamflow augmentation is more cost‐effective for higher mitigation needs associated with indoor and outdoor use and higher housing densities. Research Impact Statement: Small‐scale water storage is a cost‐effective approach for offsetting the impact of domestic wells on surface water rights. [ABSTRACT FROM AUTHOR]

Ecological infrastructure (natural ecosystems that provide important services and save on built infrastructure costs) can have an important role in securing water supply, particularly in water-scarce areas, but this importance is not reflected in investment decisions, partly due to a lack of evidence. In South Africa, one of the main threats to water supply is the proliferation of woody invasive alien plants which significantly reduce stream flow and water yields. We used existing spatial data and estimates of the impact of woody invasive plants on flows and water yields and on restoration costs to analyse the viability of investing in ecological infrastructure at the scale of major water supply areas. The analysis involved comparison of the costs and effects on water yields of catchment restoration with those of planned built infrastructure interventions designed to meet increasing water demands in the medium to long term. The cost-effectiveness analysis used the unit reference value as a measure of comparison, which is based on the discounted flows of costs and water supplied over a defined time. Restoration could supply 24% of the combined yield of planned built infrastructure interventions by 2050, and is not only cost-effective but has the added advantage of a range of co-benefits delivered by improving ecosystem health. This finding suggests that investing in ecological infrastructure should be considered ahead of new built-infrastructure projects. Significance: • Clearing invasive alien plants from South Africa’s main water catchment areas could increase water yields by 997 million m3 by 2050 relative to a business-as-usual approach, equivalent to a quarter of the yield gains through implementation of built infrastructure interventions planned over the same period. • Invasive alien plant clearing would be more cost-effective than built infrastructure interventions in all water supply systems, except one, the Orange River System. • These findings add to the growing body of literature that advocates for ecological infrastructure investments to secure hydrological ecosystem services. [ABSTRACT FROM AUTHOR]

Despite their popularity, the construction and operation of hydropower reservoirs pose challenges to water resources. This study investigated the impacts of cascading dams on streamflow in Tanzania's Rufiji River Basin. The SWAT model was developed to represent the entire Rufiji River Basin. The model simulated the streamflow for 41 years, from 1982 to 2022, and developed two main scenarios: with-dam and without-dam. To capture the influence of all dams, the results were emphasized from 2000 to 2022, when all three dams were operating. Calibration and validation were applied at the Rufiji-Stiegler and Kilombero-Swero stations with good performance. The results show that cascading dams annually decrease the streamflow by 1% at Rufiji-Stiegler station. In contrast, individually, the Mtera Dam displayed a 5% decrease while the Kidatu and Kihansi Dams exerted a 1% increase on the annual streamflow downstream at Rufiji-Stiegler. During 2000–2022, the Rufiji River Basin showed an annual reduction in streamflow contribution of 104.97 m3/s. Therefore, the reservoir's operation significantly impacts the downstream streamflow. The findings are expected to guide policymakers, water resource managers, and environmentalists in mitigating potential adverse effects while optimizing the benefits of hydropower generation and water regulation within the region. [ABSTRACT FROM AUTHOR]

This paper presents an assessment of the Soil and Water Assessment Tool (SWAT) on a glaciated (Qugaqie) and a non-glaciated (Niyaqu) subbasin of the Nam Co Lake. The Nam Co Lake is located in the southern Tibetan Plateau, two subbasins having catchment areas of 59 km2 and 388 km2, respectively. The scores of examined evaluation indices (i.e., R2, NSE, and PBIAS) established that the performance of the SWAT model was better on the monthly scale compared to the daily scale. The respective monthly values of R2, NSE, and PBIAS were 0.94, 0.97, and 0.50 for the calibration period while 0.92, 0.88, and −8.80 for the validation period. Glacier melt contribution in the study domain was simulated by using the SWAT model in conjunction with the Degree Day Melt (DDM) approach. The conjunction of DDM with the SWAT Model ensued improved results during both calibration (R2=0.96, NSE=0.95, and PBIAS=−13.49) and validation (R2=0.97, NSE=0.96, and PBIAS=−2.87) periods on the monthly time scale. Average contribution (in percentage) of water balance components to the total streamflow of Niyaqu and Qugaqie subbasins was evaluated. We found that the major portion (99.45%) of the streamflow in the Niyaqu subbasin was generated by snowmelt or rainfall surface runoff (SURF_Q), followed by groundwater (GW_Q, 0.47%), and lateral (LAT_Q, 0.06%) flows. Conversely, in the Qugaqie subbasin, major contributor to the streamflow (79.63%) was glacier melt (GLC_Q), followed by SURF_Q (20.14%), GW_Q (0.13%), and LAT_Q (0.089%). The contribution of GLC_Q was the highest (86.79%) in July and lowest (69.95%) in September. This study concludes that the performance of the SWAT model in glaciated catchment is weak without considering glacier component in modeling; however, it performs reasonably well in non-glaciated catchment. Furthermore, the temperature index approach with elevation bands is viable in those catchments where streamflows are driven by snowmelt. Therefore, it is recommended to use the SWAT Model in conjunction with DDM or energy base model to simulate the glacier melt contribution to the total streamflow. This study might be helpful in quantification and better management of water resources in data scarce glaciated regions. [ABSTRACT FROM AUTHOR]

The concept of integrated water resource management (IWRM) attempts to integrate all elements of water resources. Different tools are developed to assist in developing sound IWRM plans. One such tool is multiobjective analysis using an integrated hydro-economic model (IHEM). However, IHEM mainly deals with the optimization of river flow (blue water) in a river basin. This paper linked a distributed model of green water (landscape water uses) in the upper catchment with mainly blue water uses in the lower catchment of the Pangani Basin. The results show that agricultural water use has the highest water productivity and competes with all other objective functions in the catchment. The generation of firm energy competes with the downstream ecosystem requirements. The integrated study shows that improving rainfed cropping through supplementary irrigation has comparable marginal water values to full-scale irrigation but are much higher compared with hydropower. However, hydropower has more benefits if used in conjunction with the environment. The methodological approach has increased the understanding of trade-offs between green and blue water uses that are highly interdependent in African landscapes. [ABSTRACT FROM AUTHOR]

Research on water cooperation in the Eastern Nile Basin has focused on expanding policy and diplomacy tools for a better allocation of transboundary water resources confined to the river. Regional cooperation on water and related sectors such as energy and land expands the bargaining and areas for mutual gain, and thus enhances cooperation perspectives. This paper looks at the contribution and the potential benefits of a regional cooperation approach to addressing the underlying challenges of water diplomacy, such as complexity and distrust. It also promotes the understanding of river basins as a “resource basin” of integrated and linked resource-use issues, not always related to the river flow. The paper provides an analysis of priority issues for water–energy–food nexus in regional cooperation in the Eastern Nile Basin. This basin represents an illustrative case for regional cooperation and increased integration due to multiple comparative advantages inherent in the uneven endowments of water, energy and arable land resources, and to varying levels of economic and technological advancement among the three riparian countries: Egypt, Sudan and Ethiopia. The paper also analyzes institutional arrangements on a regional scale, and elaborates on the inherent trade-offs associated with them. [ABSTRACT FROM AUTHOR]

The upper Colorado River basin (UCRB) is one of the primary sources of water for the western United States, and increasing temperatures likely will elevate the risk of reduced water supply in the basin. Although variability in water-year precipitation explains more of the variability in water-year UCRB streamflow than water-year UCRB temperature, since the late 1980s, increases in temperature in the UCRB have caused a substantial reduction in UCRB runoff efficiency (the ratio of streamflow to precipitation). These reductions in flow because of increasing temperatures are the largest documented temperature-related reductions since record keeping began. Increases in UCRB temperature over the past three decades have resulted in a mean UCRB water-year streamflow departure of −1306 million m3 (or −7% of mean water-year streamflow). Additionally, warm-season (April through September) temperature has had a larger effect on variability in water-year UCRB streamflow than the cool-season (October through March) temperature. The greater contribution of warm-season temperature, relative to cool-season temperature, to variability of UCRB flow suggests that evaporation or snowmelt, rather than changes from snow to rain during the cool season, has driven recent reductions in UCRB flow. It is expected that as warming continues, the negative effects of temperature on water-year UCRB streamflow will become more evident and problematic. [ABSTRACT FROM AUTHOR]

The exigency of the current climate crisis demands a more comprehensive approach to addressing location-specific climate impacts. In the Passaic River Basin (PRB), two bodies of research—hydroclimatic trend detection and hydrological modeling—have been conducted with the aim of revealing the basin's hydroclimate patterns as well as the hydrologic response to recent climate change. In a rather novel application of the wavelet transform tool, we sidelined the frequently used Mann–Kendal (MK) trend test, to identify the hidden monotonic trends in the inherently noisy hydroclimatic data. By this approach, the use of MK trend test directly on the raw data, whose results are almost always ambiguous and statistically insignificant in respect of precipitation data, for instance, no longer poses a challenge to the reliability of trend results. Our results showed that, whereas trends in temperature and precipitation are increasing in the PRB, streamflow trends are decreasing. Based on results from the hydrological modeling, streamflow is more sensitive to actual evapotranspiration (ET) than it is to precipitation. In periods spanning decades with sufficient water availability, energy governs actual evapotranspiration rates, rendering streamflow more sensitive to increases in precipitation. Conversely, during meteorologically stressed decades, water availability dictates actual evapotranspiration, consequently amplifying streamflow sensitivity to fluctuations in actual evapotranspiration. We found that the choice of baseline condition constitutes an important source of uncertainty in the sensitivities of streamflow to precipitation and evapotranspiration changes and should routinely be considered in any climate impact assessment. [ABSTRACT FROM AUTHOR]

Our understanding and predictive capability of streamflow processes largely rely on high-quality datasets that depict a river's upstream basin characteristics. Recent proliferation of large sample hydrology (LSH) datasets has promoted model parameter estimation and data-driven analyses of hydrological processes worldwide, yet existing LSH is still insufficient in terms of sample coverage, uncertainty estimates, and dynamic descriptions of anthropogenic activities. To bridge the gap, we contribute the synthesis of Global Streamflow characteristics, Hydrometeorology, and catchment Attributes (GSHA) to complement existing LSH datasets, which covers 21 568 watersheds from 13 agencies for as long as 43 years based on discharge observations scraped from the internet. In addition to annual and monthly streamflow indices, each basin's daily meteorological variables (i.e., precipitation, 2 m air temperature, longwave/shortwave radiation, wind speed, actual and potential evapotranspiration), daily–weekly water storage terms (i.e., snow water equivalence, soil moisture, groundwater percentage), and yearly dynamic descriptors of the land surface characteristics (i.e., urban/cropland/forest fractions, leaf area index, reservoir storage and degree of regulation) are also provided by combining openly available remote sensing and reanalysis datasets. The uncertainties in all meteorological variables are estimated with independent data sources. Our analyses reveal the following insights: (i) the meteorological data uncertainties vary across variables and geographical regions, and the revealed pattern should be accounted for by LSH users; (ii) ∼6 % watersheds shifted between human-managed and natural states during 2001–2015, e.g., basins with environmental recovery projects in northeast China, which may be useful for hydrologic analysis that takes the changing land surface characteristics into account; and (iii) GSHA watersheds showed a more widespread declining trend in runoff coefficient than an increasing trend, pointing towards critical water availability issues. Overall, GSHA is expected to serve hydrological model parameter estimation and data-driven analyses as it continues to improve. GSHA v1.1 can be accessed at 10.5281/zenodo.8090704 and 10.5281/zenodo.10433905 (Yin et al., 2023a, b). [ABSTRACT FROM AUTHOR]

Using the drainage area ratio (DAR) method, daily or monthly flows gauged at a nearby station are transferred to an ungauged section where a dam is to be built by multiplying them by a real number resulting from raising the ratio of the drainage areas of the dam section and of the station to an exponent n. Commonly, n is taken as 1. Using the daily flows of two stations nearby a dam section on the same stream or a neighbouring stream, having a common N-year-long gauging record, it is proposed that 365 linear regressions be fitted to N pairs of 365 daily flows and relate them to the DARs. The daily flows at the dam section can then be computed using 365 values of n' each belonging to 1 day. Next, using the same analytical model applied to monthly flows, 12 monthly flows are transferred to the dam section. Five versions of the DAR method – three of which have constant exponents – were investigated. The method with n = 1 was found to be the worst performer. [ABSTRACT FROM AUTHOR]

Robust hydrologic models are needed to help manage water resources under a changing climate. In this paper, we assessed daily streamflow predictions by applying two simple conceptual models (IHACRES and GR4J) and one physically-based, semi-distributed model (SWAT) for the Yongdam Dam watershed. We compared these models' capabilities in reproducing observed streamflow in the time and quantile domains. The calibration and validation of the three models were performed using 13-year daily datasets and the k-fold cross validation method. The Particle Swarm Optimization algorithm was used to optimize flow parameters for the more complex model, whereas the Shuffled Complex Evolution algorithm was used to calibrate the two simpler models. Global sensitivity analysis was performed for the complex model to reduce the number of flow parameters; the effective hydraulic conductivity in main channel alluvium was the most sensitive parameter, followed by the baseflow alpha factor for bank storage. The results showed that GR4J was slightly better than SWAT and IHACRES, and IHACRES had equivalent model performance to SWAT, with respect to the day-to-day flow comparison focused on reproducing the exact observed discharge hydrograph. However, SWAT performed best for reproducing the frequency distribution curve. Furthermore, SWAT performance was improved for each flow range of the flow duration curve by manually changing the most sensitive model parameters. In general, the results revealed that a multimodel ensemble approach with SWAT and either GR4J or IHACRES should be utilized for regional water resources assessment in the Yongdam watershed. [ABSTRACT FROM AUTHOR]

The rapid economic development of river basins depends on the excessive use of water resources. China experienced a rapid development of hydropower projects in the last two decades and thus faces many ecological and environmental issues, especially in ecologically sensitive areas. Environmental flow is an important management tool that requires attention in the environmental impact assessment of hydropower projects. Environmental flows are of great significance for maintaining river structures and protecting the health of both aquatic ecosystems and human sustainable livelihoods. Although the government authorities have done much work in this area and attempted to consider technical requirements to address the negative externalities of hydropower projects, there are still defects in the basic procedures, calculation methods, and ultimately implementation process from policy to operationalization in terms of environmental flows. The official standards for environmental flows assessment mainly appear in two documents: 1. specification for calculation of environmental flow in rivers and lakes; and 2. code for calculation ecological flow of hydropower projects. This paper reviewed the overarching framework of the two documents and then summarized their fitness in terms of environmental flows implementation in hydropower projects. The research status of environmental flows and future directions for China were also proposed in this paper. [ABSTRACT FROM AUTHOR]

Human activities mostly impact the trend and direction of rainwater, groundwater, and other river basin resources in the watershed in Africa. These activities alter river flows and the quality of usable water supplies at both highlands and lowlands. A watershed is indeed a conserved area of land that collects rain, sleet and snow, and empties or penetrates groundwater sources. The act of managing the activities around the watershed is integrated watershed management, which considers the social, economic, and environmental issues in tandem with the human, institutional, natural, and sustainability systems, which are the key drivers as identified in this study, as well as community interests and participation, to manage groundwater resources sustainably. These watersheds, river basins, and groundwater resources provide important services for communities and biodiversity. This paper reveals that the best way to protect groundwater resources is on a watershed basis using sustainable management measures. This technique enables us to handle a variety of concerns and objectives while also allowing us to plan in a complicated and uncertain environment. Sustaining a regional and sub-regional watershed involves cooperation and participation from a wide range of community interests and water users, including municipalities, companies, people, agencies, and landowners, for stakeholders' input to be successful. All of the strategies and plans are produced with regard to one another, as well as the overall conditions of the watershed, local land uses, and specific regional transboundary issues. [ABSTRACT FROM AUTHOR]

The Alps are a key water resource for central Europe, providing water for drinking, agriculture, and hydropower production. Thus, understanding runoff generation processes of Alpine streams is important for sustainable water management. It is currently unclear how much streamflow is derived from old water stored in the subsurface, versus more recent precipitation that reaches the stream via near-surface quick flow processes. It is also unclear how this partitioning varies across different Alpine catchments in response to hydroclimatic forcing and catchment characteristics. Here, we use stable water isotope time series in precipitation and streamflow to quantify the young water fractions Fyw (i.e., the fraction of water younger than approximately 2–3 months) and new water fractions Fnew (here, the fraction of water younger than one month) in streamflow from 32 Alpine catchments. We contrast these measures of water age between summer and winter and between wet and dry periods, and correlate them with hydroclimatic variables and physical catchment properties. New water fractions varied from 9.6 % in rainfall-dominated catchments to 3.5 % in snow-dominated catchments (mean across all catchments = 7.1 %). Young water fractions were approximately twice as large (reflecting their longer time scale), varying from 17.6 % in rainfall-dominated catchments to 10.1 % in snow-dominated catchments (mean across all catchments = 14.3 %). New water fractions were negatively correlated with catchment size (Spearman rank correlation rS = 0.38), q95 baseflow (rS = -0.36), catchment elevation (rS = 0.37), total catchment relief (rS = -0.59), and the fraction of slopes steeper 40° (rS = -0.48). Large new water fractions, implying faster transmission of precipitation to streamflow, are more prevalent in small catchments, at low elevations, with small elevation gradients, and with large forest cover (rS = 0.36). New water fractions averaged 3.3 % following dry antecedent conditions, compared to 9.3 % after wet antecedent conditions. Our results quantify how hydroclimatic and physical drivers shape the partitioning of old and new waters across the Alps, thus indicating which landscapes transmit recent precipitation more readily to streamflow, and which landscapes tend to retain water over longer periods. Our results further illustrate how new water fractions may find relationships that remained invisible with young water fractions. [ABSTRACT FROM AUTHOR]

Water resources are key to economic development of the Mekong River Basin, but are threatened by climate change and affected by hydropower development. Knowledge of these drivers' integrated impact on future hydrological alterations is limited, especially with respect to flood inundation in the lower basin. This study assesses streamflow and flood extent alterations by reservoir operations and climate change using the latest climate projections. A distributed hydrologic model is used to generate discharge and flood extent. Our findings indicate substantial changes in seasonal and annual peak discharge due to reservoir operations. Under the future hydropower scenario, the discharge at Kratie will change by + 28% (− 10%) during the dry (wet) season. While the effects of hydropower operations vary by season, climate change tends to increase river discharge overall. Under the high-emission scenario, the wet seasonal flow at Kratie will increase by + 7% in the near-future (2026–2050), but change by -5% under integrated impact of climate change and reservoir operations. In the far-future, the wet seasonal flow at Kratie under climate change only (integrated impact) will increase by + 33% (+ 19%). Although climate change is the dominant driver of flow alterations, hydropower development is critical for reducing discharge and flood magnitude. Nonparametric statistical testing shows significant changes in the inundated area by up to + 37% during the projected periods. [ABSTRACT FROM AUTHOR]

Abstract The estuarine delta of the Yellow River is a region of strong land-ocean-human interactions that undergoes a unique evolutionary process. In this paper, we combined historical data and remote sensing images to better understand the evolution of the Yellow River delta. Changes in the Yellow River estuarine flow path from natural swing to artificial diversion were first studied to understand the rules and mechanism of the delta formation. The complex geomorphic structural system of the Yellow River delta is constituted by contemporary and modern deltas and their sub-deltas. Remote sensing images and measured data indicate that variations in runoff and sediment load in the Yellow River estuary will affect the deltaic erosion and siltation. The natural and artificial factors that cause runoff and sediment changes include climate change, soil erosion control in the middle reaches, building reservoirs along the Yellow River, which are used as water supply for local cities and farmland irrigation, as well as the water and sediment regulation experiment in the Xiaolangdi reservoir. Furthermore, artificial intervention of diversion channels has contributed to the maintained stability of the Yellow River estuary environment, so as to have regional development and prosperity of the Yellow River delta. Graphical abstract Image 1 Highlights • The Yellow River estuarine flow paths change from natural swing to artificial diversion, and the delta formation rule. • The variation of runoff and sediment load in the Yellow River affect the deltaic erosion and siltation. • Probe into the natural and artificial reasons of runoff and sediment changes and decrease. • The estuarine artificial control maintains relative stableenvironment and creates prosperity of the Yellow River delta. • This study reflects land-ocean-human interaction on estuarine coastal region. [ABSTRACT FROM AUTHOR]

The Paraiba do Sul River Basin (PSRB) is a vital source of water resources in Brazil, providing water for human consumption, industry, agriculture, and hydroelectric energy generation. As part of one of the most developed areas of the country, in the Southeast of Brazil, the region is vulnerable to the impacts of climate change, with evidence of extreme events such as droughts and floods affecting the availability and quality of water. Hence, this study analyzes precipitation and streamflow rates data from the PSRB between 1939–2020 to investigate the spatial variability of average patterns and extreme events, trends, and their relationship with urban growth and socioeconomic development. The analysis reveals significant spatial variations in precipitation and runoff rates, with higher altitude areas, such as the Serra da Mantiqueira, exhibiting higher average values. Moreover, the Mann–Kendall trend results showed in most of the sites no significant trend regarding precipitation data; however, about 50% of the sites in the PSRB presented a decreasing trend of runoff rates. Since the precipitation does not explain identified changes in the hydrological patterns, the evaluation of the area's urban growth and socioeconomic development throughout the decades suggested that human activities, such as those associated with urbanization, have played a significant role in altering the runoff patterns in the basin. These findings highlight the importance of sustainable land-use planning and water resource-management practices in the PSRB to mitigate the negative impacts of urbanization on the hydrological cycle and to enhance the resilience of the region's water resources. [ABSTRACT FROM AUTHOR]

The study covered water resources of two mountain streams in the Polish Carpathians. These were the Biała Woda and Czarna Woda streams, the catchments of which are adjacent to each other. Water flows in both streams were measured during the hydrological years from 2006 to 2020. Next, water outflows from the catchments were calculated. The study aimed to determine differences in the water resources of those catchments in a very small mountainous area. The study showed quantitative similarity in water resources in the entire multi-annual period but at the same time large differences in shorter periods. Instantaneous and daily outflows showed the largest differences, but differences in annual outflows of up to 20% were also recorded. Therefore, hydrological data from operational cross-sections to assess water resources of neighbouring uncontrolled watercourses should cover multi-annual mean values. It was found that during periods of increased runoff (from melting snow or precipitation), the outflow from the Biała Woda catchment was much larger, while during rain-free periods, the outflow from the Czarna Woda catchment prevailed. All shortterm flood like outflows were at least several tens of per cent higher in the Biała Woda catchment. The higher retention capacity of the Czarna Woda catchment can be attributed to the land use (mainly forest areas). The results can be used for modelling catchments of similar parameters and determining their retention capacity. [ABSTRACT FROM AUTHOR]

Recharge is a crucial section of water balance for both surface and subsurface models in water resource assessment. However, quantifying its spatiotemporal distribution at a regional scale poses a significant challenge. Empirical and numerical modeling are the most commonly used methods at the watershed scales. However, integrated models inherently contain a vast number of unknowns and uncertainties, which can limit their accuracy and reliability. In this work, we have proposed integrated SWAT-MODFLOW and Transient Water Table Fluctuation Method (TWTFM) to evaluate the spatiotemporal distribution of groundwater recharge in Anyang watershed, South Korea. Since TWTFM also uses SWAT model percolation output data, calibration was performed for individual models and a coupled model. The coupled model was calibrated using daily streamflow and hydraulic head. The SWAT-MODFLOW model performed well during the simulation of streamflow compared to the SWAT model. The study output showed that the study watershed had significant groundwater recharge variations during the simulated period. A significant amount of recharge happens in the wet season. It contributes a significant amount of the average annual precipitation of the region. The direct flow components (surface and lateral) showed significant contributions when the water balance components were evaluated in the region. TWTFM showed a glimpse to estimate recharge, which requires representative monitoring wells in the study region. Comprehensively, the SWAT-MODFLOW model estimated groundwater recharge with reasonable accuracy in the region. [ABSTRACT FROM AUTHOR]

Seasonally snow‐covered catchments in the western United States supply water to growing populations as both annual snowmelt‐driven streamflow and multi‐year groundwater recharge. Although interannual variability in streamflow is driven largely by precipitation, runoff efficiency (the ratio of streamflow to precipitation) in individual catchments varies by 50% or more. Recent work suggests that interannual variability in groundwater storage, inferred from winter baseflow, is a primary control on runoff efficiency, highlighting a need to quantify both the time scales on which groundwater storage varies and the hydro‐climatic drivers of storage. Using over a century of daily stream discharge data from 10 seasonally snow‐covered catchments in northern Utah, we find that temporal variability in winter baseflow, an index of groundwater storage, measured from mean daily January discharge, exhibits a 2–5‐ and 12–15‐year periodicity, driven by regional precipitation patterns and snowmelt dynamics. Specifically, multiple linear regression (MLR) modeling using antecedent hydro‐climatic variables demonstrates that winter baseflow (groundwater storage) was positively related to 3–4 years of antecedent annual precipitation, negatively related to the previous year's mean annual temperature, and positively related to 1–4 antecedent years of snowmelt rate and duration. Because antecedent baseflow (groundwater storage) is strongly related to runoff efficiency, these results suggest that more frequent and longer droughts in a future climate will reduce surface water supplies faster than otherwise expected. More broadly, these results highlight the importance of including the influence of antecedent climate on groundwater storage when modeling and managing water supplies from seasonally snow‐covered catchments. Key Points: Interannual changes in groundwater storage inferred from baseflow in 10 headwater catchments exhibit coherent 2–5 and 12–15 year periodicityInterannual variability in groundwater storage is related to 1–4 years of antecedent precipitation (+) snowmelt rate (+) and temperature (−)Groundwater storage in warmer/drier catchments is related to longer periods of antecedent climate than cooler/wetter catchments [ABSTRACT FROM AUTHOR]

This study aims at assessing runoff characteristics and water availability between 1955 and 2019 in the Nigerian sub-catchment of River Niger upstream of Baro, central Nigeria, and the implication for food security in the nation. The discharge regime of the sub-catchment is characterized by single peak, which occurs in September. The sub-catchment's runoff coefficient ranged between 23% and 32% for the period 1955−1970; 9% and 22% for 1971–1990; 45% and 55% for 1994–1999; and 15–33% for 2000–2019. The study observed a decline in river flow and water availability due to climate variability. This indicates impending problems of water crises and food insecurity. [ABSTRACT FROM AUTHOR]

This paper presents a comparison of the predictive capability of three hydrological models, and a mean ensemble of these models, in a heavily influenced catchment in Peninsular India: GWAVA (Global Water AVailability Assessment) model, SWAT (Soil Water Assessment Tool) and VIC (Variable Infiltration Capacity) model. The performance of the three models and their ensemble were investigated in five sub-catchments in the upstream reaches of the Cauvery river catchment. Model performances for monthly streamflow simulations from 1983–2005 were analysed using Nash-Sutcliffe efficiency, Kling-Gupta efficiency and percent bias. The predictive capability for each model was compared, and the ability to accurately represent key catchment hydrological processes is discussed. This highlighted the importance of an accurate spatial representation of precipitation for input into hydrological models, and that comprehensive reservoir functionality is paramount to obtaining good results in this region. The performance of the mean ensemble was analysed to determine whether the application of a multi-model ensemble approach can be useful in overcoming the uncertainties associated with individual models. It was demonstrated that the ensemble mean has a better predictive ability in catchments with reservoirs than the individual models, with Nash-Sutcliffe values between 0.49 and 0.92. Therefore, utilising multiple models could be a suitable methodology to offset uncertainty in input data and poor reservoir operation functionality within individual models. [ABSTRACT FROM AUTHOR]

• BMA improved estimation of prediction uncertainty for daily streamflow simulations. • Applying BMA provided a better understanding of SWAT-HS reliability and precision. • BMA models had similar estimations for model reliability, but different for precision. • BMA showed that MNSE had the highest predictive ability in simulating streamflow. In this paper, multiple objective functions were used to conduct uncertainty analysis on calibrated streamflow simulations of SWAT-hillslope model (SWAT-HS), which is a semi-distributed model suited for humid, mountainous regions. SWAT-HS was set up for six watersheds of NYC water supply system and streamflow was simulated at a daily time step. Each SWAT-HS model setup was then passed through a calibration process for parameter optimization. This process involved constraining the model using several alternative objective functions which produce statistically acceptable streamflow simulations, which served as an ensemble of predictions for uncertainty analysis. We hypothesize that the uncertainty introduced by selecting a single objective function can be large with each objective function being sensitive to a specific hydrologic signature. Using multiple objective functions result in a wider range in optimum parameter values and an ensemble of simulations based on different objective functions can capture the overall prediction uncertainty through differences in parameter estimates. The Bayesian Model Averaging (BMA) method was then applied to ensemble predictions to quantify overall prediction uncertainty. The uncertainty analysis showed the similarity of uncertainty interval characteristics for all BMA based predictions across all watersheds. More than 94–97% of the observations were covered by uncertainty intervals estimated using multiple objective functions when compared to 73–79% of the observations when using a single objective function. No significant differences were observed among the six watersheds, and the results of their uncertainty analysis were similar. We propose the use of multiple objective functions as an option in ensemble modeling of streamflow and uncertainty analysis. [ABSTRACT FROM AUTHOR]

The assessment of the hydrological response to projected changes in climatic variables is imperative for water resources management, especially in watersheds where snowmelt represents a significant source of runoff. In this study, we modify the source code of the snow accumulation and melting algorithm of the Soil and Water Assessment Tool (SWAT) model to improve runoff simulations in snow dominated basins. A sinusoidal snowmelt function under the degree-day factor method was adopted with its parameters calibrated based on historical data. River flow simulations were compared to measured data under the modified and unmodified SWAT models. Model differences in future predictions of river flows (2032- RCP 4.5) were also assessed. The results showed that the modifications improved runoff simulations by better capturing flow dynamics as represented by daily flows and corresponding variability during the snowmelt period. The modified model increased the Nash–Sutcliffe Efficiency (from 0.64 to 0.79; 0.60 to 0.80; 0.70 to 0.75) and the coefficient of determination (from 48 to 67%, 48 to 69%, 58 to 70%) at three gauging stations. While both models predicted a decrease of water availability in the basin, future simulations with the modified snowmelt algorithm predicted that the drop in water availability as compared to baseline year (2008) will be less dramatic (24%) compared to predictions from the unmodified SWAT (31%). We argue that the proposed source code modifications to the snowmelt algorithm of SWAT provide better insights about future water availability in snow-dominated watersheds. [ABSTRACT FROM AUTHOR]

Understanding the available resources and the needs of those who use them is necessary for the evaluation and allocation of water resources. The main sectors utilizing the basin water resources are agriculture, drinking water, animal husbandry, and industries, and the efficient and rational management of water resources to be distributed among those different sectors of activity is vital. This study attempts to develop an integrated water resource management system for the Dhasan River Basin (DRB) by employing a scenario analysis approach in conjunction with Water Evaluation and Planning Model (WEAP) to analyze trends in water use and anticipated demand between 2015 and 2050, simulating five possible scenarios (I, II, III, IV, and V) as for external driving factors. For the WEAP modeling framework, 2015 was chosen as a current (base) year for which all available information and input data were given to the model and the future demand situation was analyzed for the period 2016–2050 (forecasting period). From the findings, it was observed that for the forecasting period, total water demand, unmet demand, and streamflow were 185.29 Bm3, 117.35 Bm3, and 58.26 Bm3, respectively, in the case of scenario I; 232.34 Bm3, 162.17 Bm3, and 59.87 Bm3 in case of scenario II; 139.40 Bm3, 84.37 Bm3, and 58.15 Bm3 in case of scenario III; 186.15 Bm3, 118.76 Bm3, and 56.98 Bm3 in case of scenario IV; and 181.89 Bm3, 96.87 Bm3, and 53.11 Bm3 in case of scenario V. Results of the study indicated that by 2050, increasing population growth, industrial development, and an increase in the agricultural area will rise the water demand dramatically, posing threats to the environment and humans. Therefore, implementing improved irrigation technologies, advancing agricultural practices on farms, and constructing water conservation and retaining structures could significantly reduce the unmet demands and shortfalls in DRB. Overall findings reveal that the pressure on the Dhasan water resources would increase in the future, and thus several suggestions have been provided to assist decision-makers in sustainable planning and management of water resources to meet future demands. [ABSTRACT FROM AUTHOR]

Knowledge of the historical changes in streamflow is essential for operating and planning water structures. Hence, the monthly mean streamflow trends of 20 streamflow gauge stations in Turkey's Mediterranean basins, which are sensitive to climate change, are examined using the Mann–Kendall test, and trend slopes are calculated using Sen's slope method. Furthermore, the innovative trend significance test (ITST) and recently proposed innovative polygon trend analysis (IPTA) method are used to analyse the historical changes in streamflow values at the stations. The trend analysis results of a total of 240-time series between 1977 and 2015 are evaluated and compared with three different trend methods. As a result of the study, the MK method determined significant trends in only 24% (3 increasing, 55 decreasing), ITST in 83% (41 increasing, 158 decreasing), and IPTA in 82% (38 increasing, 158 decreasing) of the 240-time series. The IPTA and ITST methods are more sensitive to determining significant monthly streamflow trends than the MK method. In addition, there is a more significant decrease in the streamflow values of the stations located east of the basin, and the trend slope value reaches − 26%/decade. Therefore, the results of the streamflow trend in the Mediterranean basins will benefit decision-makers in planning the efficient use of water resources in the region. [ABSTRACT FROM AUTHOR]

Anthropically-induced land-use/land cover (LULC) changes create an imbalance between water and energy fluxes by affecting rainfall-runoff partitioning. This alters the catchment's flow regime, generating increased highs and reduced low flows, triggering socio-economic and environmental damages. The focus of this study is two-fold (i) to quantify the hydrological changes induced in the urbanizing watershed and (ii) to analyze changes in streamflow variability and generation of extremes (high- and low-flow), using the soil and water assessment tool (SWAT) for Peachtree Creek, USA. The results indicate that the change in LULC significantly influences the availability of soil moisture, ET, and contribution to groundwater flow. It is observed that the variations in these processes regulate the water availability from the surface and sub-surface sources, thus affecting the generation of extreme flows. The spatio-temporal analysis, in response to LULC changes, indicates that (i) urbanization significantly affects baseflow, and its variability depends on the degree of urbanization and the predominant land-use class of the subwatersheds, and (ii) the seasonal variations in the baseflow contribution to the streams depend on ET and the timing and magnitude of groundwater outflow to streams. These variations in ET and groundwater lead to water excess/deficit regions, thus increasing the susceptibility to floods during heavy precipitation events and reducing the reliability of streams during dry periods. Thus, in an urbanizing watershed, the hydrological regime of the watershed may not always be a function of changes in the surface runoff, but will be modified by ET and groundwater dynamics. Further, the study shows that the changes in model parameters can provide insight into the implications of LULC changes on hydrological processes and flow regimes. Evaluating the implications on the basin water balance is paramount for deriving any management operations and restoration activities. The study also outlines the significance of analyzing the spatial and temporal scale streamflow variations for managing water resources to reduce damage to lives and properties. [ABSTRACT FROM AUTHOR]

Quantification of the changes of water balance components is significant for water resource assessment and management. This paper employed the Soil and Water Assessment Tool (SWAT) model to estimate the water balance in a mountainous watershed in northwest China at different spatial scales over the past half century. The results showed that both Nash-Sutcliffe efficiency (NSE) and determination coefficient (R2) were over 0.90 for the calibration and validation periods. The water balance components presented rising trends at the watershed scale, and the total runoff increased by 30.5% during 1964 to 2013 period. Rising surface runoff and rising groundwater flow contributed 42.7% and 57.3% of the total rising runoff, respectively. The runoff coefficient was sensitive to increasing precipitation and was not significant to the increase of temperature. The alpine meadow was the main landscape which occupied 51.1% of the watershed and contributed 55.5% of the total runoff. Grass land, forest land, bare land, and glacier covered 14.2%, 18.8%, 15.4%, and 0.5% of the watershed and contributed 8.5%, 16.9%, 15.9%, and 3.2% of the total runoff, respectively. The elevation zone from 3500 to 4500 m occupied 66.5% of the watershed area, and contributed the majority of the total runoff (70.7%). The runoff coefficients in the elevation zone from 1637 to 2800 m, 2800 to 3500 m, 3500 to 4000 m, 4000 to 4500 m, and 4500 to 5062 m were 0.20, 0.27, 0.32, 0.43, and 0.78, respectively, which tend to be larger along with the elevation increase. The quantities and change trends of the water balance components at the watershed scale were calculated by the results of the sub-watersheds. Furthermore, we characterized the spatial distribution of quantities and changes in trends of water balance components at the sub-watershed scale analysis. This study provides some references for water resource management and planning in inland river basins. [ABSTRACT FROM AUTHOR]

Determining the mean transit times (MTTs) and water sources in catchments at different flow conditions helps better understand river functioning, and manage river health and water resources. Despite being common in a range of environments, the MTTs and water sources in intermittent streams are much less well understood compared to perennial streams. Major ion geochemistry, stable isotopes, 14 C, and 3 H were used in this study to identify water sources and MTTs of the periodically intermittent upper Wimmera River from southeast Australia at different flow conditions, including zero-flow periods. The disconnected pool waters during the zero-flow period in the summer months of 2019 had 3 H activities of 0.64 to 3.29 TU. These and the variations in total dissolved solids and stable isotopes imply that these pools contained a mixture of older groundwater and younger stream water impacted by evaporation. 3 H activities during the high-flow period in July 2019 were 1.85 to 3.00 TU, yielding MTTs of up to 17 years. The 3 H activities at moderate and low-flow conditions in September and November 2019 ranged from 2.26 to 2.88 TU, implying MTTs of 1.6 to 7.8 years. Regional groundwater near the Wimmera River had 3 H activities of < 0.02 to 0.45 TU and 14 C activities of 57 to 103 pMC, and was not recharged by the river at high flows. The Wimmera River and other intermittent streams in southeast Australia are sustained by younger catchment waters from relatively small near-river stores than comparable perennial streams, which have older deeper regional groundwater inputs. This results in these intermittent streams being more susceptible to short-term changes in climate and necessitates the protection of near-river corridors to maintain the health of the riverine systems. [ABSTRACT FROM AUTHOR]

Runoff from the Upper Colorado River Basin (UCRB) is an important water resource in the western United States. The majority of annual runoff is derived from spring snowmelt, and therefore April–July water supply volume (WSV) forecasts produced by the Colorado Basin River Forecast Center are critical to basin water management. The primary objective of this study was to evaluate the impact of snow data assimilation (DA) in a distributed hydrologic model on WSV forecasting error and skill in headwater catchments of the UCRB. To do this, a framework was built to use the Localized Ensemble transform Kalman filter to update modeled snow water equivalent (SWE) states in the Hydrology Laboratory‐Research Distributed Hydrologic Model with SNOTEL SWE observations and spatially and temporally complete MODIS Snow Covered‐Area and Grain size data. The DA approach was assessed by evaluating ensemble streamflow prediction forecasts over a 20‐year period for 23 catchments in the UCRB. Overall, the DA approach improved water supply forecast skill in 80% of pilot basins with an average improvement in the median continuous rank probability skill score of 4%. A research‐to‐operations transition was facilitated by automating the implementation of the DA approach. This work demonstrates the capacity for gridded and point snow products to be used to objectively update model states in an operational forecasting setting. [ABSTRACT FROM AUTHOR]

While bushfires are often regarded as a vital trigger that alters the partitioning of hydrological fluxes, their role in water balance changes remains poorly quantified, especially in regions where the impacts of frequent bushfires and climate variability overlap. Here, we estimated the fire‐induced water balance changes based on a modified paired catchment method that considers the partial effect of annual precipitation differences. In the application for eight forested catchments impacted by the 2009 Victorian Bushfires with multiple burned areas (12%–89%), we found that evapotranspiration declined by 33 ± 20 mm yr−1 (mean ± standard deviation) and streamflow increased by 68 ± 32 mm yr−1 during the post‐fire decade. For the interannual changes within this decade, the decline in evapotranspiration due to fires gradually recovered after the first year since bushfires, while the increase in streamflow mostly peaked in the second or third year and diminished in subsequent years. We surmised that such asynchronous responses of the two fluxes to bushfires occurred with the initial increase and the later decrease in terrestrial water storage. Averaged for the post‐fire decade, there seems to be an overall decline in terrestrial water storage for burned catchments relative to unburned catchments. Plain Language Summary: Apart from high temperature and burning biomass, bushfires can induce dramatic changes in plant water use and water yield in catchments during post‐fire years. Its impact on water resources is often compounded with climate change. In early 2009, large forest fires swept many parts of Victoria, Australia. Using a set of burned and unburned catchments and high‐resolution water balance data, we found that bushfires caused asynchronous decreases in evapotranspiration and increases in streamflow. The different magnitudes of the changes in the two fluxes may indicate a decline in terrestrial water storage for burned catchments when compared with that of unburned catchments. These findings advance our understanding of the role of bushfires on hydrological fluxes and terrestrial water storage in a changing climate. Key Points: Paired catchment method is modified for the use of detecting changes in catchment water balance, including water storage changeThe decreased evapotranspiration and the increased streamflow due to the 2009 Victorian Bushfires are temporally asynchronousThe water storage increase in a wetter climate may be less than expected for burned catchments [ABSTRACT FROM AUTHOR]

Iran has experienced a severe drought during the past two decades. Increasing water demand and decreasing available water resources have led the country to critical conflicts, particularly in shared watersheds. In these basins, the precise measurement of releasing water from upstream to downstream can be a key data for resolving water conflicts. The Iranian Ministry of Energy (MOE) makes the streamflow measurements through regional water administrative offices, mainly using the classical rating curve (RC) method. The documented measurements of runoff by adjacent offices usually present different estimations of the water resources, due to high uncertainties of the RC approach. Fluvial acoustic tomography (FAT) is a reliable technology for continuously monitoring streamflow, which can be implemented to solve this problem of shared basins. In this study, FAT is applied to the Zayanderud basin, an Iranian shared watershed and its results are compared with the RC methods. The main terms of FAT streamflow measurement uncertainty are velocity resolution component, and the selected flow angle between the stream direction and acoustic transmission path. Therefore, a new equation is proposed to carefully choose accurate angles for different river widths to decrease FAT measurement error in this paper. The obtained results show that the measured streamflow by FAT and RC are 9.89 m3/s and 10.3 m3/s, respectively, and the largest possible error of FAT is less than 15%. [ABSTRACT FROM AUTHOR]

Rational allocation of water resources is very essential to cope with water scarcity. The optimal allocation of limited water resources is required for various purposes to achieve sustainable development. The Awash River Basin is currently faced with a scarcity of water due to increasing demands, urbanization, irrigation expansion, and variability of climates. The excessive abstraction of water resources in the basin without proper assessing of the available water resources contributed to water scarcity. This paper aimed to develop a water evaluation and planning (WEAP) model to allocate the water supplies to demanding sectors based on an economic parameter to maximize the economic benefits. The water demands, water shortages, and supply alternatives were analyzed under different scenarios. Three scenarios were developed, namely reference (1981–2016), medium-term development (2017–2030), and long-term development (2031–2050) future scenarios with the baseline period (1980). The results of this study showed that the total quantity of water needed to meet the irrigation demands of all the stations was 306.96 MCM from 1980 to 2016. Seasonally, March, April, May, and June require the maximum irrigation water demand. However, July, August, and September require minimum demand for water because of the rainy season. The seasonal unmet demand is observed in all months, which ranged from 6 × 106 m3 to 35.9 × 106 m3 in August and May respectively. The trend of streamflow in Melka Kuntre was a statistically significant increasing trend after 2008 (Z = 5.33) whereas the trends in other gauge stations showed a relatively decreasing trend. The results also showed that future water consumption would greatly increase in the Awash River Basin. The prevention of future water shortages requires the implementation of water-saving measures and the use of new water supply technologies. The findings of this study will serve as a reference for water resources managers and policy and decision makers. [ABSTRACT FROM AUTHOR]

The influence of climate change on the catchment-scale hydrologic processes can have a profound impact on river flow and the availability of fresh water. Here, we develop an integrated modeling framework to assess the near- and long-term hydrological response to climate change in a mountain river basin. The framework samples climate model outputs under different representative concentration pathways to force the calibrated hydrologic model and generate daily streamflow projections. We implement a framework in the Modi River basin, with an elevation ranging from 750 m to over 8000 m above sea level. Under the highest warming scenario, the mean annual precipitation and temperature are projected to vary to be as high as 4531 mm and 25.7 °C, respectively. The study results show that the future streamflow of the Modi River basin will increase during the latter time windows, i.e., far future (2075–2099) > > mid future (2050–2074) > > near future (2025–2049). Exploring how climate change can alter different hydrological processes can help improve the fundamental understanding of water balance and hydrologic controls, which are critical in ensuring the functionality of the natural ecosystems. [ABSTRACT FROM AUTHOR]

In developing countries with data scarcity challenges, an integrated approach is required to enhance the estimation of streamflow variability for the design of water supply systems, hydropower generation, environmental flows, water allocation and pollution studies. The Flow Duration Curve (FDC) was adopted as a tool that is influenced by topography, land use land cover, discharge and climate change. The data from Global Climate Model (GCM) projections, based on Representative Concentration Pathways (RCP) 4.5 and RCP 8.5 climate scenarios, were used as input data for the SWAT model for the simulation of streamflow. The FDCs were then derived from the simulated streamflow. The FDC for RCP 4.5 showed insignificant differences, whilst for RCP 8.5 it showed an increase of 5–10% in FDC from the baseline period, which is likely to increase the hydropower generation potential with some considerable streamflow variability. The integrated approach of utilizing FDC, GIS and SWAT for the estimation of flow variability and hydropower generation potential could be useful in data scarce regions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Wildfires are becoming more severe and frequent and their effects increasingly expose water resources to stress. • Climate, wildfire, streamflow records (1975 to 2018), topography, and landcover data from 92 catchments were combined to quantify the contribution of wildfire to streamflow variability across Australian temperate forests. • On average wildfire regimes relatively results in 8.8% increases in streamflow across Australian temperate forests. • Wildfire effects on regional streamflow varied spatially depending on hydroclimate factors, particularly precipitation. As wildfires become more frequent and severe, there are concerns regarding their impacts on water yield from forested catchments. While there are many studies in Australia about the effects of individual wildfires on streamflow at fine scales (<1 km2) in specific geographic settings, the effects of wildfire regimes on streamflow at broad spatial scales across temperate forests in Australia are not well understood. In this paper, we combined climate, wildfire, streamflow records (1975 to 2018), topographic, and landcover data from 92 catchments (74 – 4740 km2) in the Australian temperate zone to quantify the contribution of wildfire regimes over time to streamflow variability in different hydroclimatic settings (humid, dry sub-humid, and semi-arid) and geographic regions (Southeast Australia (SEA), Southwest Western Australia (SWWA), and Tasmania (TAS)). Wildfire regimes were represented by two metrics: the burnt area to drainage area (BDA) ratio for wildfire events in each year and a spatially averaged metric of Time Since Fire (TSF), which is a spatial average of time since the last wildfire in the catchment. By comparing prefire and postfire runoff ratios our study found that on average there was a short-term increase in runoff ratio (∼3% in year 1 and ∼6% in year 2 post-fire) after wildfires with BDA > 25%. No influence of fire was found in the long-term (15–20 years after the wildfire) runoff ratio. We found that wildfire regime, measured by TSF, explained ∼8.8% of the variation in annual streamflow across the Australian temperate zone, and that with decreasing TSF (i.e., increased wildfire impact), the average streamflow increased. Streamflow variation explained by wildfire regimes varied with hydroclimate. The explained variance of streamflow by wildfire regimes in semi-arid catchments (23%) and dry sub-humid catchments (13%) were higher than humid catchments (5%). Our results provide a broad-scale understanding of how wildfire regimes influence streamflow variability at broad temporal and spatial scales, and provided important context and baseline information for determining the implications of changes in climate and fire regimes for regional water availability. [ABSTRACT FROM AUTHOR]

Global change is expected to have a strong impact in the Himalayan region. The climatic and orographic conditions result in unique modelling challenges and requirements. This paper critically appraises recent hydrological modelling applications in Himalayan river basins, focusing on their utility to analyse the impacts of future climate and socio-economic changes on water resource availability in the region. Results show that the latter are only represented by land use change. Distributed, process-based hydrological models coupled with temperature-index melt models are predominant. The choice of spatial discretisation is critical for model performance due to the strong influence of elevation on meteorological variables and snow/ice accumulation and melt. However, the sparsity and limited reliability of point weather data, and the biases and low resolution of gridded datasets, hinder the representation of the meteorological complexity. These data limitations often limit the selection of models and the quality of the outputs by forcing the exclusion of processes that are significant to the local hydrology. The absence of observations for water stores and fluxes other than river flows prevents multi-variable calibration and increases the risk of equifinality. The uncertainties arising from these limitations are amplified in climate change analyses and, thus, systematic assessment of uncertainty propagation is required. Based on these insights, transferable recommendations are made on directions for future data collection and model applications that may enhance realism within models and advance the ability of global change impact assessments to inform adaptation planning in this globally important region. [ABSTRACT FROM AUTHOR]

Evaluating the hydrological application of reanalysis datasets is of practical importance for the design of water resources management and flood controlling facilities in regions with sparse meteorological data. This paper compared a new reanalysis dataset named CMADS with gauge observations and investigated the performance of the hydrological application of CMADS on daily streamflow, evapotranspiration, and soil moisture content simulations. The results show that: CMADS can represent meteorological elements including precipitation, temperature, relative humidity, and wind speed reasonably for both daily and monthly temporal scales while underestimates precipitation compared with gauge observations slightly (<15%). The hydrological model using CMADS dataset as meteorological inputs can capture the daily streamflow chracteristics well overall (with a NS value of 0.56 during calibration period and 0.61 during validation period) but underestimates streamflow obviously (with a BIAS of − 42.42 % during calibration period and a BIAS of − 33.29 % during validation period). The underestimation of streamflow simulated with CMADS dataset is more seriously in dry seasons ( − 48.40 %) than that in wet seasons ( − 39.41 %) for calibration period. The model driven by CMADS estimates evapotranspiration and soil moisture content well compared with the model driven by gauge observations. [ABSTRACT FROM AUTHOR]

Under normal precipitation conditions, increasing water demand causes groundwater level to deplete and streamflow to decrease. This results in more frequent intermittency. The so‐called intermittency cycle hypothesis is examined in this study. It is a vicious circle in hydrology making both surface and groundwater resources less efficient in supplying ever‐increasing water demand. To test the hypothesis, a data‐based analysis is applied to Gediz, Kucuk Menderes, and Ergene river basins in western Turkey. Results at basin‐ and station‐scale show no change in precipitation while streamflow fluctuates with a tendency to decrease and groundwater steeply declines. Groundwater is used excessively to meet the drastically increased water demand. With the overuse of groundwater, water table descends, streamflow decreases and intermittency increases. Intermittency ratio of a river is found related to the decrease in streamflow and the drop in groundwater level. It can trigger drought in streamflow and groundwater. It is concluded that intermittency can be taken as a quantitative indicator of drought in streamflow and groundwater. [ABSTRACT FROM AUTHOR]

In a warming climate, periods with lower than average precipitation will increase in frequency and intensity. During such periods, known as meteorological droughts, the decline in annual runoff may be proportionally larger than the corresponding decline in precipitation. Reasons behind this exacerbation of runoff deficit during dry periods remain largely unknown, and this challenges the predictability of when this exacerbation will occur in the future and how intense it will be. In this work, we tested the hypothesis that runoff deficit exacerbation during droughts is a common feature across climates, driven by evaporation enhancement. We relied on multidecadal records of streamflow and precipitation for more than 200 catchment areas across various European climates, which distinctively show the emergence of similar periods of exacerbated runoff deficit identified in previous studies, i.e. runoff deficit on the order of -20 % to -40 % less than what expected from precipitation deficits. The magnitude of this exacerbation is two to three times larger for basins located in dry regions than for basins in wet regions, and is qualitatively correlated with an increase in annual evaporation during droughts, in the order of +11 % and +33 % over basins characterized by energy-limited and water-limited evaporation regimes, respectively. Thus, enhanced atmospheric and vegetation demand for moisture during dry periods induces a nonlinear precipitation-runoff relationship for low-flow regimes, which results in an unexpectedly large decrease in runoff during periods of already low water availability. Forecasting onset, magnitude, and duration of these drops in runoff have paramount societal and ecological implications, especially in a warming climate, given their supporting role for safeguarding water, food, and energy. The outcome that water basins are prone to this exacerbation of runoff deficit for various climates and evaporation regimes makes further understanding of its patterns of predictability an urgent priority for water-resource planning and management in a warming and drier climate. [ABSTRACT FROM AUTHOR]