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Estimating the streamflow corresponding to a particular probability is of great importance in many hydrological studies, such as determining hydroelectric water potential, assessing water quality, and investigating sedimentation and drought. This paper aims to effectively estimate low-flow quantiles since hydrologic droughts motivate the study. The study illustrates a methodology, where droughts are characterized by the lower part of the flow--duration curve (FDC), and offers a perspective estimating low-flow quantiles related to the basin characteristics. Low-flow quantiles are derived both from traditional FDCs curves and median annual FDCs (AFDCs). As an innovation, the concept of areal scale factor, which represents a scaling ratio between the basin area and flow quantiles, was introduced. Unlike many other parametric approaches, this study models the streamflow quantiles depending on the basin characteristics instead of the parameters in the analytical equation of FDCs. The methodology was evaluated for the Western and Southwestern Anatolia regions in Turkey. The outcomes were compared for two types of FDCs in two regions. The approach gave similar results for both study regions. AFDCs provided a distinct advantage over traditional FDCs, especially for low-flow quantiles, due to the superiority of AFDCs in estimating streamflow quantiles of intermittent streams. [ABSTRACT FROM AUTHOR]

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]

This paper investigates how hydrological drought affected suspended sediment yield (SSY) in the Mediterranean Ofanto River basin, southern Italy, and in its five sub-basins from 1951 to 1989. The Standardized Precipitation Evaporation Index over a scale of 12 months (SPEI12) has been used to compute the hydrological drought in the investigated span of time. SPEI12, mean and maximum monthly discharge (Qmean and Qmax), monthly rainfall erosivity, monthly simple daily intensity index and monthly SSY were used to assess the relationships between dry/wet cycles, streamflow and SSY through the Pearson correlation matrix. Qmean and Qmax are significantly correlated with SSY, while SPEI12 and rainfall intensity do not show good correlation with SSY. Furthermore, from the overall analyses it emerges that sediment yield estimations were higher mainly during the wet period following a period of drought or during a drought period. [ABSTRACT FROM AUTHOR]

In the paper, the frequency and spatial distribution of atmospheric and hydrological droughts over the territory of Belarus in 1945–2020 is assessed. Aridity is evaluated using the standardized precipitation and streamflow indices (SPI and SSFI). Trends in monthly total precipitation are determined, the features of the streamflow formation during the open-channel period are revealed. The frequency of anticyclones that determines hydrometeorological conditions on the territory of Belarus is calculated. The statistically significant increase in the number of the days with anticyclonic weather over the country during the warm season is found. The increasing frequency of dry conditions in summer months against the background of a rise in the precipitation intensity is highlighted. A simultaneous increase in the frequency of very-low-flow periods on rivers and an increase in the height of warm rainfall floods in some river basins are revealed. [ABSTRACT FROM AUTHOR]

Tree rings have been central to the understanding of variability of flow of the Colorado River. Spurred by steadily declining flows after the 1920s, early tree‐ring research drew attention to the importance of climate variability to water supply by identifying episodes in the past that were even drier. Application of modern statistical methods to tree‐ring data later yielded a reconstruction of annual flows at Lees Ferry back to the early 1500s that highlighted the unprecedented wetness of the base period for the 1922 Colorado River Compact. That reconstruction served as the framework for a collection of papers in a 1995 special issue of Water Resources Bulletin on coping with severe sustained drought on the Colorado River. This retrospective paper reviews historical aspects of the dendrohydrology of the Colorado River, and the updates since 1995. A constantly expanding tree‐ring network has been subjected to an array of new statistical approaches to reconstruction. Climate change and increasing demand for water have meanwhile driven increased interest in the processing and presentation of reconstructions for optimal use in water resources planning and management. While highlighting the robustness of main findings of earlier studies, recent research yields improved estimates of magnitudes of flow anomalies, extends annual flows to more than 1200 years, and underscores unmatched drought duration in the medieval period. [ABSTRACT FROM AUTHOR]

The Murray-Darling River system is perhaps Australia's most important, with significant social, cultural and environmental values including 16 Ramsar listed wetlands. The MDB is home to 2.6 million people and produces about $24 billion worth in agricultural production each year (about one-third of total value for Australia). Hydrologic issues, typified by water availability and quality, have existed for many years, peaking during the Millennium drought from 1997 to 2010. Competing interests (i.e. irrigation, tourism, environmental heath), and the declining flows and water quality during droughts, led governments and water management agencies to consider the risks to water resources in the system in the early-mid 2000s. This paper reviews changes to risks associated with forest dynamics, as identified by - afforestation and bushfire – and considers new issues that have emerged since that analysis. It was found that the potential impacts of bushfire on stream flows were over-estimated in past studies, and that a planned significant afforestation expansion into agricultural and grazing land that was projected to reduce stream flows did not occur. While these two risks now do not seem likely to have significant future impacts on flows, or consequent effects on downstream users, the interaction of elevated CO2 and increasing temperatures on vegetation functioning and subsequent hydrologic consequences at catchment scale require further research and analysis. Reduced rainfall and increased temperatures under future climate change are likely to have an impact on inputs and flows. Uncertainties in how these changes, and feedbacks between climate, drought, more frequent fire and vegetation responses, impact on system hydrology also require further investigation. [ABSTRACT FROM AUTHOR]

For an effective reservoir operation during drought, the variations of both water supply and water demand which depend on hydrological and meteorological conditions need to be dealt with. This paper aimed to consider these variations in the Aharchay basin (Iran) by coupling a hedging rule (HR)-based reservoir operation model (HRROM) with a climate-based irrigation scheduling model (CBISM) at the farm level. Through the HRROM, optimal long-term decisions for Sattarkhan reservoir were made by considering the probable streamflow scenarios in the system. Given the variable agricultural demands (VAD) in the CBISM, the irrigation water was optimally allocated to the crops using several evapotranspiration (ET) scenarios. The CBISM employs three sub-models including linear programming (LP), nonlinear programming (NLP), and particle swarm optimization (PSO) to maximize the total income of the Aharchay agricultural network as a function of the climate factors and the supplied water. To this end, the daily weather and discharge data from 1990 to 2015 were used in this study. The standardized precipitation-evapotranspiration index (SPEI) and the streamflow drought index (SDI) were used to detect the meteorological and hydrological droughts, respectively. The SPEI was calculated based on the high-resolution-gridded datasets of the Climatic Research Unit (CRU). The findings demonstrated that the HRROM-CBISM generally managed to increase the time-based (αt) and volume-based (αv) reliability indices by 20% and 44%, respectively, compared with the conventional standard operation policy (SOP). For more investigations, the three major droughts of 2000–2002, 2004–2006, and 2008–2014 were separately analyzed. The average values of αt, αv, and vulnerability (V) for SOP were 0.33, 0.51, and 0.48, respectively. With the HRROM-CBISM, these values were about 0.5, 0.55, and 0.45, respectively. Among these indices, αt had the highest variations, while αv had the lowest variations in both the SOP and HRROM-CBISM approaches. The average water shortage for the mentioned droughts was significantly decreased from 89 (SOP) to 75 MCM (HRROM-CBISM). [ABSTRACT FROM AUTHOR]

Portions of the northeastern United States (NE) have experienced drought every year since 2016. The U.S. Drought Monitor (USDM) has played an important role in drought characterization and management by providing weekly drought maps across the entire United States, including the NE. Unfortunately, the USDM lacks consistency between input variables leading to difficulties in defining boundaries between drought categories. This paper evaluates the National Water Model's (NWM) ability to model streamflow and soil moisture, two important hydrological products that are frequently incorporated in drought indices. Using a 26-yr NWM retrospective simulation, comparisons were conducted between NWM output and observations of streamflow and soil moisture, as well as between drought categories derived from the NWM and observations and the USDM. Results indicate that NWM provides moderate predictions of streamflow at NE stations when comparing to historical observations, that NWM streamflow estimators are generally upwardly biased, and performance is worse at lower streamflow magnitudes. The NWM's ability to predict soil moisture is worse than streamflow, with again a positive bias at most sites and strong variations in anomaly correlation across sites. When predicting drought categories, NWM streamflow is as strong a predictor of USDM drought categories as observed streamflow. Extending the NWM streamflow series using a maintenance of variance technique and only past records provides slight improvements over drought categories derived from the entire 26-yr retrospective simulation. Output from the NWM appears to have some skill in characterizing drought in the NE and provides a spatial resolution to improve the designation of drought boundaries. [ABSTRACT FROM AUTHOR]

Dominant hydrological processes of a catchment could shift due to a changing climate. This climate-induced hydrological nonstationarity could affect the reliability of future runoff projection developed using a hydrological model calibrated for the historical period as the model or parameters may no longer be suitable under a different future hydroclimate. This paper explores whether competing parameterization approaches proposed to account for hydrological nonstationarity could improve the robustness of future runoff projection compared to the traditional approach where the model is calibrated targeting overall model performance over the entire historical period. The modeling experiments are carried out using climate and streamflow datasets from southeastern Australia, which has experienced a long drought and exhibited noticeable hydrological nonstationarity. The results show that robust multicriteria calibration based on the Pareto front can provide a more consistent model performance over contrasting hydroclimate conditions, but at a slight expense of increased bias over the entire historical period compared to the traditional approach. However, the robust calibration does not necessarily result in a more reliable projection of future runoff. This is because the systematic bias in any parameterization approach would propagate from the historical period to the future period and would largely be cancelled out when estimating the relative runoff change. Ensemble simulations combining results from different parameterization considerations could produce a more inclusive range of future runoff projection as it covers the uncertainties due to model parameterization. [ABSTRACT FROM AUTHOR]

The paper discusses the impact of atmospheric circulation on the occurrence of droughts. The research includes mean monthly discharges for 7 rivers in 1954–2018. Dry periods were determined with Standardised Streamflow Indices (SSI-12). Additionally, the circulation type calendar for Central Poland was used to determine the atmospheric circulation indices: western zonal (W), southern meridional (S) and cyclonicity (C). The analyses indicated a variation in the duration and intensity of droughts in the rivers. 2014–2017 was the driest period with the lowest SSI-12 for most rivers and the highest number of extremely dry months. The advection of air from the West and the South prevailed and anticyclonic synoptic situations dominated over the cyclonic types. Drought spells occurred at a dominance of anticyclonic circulation, with the inflow of air from the North and with increased western zonal circulation. [ABSTRACT FROM AUTHOR]

In drought risk management, the regional analysis of drought is significant. In this paper, this important issue is investigated by presenting the new hydrological regional drought index (RDI). For this purpose, the Ajichai basin was selected as the study area. First, the time series of the streamflow drought index (SDI) was calculated for each of the hydrometric stations in the basin f regional analysis of hydrological drought. Then, to determine the homogeneous regions in terms of hydrological drought, the k-means method was used for clustering analysis. Based on the clustering results, 6 Homogeneous regions were identified in the basin. For each of these regions, the time series of the RDI index was calculated from 1365 to 1393. The results showed that during the study period in each of the regions 1, 2, 3, 4, 5, and 6, mild Wet and mild drought has occurred at 82.1, 80.1, 78.9, 83.3, and 84.3 percent of regions, respectively. Also, the total percentage of drought events (moderate and high) is higher than the total percentage of wet events (moderate and high) in all regions. So, during the study period, the total percentage of drought events (moderate and high) is more than twice the total percentage of wet events (moderate and high) in regions 2 and 3. [ABSTRACT FROM AUTHOR]

• Standardized River Stage Index (SRSI) is proposed to measure drought. • SRSI is calculated using satellite altimetry without in-situ observations. • SRSI captures variation in drought of river water and identifies drought events. • SRSI reveals time lags in drought propagation. The lack of in-situ hydrological observations, especially in remote and developing areas, is one of the main challenges of drought monitoring. This paper proposes a novel drought index, the Standardized River Stage Index (SRSI), based on data retrieved from satellite altimetry missions. The novelties of the SRSI are that it can quantitatively monitor drought of river water deficits at multiple timescales and that it does not require in-situ observations. The Mekong River is used as the case study to demonstrate and validate the SRSI calculated from the multi-mission satellite altimetry data (Envisat and Jason-2/3). The log-logistic distribution is best fitted for SRSI calculation for the highest goodness-of-fit for river stages for the Mekong River. The comparison with the Standardized Streamflow Index (SSI) from the in-situ discharge observations shows that the SRSI can reasonably capture the drought of river water deficit. It is effective in identifying the timing and intensity of severe drought events, such as the 2019 Thailand drought. The SRSI is also compared with the Standardized Precipitation Evapotranspiration Index and the GRACE Drought Index in revealing drought features such that the river water deficit of Mekong River has time lags of 2 to 3 months for precipitation deficits and water storage deficits. Hence, the SRSI is more suitable for monitoring drought of river water deficit, because it directly links to the streamflow via physical stage-discharge relationship. Overall, the SRSI provides an effective indicator of drought of river water deficit when in-situ observations are unavailable. It can serve in the real-time drought early warning in remote and developing areas. [ABSTRACT FROM AUTHOR]

• Standardized River Stage Index (SRSI) is proposed to measure drought. • SRSI is calculated using satellite altimetry without in-situ observations. • SRSI captures variation in drought of river water and identifies drought events. • SRSI reveals time lags in drought propagation. The lack of in-situ hydrological observations, especially in remote and developing areas, is one of the main challenges of drought monitoring. This paper proposes a novel drought index, the Standardized River Stage Index (SRSI), based on data retrieved from satellite altimetry missions. The novelties of the SRSI are that it can quantitatively monitor drought of river water deficits at multiple timescales and that it does not require in-situ observations. The Mekong River is used as the case study to demonstrate and validate the SRSI calculated from the multi-mission satellite altimetry data (Envisat and Jason-2/3). The log-logistic distribution is best fitted for SRSI calculation for the highest goodness-of-fit for river stages for the Mekong River. The comparison with the Standardized Streamflow Index (SSI) from the in-situ discharge observations shows that the SRSI can reasonably capture the drought of river water deficit. It is effective in identifying the timing and intensity of severe drought events, such as the 2019 Thailand drought. The SRSI is also compared with the Standardized Precipitation Evapotranspiration Index and the GRACE Drought Index in revealing drought features such that the river water deficit of Mekong River has time lags of 2 to 3 months for precipitation deficits and water storage deficits. Hence, the SRSI is more suitable for monitoring drought of river water deficit, because it directly links to the streamflow via physical stage-discharge relationship. Overall, the SRSI provides an effective indicator of drought of river water deficit when in-situ observations are unavailable. It can serve in the real-time drought early warning in remote and developing areas. [ABSTRACT FROM AUTHOR]

• Recovery of hydrological droughts can be 4 times longer than that of meteorological ones. • Recovery time is longest and more severe in dry regions and regions with high water use. • A multi-indicator approach is key to the proper characterization of drought development and propagation. Meteorological droughts propagate through the hydrological cycle causing hydrological droughts and societal impacts. However, the effects of climate and basin characteristics on hydrologic drought propagation vary regionally and remain largely unclear. In this paper, we characterize meteorological and hydrological droughts in 457 basins in Brazil. Using the Standardized Precipitation Evapotranspiration Index (SPEI) and Standardized Streamflow Index (SSI), we investigate hydrologic drought propagation based on differences in drought onset, the center of mass, and time to peak. Additionally, we estimate the recovery time of meteorological and hydrologic drought events. The results indicate that hydrological droughts are usually more long-lasting, severe, and with a slower recovery time compared to meteorological droughts. While the most severe meteorological droughts are observed in humid regions (e.g., Amazon and Southern Brazil), the most severe and long-lasting hydrological droughts are found in the driest region (i.e., Northeast Brazil) or mostly impacted by human activities (i.e., Southeast Brazil). Hydrological droughts in dry regions can take four times longer to recover than meteorological ones. For most regions, the propagation time was slightly different considering the different approaches. Our results highlight the importance of a multi-indicator approach to fully characterize the mechanisms controlling the development and propagation of droughts through the water cycle. [ABSTRACT FROM AUTHOR]