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[Display omitted] • An integrated approach involving HGS modeling and isotope analysis is developed. • The contribution of the subsurface water to the stream is 32% for one rainfall event. • Rainfall infiltration-mixing-exfiltration process is vital to runoff generation. • Exfiltration-induced overland flow is the dominant water source for a rainfall event. The majority of runoff studies to date have reported a similar pattern of rapid rain-to-runoff conversion via overland flow and shallow subsurface water flow, as well as the ubiquity of thresholds and hysteresis in rainfall-runoff response. But each of those approaches has limitations in terms of inference and process description. Processes of water transport and mixing and how they influence runoff generation are still not well understood or quantified. In this study, HydroGeoSphere, a fully-integrated surface/subsurface flow model, was used together with stable isotopes for disentangling runoff generation mechanisms in a headwater catchment. The study area (0.21 km2) is the first-level tributary of the Xin'an Jiang River located in a humid climate region of eastern China. Water flow simulation elucidated the spatial water transport process, and isotope tracing quantified the complex water mixing process between the shallow soil and hillslope surface. This study provides insights into the rapid transformation of rainfall infiltration and mixing in soil and exfiltration to a hillslope. Results show that the stream runoff was separated into rainfall-induced overland flow (9 %), exfiltration-induced overland flow (including the 53 % of mixed water from rainfall infiltration and 6 % of the stored soil water), and subsurface flow (32 %). Exfiltration-induced overland flow was the dominant water source in the headwater catchment during a rainfall-runoff event. The mutual influence of rainfall infiltration and soil water exfiltration caused the shallow soil to rapidly become saturated, thus forming saturation excess overland flow. The consistence of estimated soil water velocity based on the integrated isotope and numerical modeling approach, at about 0.4 m/d nearby the stream channel, improved the reliability of model visualizations. Further analysis indicates that water flow paths and velocities responding to the rainfall event were attributed to the soil moisture conditions and topography. This study demonstrates that identifying the rapid mixing processes between the rain and the soil water was crucial for understanding hillslope overland flow and runoff generation in headwater catchments. [ABSTRACT FROM AUTHOR]

Basin-scale runoff changes reflect the comprehensive impact of various environmental variables including climate and vegetation. Among them, vegetation plays an important role in the process of runoff generation and is one of the key links affecting the hydrological cycle. However, the contribution of vegetation to runoff changes is still unclear. In order to analyze the main driving factors of runoff changes in the Yanhe River Basin, this research builds a BPG model based on the Budyko framework, Porporato model, and Guswa model to evaluate the sensitivity coefficient of runoff to vegetation change, and quantitatively reveals the contribution of vegetation to runoff change. The results showed that, the runoff in the Yanhe River Basin decreased significantly from 1982 to 2018, and a mutation point occurred in 1999. After that, the runoff decreased by 29.88% and the NDVI increased by 15.30%. In terms of the sensitivity to runoff changes, runoff is more sensitive to changes in basin characteristics including vegetation than the climate change. According to the concepts of relative contribution and absolute contribution proposed in this research, the relative contribution rate and absolute contribution rate of climate change and basin characteristic changes were − 18.13%, 117.67% and 43.22%, 56.78%, respectively. Changes in basin characteristics such as vegetation, terraces and check dams caused by human activities are the main causes for runoff changes. The relative contribution rate and absolute contribution rate of vegetation change to runoff change were 34.98% and 16.88%, respectively, which is one of the dominant factors for basin characteristics change. And basin characteristic factors such as artificial water withdrawal, terraces and silt dams are the other main contributors to runoff changes. The research results can provide a theoretical basis for guiding future ecological restoration projects and water resources planning in the basin. • BPG model was built to evaluate the sensitivity and quantitatively reveal the contribution of vegetation to runoff change. • Proposed the concepts of relative contribution and absolute contribution of runoff change. • The runoff was more sensitive to basin characteristics change than climate change. • Climate change led to runoff increase while basin characteristics change led to runoff reduction in Yanhe River. • Changes in basin characteristics such as vegetation are the dominant factors of runoff changes. [ABSTRACT FROM AUTHOR]

Abstract: The application of artificial neural network (ANN) to rainfall-runoff simulations has provided promising results in recent years. However, it is difficult to obtain satisfying results by using raw data for the direct prediction of the time series of streamflows. To improve simulating daily streamflow with back-propagation (BP) neural networks, the whole data set in this study is divided into two independent groups, flood period and non-flood period. The approaches and techniques of applying the division-based BP (DBP) in runoff simulation are presented in this paper. A comparison of the DBP model to the primitive BP model and the Xinanjiang model was also conducted to evaluate the effectiveness of the improvement. The numerical experimental results indicate that DBP model still overestimated flow peak, but improved considerably the streamflow simulation in the non-flood period. [Copyright &y& Elsevier]

The effect of soil and water conservation measures (SWCMs) is usually dependent on time. Thus the trend in reducing runoff and sediment over time is a very important theoretical problem for evaluating the effectiveness of SWCMs. Moreover, there is still a lack of comprehensive assessment of water erosion dynamics following implementing SWCMs despite their ecological significance. Therefore, the long-term impact of SWCMs on runoff and sediment and their relationships was assessed for an orchard on sloping red soil in southern China. Overland flow and erosion sediment were continuously observed for 15 years on citrus experimental s under one of four treatments: grass strips, strip intercropping, level terrace and clean-tillage. By means of Mann–Kendall trend tests and double cumulative curves, the time series of runoff and sediment under the different treatments were analyzed. Furthermore, we linked the effect of soil conservation and the relationship between runoff and sediment variation to determine the mechanism of conservation measures on sediment reduction. The results showed that the first 4 years was the key period to prevent soil erosion for this orchard, and then the intensity of soil erosion decreased below 500 t·km–2·a–1. Considering economic costs and ecological effect, grass strips were the best protective measure for this test situation. The fitted curves of the effect of SWCMs on sediment reduction over time showed an ‘L’ form, but on runoff there was an approximately horizontal line. The SWCMs did not change the rainfall–runoff relationship, but did change the runoff–sediment erosion relationship. The erosion reduction mechanism of SWCMs in the early phase was a joint function of reducing runoff and changing the runoff–sediment relationship, and in the post-stable phase it worked mainly by reducing runoff. The results provide the basis for rational allocation of SWCMs considering location and time. [ABSTRACT FROM AUTHOR]

• Flood season and autumn are the leading periods of annual runoff reduction. • The underlying surface change is the dominant factor for annual runoff change. • Climate change dominates runoff changes in flood season, summer and autumn. • Underlying surface dominates runoff changes in non-flood season, spring and winter. Accurately understanding the intra-annual variation in runoff evolution attribution is essential for basin-scale water resources management. In this study, the runoff in the Yellow River source area was divided into two intra-annual time scales: flood season, non-flood season and spring, summer, autumn and winter, and the sensitivity and attribution differences of runoff changes in the Yellow River source area at different intra-annual time scales were quantitatively assessed based on the time-varying Budyko framework. Results show that the runoff in the Yellow River source area decreases during the flood season and spring, summer and autumn, and increases during the non-flood season and winter from 1960 to 2020, with an insignificant decreasing trend in annual runoff. Flood season and autumn runoff changes are the main reasons for the reduction in annual runoff. Runoff in the Yellow River source area is most sensitive to the underlying surface such as vegetation and soil freeze–thaw change, and in terms of climate change, non-flood season and autumn runoff is more sensitive to changes in precipitation, while flood season and summer runoff is more sensitive to changes in potential evaporation. The underlying surface change is the dominant factor for annual runoff change. And for the intra-annual runoff change, the non-flood season, spring and winter runoff in the Yellow River source area is dominated by the underlying surface change, and flood season, summer and autumn runoff is dominated by the climate change. These findings can provide theoretical support for the scientific response to environmental change and the enhancement of water conservation capacity in the Yellow River source area. [ABSTRACT FROM AUTHOR]

Most glaciers in the Tibetan Plateau ( TP) are not closely monitored for mass balance (MB) due to their inaccessibility, which makes it difficult to better understand the dynamics of glacial advancement or retreat. Surface energy budget, MB, and the resulting melt runoff were calculated for Zhadang glacier (5,710 m above sea level) of the central TP. Energy balance was calculated on 30-m square grids for the summers of 2007 and 2008. On average, net radiation dominated the total energy source (66%) while the residual was supplied by sensible heat flux. More than 67% of the energy sink was available for melting on the glacier. Thus, less than 33% of the total energy was consumed by latent heat flux. A large and a slightly negative summer MB were calculated for the 2007 and 2008 summers, respectively. The high sensitivity of the glacier to air temperature may indicate that the lower than average seasonal temperature was more important than the increased precipitation for the slightly negative MB in the summer of 2008. Comparisons of glacial melt runoff indicated that rainfall and snowmelt were the dominant contribution to total runoff in the glacierized basin and the ice melting is also very important. Glacial melt calculation provides a basis for quantifying glacial melt-runoff contribution to the river streamflow in the TP. [ABSTRACT FROM AUTHOR]

Summary Improving the predictive capabilities of rainfall-runoff models in ungauged catchments is a challenging task but has important theoretical and practical significance. In this study, we investigated the predictive capabilities of the conceptual Xinanjiang model (XAJ), which is widely used for flood forecasting and simulation in China, in ungauged catchments. We further produced a hybrid rainfall-runoff model (named XAJ-GIUH) by coupling the XAJ model with the geomorphologic instantaneous unit hydrograph (GIUH) to achieve improved flood predictions in ungauged catchments. The flood prediction capabilities of the original XAJ model and the XAJ-GIUH model were investigated and compared at an hourly time scale in a mountainous catchment with six nested catchments located in the south of Anhui province, China. The two models were first calibrated for each individual catchment by comparing with the observed streamflows. Then, the nested catchments were treated as ungauged and modeled using the parameter values regionalized by transposition from the downstream catchments. The results show that the performance of both models is comparable and satisfactory on different catchment scales in the case that model parameters are calibrated in each catchment. However, the models perform differently in the case that model parameters are transposed from the downstream catchments. The XAJ-GIUH model produced markedly improved estimates of peak discharge and peak time as compared to the original XAJ model in the latter case, indicating that the runoff routing is a major uncertainty source for the application of the XAJ model in this case. Coupling XAJ with topography-based GIUH has the potential to substantially improve the flood prediction capability of the XAJ model in ungauged catchments. Our analysis further reveals that the models do not necessarily perform better when the parameter values are transposed from closer donor catchment. Instead, adopting the parameter values from the catchment with more similar topographic characteristics is more likely to produce better model performance. This study implicates the necessity of including remotely sensed geomorphologic characteristics of ungauged catchments to improve flood predictions in these regions. [ABSTRACT FROM AUTHOR]

The concept of elasticity has been widely employed to quantify the hydrological response to changes in climate and catchments properties. To separate the effect of different climatic variables on runoff, the potential evaporation (E 0 ) elasticity of runoff needs to be presented in term of observed climate variables. To fully reflect the effects of maximum and minimum temperatures and reduce the influence of the correlations of radiation with sunshine duration and relative humidity on the assessment results, we decompose the E 0 elasticity into five evaporation-related elasticities (i.e., sunshine duration, maximum and minimum temperature, wind speed and relative humidity) via the first-order differentiation of the FAO 56 Penman equation. As the catchment runoff is frequently affected by the land use/cover change, we also consider changes in catchment characteristics and derive a catchment alteration elasticity based on the Budyko framework. An application was carried out in 30 catchments with widespread climatic types in China. For the two periods (i.e., the baseline period and the changed period) divided by the Pettitt test, the contributions of different climatic variables and land use/cover conditions to runoff change were quantified. In general, the alteration of catchment characteristics and climatic change should be mainly responsible for changes in runoff in water-limited and humid basins, respectively. Although the elasticity of maximum temperature are usually higher than that of minimum temperature, the contributions to runoff change present the opposite direction. Furthermore, additional analysis indicated some overestimation in relative humidity elasticities in the previous studies, further emphasizing the necessity of our extension to alleviate the influence of correlation between climatic variables to the assessment results. Moreover, the results of model performance versus model complexity showed that the choice of model complexity still depends on the goal in the climatic and anthropogenic effects on runoff. If the climatic effect as a whole was of interest, the method based on two parameters elasticities would be more applicable compared to that based on multi-parameter elasticities because the simple model provides higher confidence in modelling the overall change in runoff. [ABSTRACT FROM AUTHOR]

Climate change in North China would result in significant changes in temperature, precipitation and their spatial/temporal distributions. Consequently, these induced changes will have profound effects on the hydrological cycle and water resources in both agricultural and natural ecosystems. Panjiakou reservoir in the middle Luanhe River basin-a tributary of the Haihe River basin-is one of the important sources of water for industrial and agricultural development in Beijing, Tianjin and Hebei province, China. Any significant change in the magnitude and/or timing of runoff from the reservoir induced by changes in climatic variables would have significant implication for the economic prosperity in North China. This paper investigates the impacts of climate change on hydrological processes in the Luanhe River basin as follows. Firstly, spatial and temporal patterns of precipitation, temperature and runoff at both annual and seasonal scales from 1957 to 2000 in the Luanhe River basin are analyzed using Mann-Kendall trend analysis, linear regression methods and inverse distance weighted interpolation. For the impact study, four Global Climate Models (GCMs) (named CSIRO, HadCM3, CNRM and GFDL) were used to produce precipitation and temperature data under A2 scenario by mean of a widely used quantile-quantile transformation. Projected meteorological variables were used to force a two-parameter hydrologic model to simulate the hydrological response to climate change in the future (2021-2050). Moreover, a sensitivity analysis is conducted to assess how precipitation and temperature affect the runoff. Results suggested that most part of the Luanhe River basin was dominated by significant increasing trends of temperature and no significant trends of precipitation in annual and seasonal scale during the past decades. Annual, spring and autumn runoffs present significant decreasing trends in the Panjiakou reservoir basin. Meanwhile, runoff is more strongly related to precipitation than to temperature. All GCMs projected precipitation and temperature series after bias correction indicated increasing temperature and increasing precipitation trends for the period 2021-2050 except that CNRM showed a slight decreasing trend in precipitation. Great enhancements can be found in projected runoff except CNRM by driving the two-parameter water balance model. The study provides valuable information on the assessment of the impact of the climate change on water resources in the Luanhe River basin as well for allocating and designing water resources projects. [ABSTRACT FROM AUTHOR]

Summary: High suspended sediment (SPS) concentration exists in many Asian rivers. In addition, human activities and climate change can change river runoff, leading to the variation of SPS and pollutant concentrations. In this research, the effects of SPS and dissolved organic nitrogen (DON) concentration on DON transformation in river systems were studied through simulation experiments with samples collected from the Yellow River which is famous for its high SPS concentration. The results indicated that high DON concentration resulted in a longer retention time of and in the system due to the inhibition effect of ammonia on nitrification. The re-suspension of sediment accelerated DON transformation, and both the ammonification and nitrification rates increased with SPS concentration. The ammonification rate constants obtained from the first-order kinetics were 0.286, 0.332, 0.538day−1; the nitrification rate constants obtained from the Logistic model were 0.0018, 0.0038, 0.005day−1 μmol−1 L−1 for the systems with SPS concentration of 0, 5, 10gL−1, respectively. Bacteria tended to attach onto SPS, and the specific growth rate in the systems with SPS was approximately two orders of magnitude higher than that without SPS in the first 3days of cultivation, which resulted in an increase of DON transformation rate with SPS concentration. This study implied that DON transformation rate may be lower in the dry season than that in the wet season, and nitrogen transformation will be affected by the variation of river runoff and SPS concentration. [Copyright &y& Elsevier]

For sake of improving our current understanding on soil erosion processes in the hilly-gully loess regions of the middle Yellow River basin in China, a digital elevation model (DEM)-based runoff and sediment processes simulating model was developed. Infiltration excess runoff theory was used to describe the runoff generation process while a kinematic wave equation was solved using the finite-difference technique to simulate concentration processes on hillslopes. The soil erosion processes were modelled using the particular characteristics of loess slope, gully slope, and groove to characterize the unique features of steep hillslopes and a large variety of gullies based on a number of experiments. The constructed model was calibrated and verified in the Chabagou catchment, located in the middle Yellow River of China and dominated by an extreme soil-erosion rate. Moreover, spatio-temporal characterization of the soil erosion processes in small catchments and in-depth analysis between discharge and sediment concentration for the hyper-concentrated flows were addressed in detail. Thereafter, the calibrated model was applied to the Xingzihe catchment, which is dominated by similar soil erosion processes in the Yellow River basin. Results indicate that the model is capable of simulating runoff and soil erosion processes in such hilly-gully loess regions. The developed model are expected to contribute to further understanding of runoff generation and soil erosion processes in small catchments characterized by steep hillslopes, a large variety of gullies, and hyper-concentrated flow, and will be beneficial to water and soil conservation planning and management for catchments dealing with serious water and soil loss in the Loess Plateau. [ABSTRACT FROM AUTHOR]

The precipitation rate dominates meteoric 10Be in soils rather than soil dynamics, and the relation of the two is indicated in a binomial way with positive correlation in low precipitation regions (<1200 mm per year) and negative in high precipitation regions (>1200 mm per year). • 10Be concentrations in topsoil are influenced by precipitation and soil dynamics. • Precipitation shows more control on 10Be using PCA and correlation analysis. • 10Be concentrations-precipitation shows different trends in dry and humid regions. • Underlying surface affects 10Be without changing 10Be-precipitation relationships. Meteoric beryllium-10 (10Be) is commonly used as a proxy of landscape dynamics (erosion and sedimentation rates) and soil development. Soil represents the first-stage reservoir of meteoric 10Be, and variability in the concentration of the isotope in soils may be affected by soil properties and atmospheric deposition. Although many investigations have targeted this issue, there are still problems in estimating the atmospheric input of the isotope in different soil environments. Here, we used 10Be data measured in soils distributed across China to explore the potential influence of meteorological and pedological conditions on the isotope concentration and related applications. In addition, to determine the mechanisms controlling 10Be concentrations in topsoil on a regional scale, the soil samples were sub-divided into 18 different catchments according to fluvial systems. The results indicated that there were significant negative correlations between precipitation and the soil 10Be concentration in high-precipitation regions (>1200 mm·y−1) and significant positive correlations for soils in low precipitation regions (<1200 mm·y−1). The data also revealed that precipitation is the most important variable controlling the 10Be concentration in soils of China when compared with the effects of soil properties such as grain size, mineralogy, pH, and cation exchange capacity. Land use and soil erosion may have limited impacts on the distribution of 10Be in soils. [ABSTRACT FROM AUTHOR]