Stochastic Ecohydrological Perspective on Semi‐Distributed Rainfall‐Runoff Dynamics

The Budyko curve is a widely used framework for predicting the steady-state water balance –solely based on the hydro-climatic setting of river basins. While this framework has been tested and verified across a wide range of climates and settings around the globe, numerous catchments have been reported to considerably deviate from the predicted behavior. Here, we hypothesize that storage capacity and field capacity of the root zone are important controls of the water limitation of evapotranspiration and thus deviations of the mean annual water balance from the Budyko curve. For testing our hypothesis, we selected 16 catchments of different climatic settings and varied the corresponding parameters of a simple water balance model that was previously calibrated against long-term data and investigated the corresponding variations of the simulated water balance in the Budyko space. We found that total soil storage capacity –by controlling water availability and limitation of evapotranspiration– explains deviations of the evaporation ratio (EVR) from the Budyko curve. Similarly, however to a lesser extent, the evaporation ratio showed sensitivity to alterations of the field capacity. In most cases, the parameter variations generated evaporation ratios enveloping the Budyko curve. The distinct soil storage volumes that matched the Budyko curve clustered at a normalized storage capacity equivalent to 5–15 % of mean annual precipitation. The second, capillarity-related soil parameter clustered at around 0.6–0.8, which is in line with its hydropedological interpretation. A simultaneous variation of both parameters provided additional insights into the interrelation of both parameters and their joint control on offsets from the Budyko curve. Here we found three different sensitivity patterns and we conclude the study with a reflection relating these offsets to the concept of catchment coevolution. The results of this study could also be useful to facilitate evaluation of the water balance in data-scarce regions, as they help constrain parameterizations for hydrological models a priori using the Budyko curve as a predictor.

The Budyko curve is a widely used framework for predicting the steady-state water balance –solely based on the hydro-climatic setting of river basins. While this framework has been tested and verified across a wide range of climates and settings around the globe, numerous catchments have been reported to considerably deviate from the predicted behavior. Here, we hypothesize that storage capacity and field capacity of the root zone are important controls of the water limitation of evapotranspiration and thus deviations of the mean annual water balance from the Budyko curve. For testing our hypothesis, we selected 16 catchments of different climatic settings and varied the corresponding parameters of a simple water balance model that was previously calibrated against long-term data and investigated the corresponding variations of the simulated water balance in the Budyko space. We found that total soil storage capacity –by controlling water availability and limitation of evapotranspiration– explains deviations of the evaporation ratio (EVR) from the Budyko curve. Similarly, however to a lesser extent, the evaporation ratio showed sensitivity to alterations of the field capacity. In most cases, the parameter variations generated evaporation ratios enveloping the Budyko curve. The distinct soil storage volumes that matched the Budyko curve clustered at a normalized storage capacity equivalent to 5–15 % of mean annual precipitation. The second, capillarity-related soil parameter clustered at around 0.6–0.8, which is in line with its hydropedological interpretation. A simultaneous variation of both parameters provided additional insights into the interrelation of both parameters and their joint control on offsets from the Budyko curve. Here we found three different sensitivity patterns and we conclude the study with a reflection relating these offsets to the concept of catchment coevolution. The results of this study could also be useful to facilitate evaluation of the water balance in data-scarce regions, as they help constrain parameterizations for hydrological models a priori using the Budyko curve as a predictor.

Zhang S., Yang X., Lovdahl L. (2016): Soil management practice effect on water balance of a dryland soil during fallow period on the Loess Plateau of China. Soil & Water Res., 11: 64–73. To understand the mechanisms affecting water balance partitioning during fallow on drylands could improve the fallow management practices in arable land ecosystems. A three-year field experiment was conducted to evaluate the effects of field management regimes on water balance partitioning and fallow efficiency during the fallow periods under a winter wheat ( Triticum aestivum L.) fallow system on the Loess Plateau, China. The fallow management regimes tested were: ( i) conventional practice, ( ii) catch cropping, and (iii) no tillage with wheat straw mulching. A process-oriented ecosystem model (CoupModel) was calibrated with field measure ments and then used to generate comparative simulations of the water balance partitioning. The simulations indicated that mulching increased the soil water storage change by 38–71 mm during the three fallow periods, thus resulting in higher fallow efficiency by 9–12%, and decreased soil evaporation by 22–72 mm, compared with the conventional practice. Furthermore, water reached deeper horizons, resulting in 7 mm deep percolation in a wet year under mulching but not under conventional practice or catch cropping. The simulation results also showed that the catch cropping decreased the soil water storage change by 13–21 mm , although it lowered soil evaporation by 11–51 mm, and altogether reduced the fallow efficiency by 3–9%, compared to conventional practice. On the Loess Plateau of China mulching proved to be a sound measure for ensuring certain fallow ef ficiency and possibly benefit to the water cycle, while catch cropping negatively partitioned the water balance. The catch cropping under mulching might be another management regime to be considered.

Water management for crop irrigation has become a major consideration given the global scale implications of climate change and its impacts on water scarcity and security, with an increasing frequency of water restrictions in many locations during the summer months. As a consequence, decision-support tools are needed to evaluate scenarios of various agricultural practices in order to provide improved water management strategies. In this study, water requirements of orchards were assessed at the plot scale using the Simulation of Evapo-Transpiration of Applied Water (SIMETAW) model. This model was applied across a small Mediterranean watershed to evaluate and compare various water use scenarios. The standard version of SIMETAW integrates a simplified water balance model with a single uniform soil layer to calculate the crop water needs at the plot scale (Quantity of Irrigation QI). Evapotranspiration is computed using a crop coefficient specific to each crop type (Kc) and a water stress coefficient (Ks) derived from the water available in the soil (SWC). In this standard version, plots are categorized by crop type, with fixed Kc unchanged across years and between plots of the same crop type. To better capture the spatial variability of crops at the watershed scale, SIMETAW was modified to incorporate remote sensing data. Kc for each crop in the basin was estimated using the relationship between normalized difference vegetation index (NDVI) and fractional vegetation cover (FCOVER) derived from Sentinel-2 images. The modified SIMETAW model was then compared to the standard version in estimating irrigation water requirements, from the field level to the watershed scale. Model calibration was undertaken using distributed soil moisture measurements collected from 2021 to 2024. Data on water volumes used for irrigation together with farmer surveys were used to assess model performance in simulating QI both at the plot level, for thirteen distributed farms, and at the basin scale. Comparisons between the standard model version and the modified version (i.e. using Kc from remote sensing) are examined in relation to the accuracy of obtained water volumes. The model simulated soil water content (SWC) across the monitored orchards with good accuracy, providing R² values of 0.7 and 0.9 for simulation in 2022 and 2023 respectively. The simulation of the quantity of irrigation water in farms shows a strong correlation with reported data (R= 0.81). The application of the SIMETAW model to all the studied crop types offered an improved performance by incorporating the remote sensed Kc compared against the standard version, with an 17 % improvement of the water irrigation volume distributed against basin averaged irrigation volumes. Overall, the incorporation of Sentinel-2 data significantly enhanced the performance of the model by accounting for the variability in crop development across the catchment scale. By considering a typology of farms according to the irrigation practices (3 classes were defined according to observations and surveys, high, medium and low irrigation), the model was able to estimate water requirements with 10 % difference to the values provided by the manager of the water resources at regional scale (“Association Syndicale Autorisée” - ASA). Such information can prove particularly useful in helping to guide water managers in making informed decisions with regards to the dynamic management of water resources. • The simplified water balance model (SIMETAW) was evaluated on Mediterranean orchards. • To better capture the spatial variability of crops, the crop coefficient ( Kc ) is now computed from Sentinel-2 data. • The model was calibrated from soil moisture measurements acquired on various orchards. • The introduction of remote sensing data improved the model performances. • Various irrigation strategies were compared to assess water use at the watershed scale.

A statistical dynamic formulation of the vertical water budget at a land‐atmosphere interface is outlined. Physically based dynamic and conservation equations express the infiltration, exfiltration, transpiration, percolation to groundwater and capillary rise from the water table during rainstorms and interstorm periods in terms of independent variables representing the precipitation, potential evapotranspiration, soil and vegetal properties, and water table elevation. Uncertainty is introduced into these equations through the probability density functions of the independent climatic variables and yields derived probability distributions of the dependent water balance elements: surface runoff, evapotranspiration, and groundwater runoff. The mean values of these quantities give a long‐term average water balance which, to the first order, defines the annual water yield and water loss in terms of the annual precipitation and potential evapotranspiration and in terms of physical parameters of the soil, vegetation, climate, and water table. This analytical framework provides physical insight into the dynamic coupling of climate‐soil‐vegetation systems. Details are presented in a series of subsequent papers.

了解流域水文过程（水量和水质）是流域综合管理的基础。城市化引起的生态环境问题已成为目前和未来一段相当长的时期内人类社会面临的重大问题。然而，城市化（土地利用/覆被变化、新污染物产生）、水文（降水、入渗、蒸散、径流过程）和生态系统服务（产水服务、调节气候、土壤保持、初级生产力、维持生物多样性等）在不同时空尺度之间的相互作用还存在知识空白。从城市化对流域生态系统结构和功能的影响、城市化对地表能量平衡与水量平衡的影响、城市化对水质和水生生物的影响、以及城市土地利用/覆被变化的大气环境效应等多方面系统总结了城市化影响流域生态水文过程的研究进展。研究发现，城市"热岛"、"干岛"、暴雨径流引起的城市内涝、水污染等环境现象都与生态水文过程密切相关。强调现代城市规划需要遵循生态水文学规律，从全流域生态系统角度认识近年来新出现的不同尺度的城市环境效应。城市最佳管理措施应以流域为单元实施，以调节土地利用/覆被、保护湿地（包括自然与人工湿地）为手段，充分发挥自然生态系统调节功能（如植被蒸散和净化水质）。未来城市生态水文学应围绕"低影响开发"以及"基于自然的解决方案"等城市流域管理措施，在稳定城市小气候、缓解洪涝干旱等极端水文气象灾害风险以及减轻城市水污染等方面开展多尺度综合研究。;Ecohydrology is the most basic science for guiding integrated watershed management. Rapid urbanization permanently alters land cover and the structure and functions of terrestrial ecosystems in a short period of time. Consequently, urbanization affects hydrological processes such as surface evaporation, runoff, and local and regional climate, and ecosystem services (e.g., water supply, carbon sequestration, biodiversity). Urbanization directly affects the physical properties of the near-surface atmosphere, the energy exchange between land and atmosphere, and the water budget of the ecosystems. These changes bring or aggravate a series of water quantity, water quality, and climatic effects, e.g., heat island, dry island, wet island, rain island, and turbid island. This study systematically summarized the research progress in understanding the impacts of urbanization on watershed ecohydrological processes with a focus on the consequences of urbanization on surface energy balance, water balance, water quality and aquatic organisms, and the meteorological effects of urban land use/cover change. The most direct impact of urban land use/cover change on the environment is the alternation of the energy and water balance at multiple scales. The unique physical properties of the urban surface and associated energy balance distribution characteristics are different from natural ecosystems, resulting in unique microclimate and eco-hydrology in an urban environment. Urbanization-driven surface processes have direct or indirect impacts on hydrological processes such as runoff, infiltration, evapotranspiration, groundwater recharge, and river network confluence at different scales by increasing impervious surface and changing vegetation cover conditions. Large-scale urbanization aggravates the peak flow, rainstorm runoff, and annual total water yield as a result of evapotranspiration reduction caused by the significant reduction of vegetation and surface soil infiltration rate. The influence of urbanization on non-point source water pollution is mainly reflected in the change of ‘source’, ‘process’, and ‘sink’ of non-point source pollutants. Urbanization aggravates water quality problems by the increase in impervious surface that elevates pollutant concentration and load and alters physical and chemical processes in aquatic systems. Large-scale conversion of forest lands to urban uses leads to a significant reduction in evapotranspiration, which reduces its ‘biological drainage’ function, coupled with an increase in the impervious surface, and is likely to exacerbate the risk of flooding or the risk of ‘light rainstorms, heavy waterlogging’ phenomena in urban watersheds. The conversions of vegetated ecosystems reduce their ‘air conditioner’ functions and are likely to exacerbate the ‘urban heat island’ and ‘urban dry island’ effects. We summarized measures and strategies centered on ‘Low Impact Development’ to mitigate the urbanization risks through reducing extreme hydrological effects of impervious surface, enhancing the regulating function of natural ecosystems, and implementing the Urban Best Management Practices at a watershed scale. We concluded that future urban ecohydrological science should focus on the studying the mechanisms of ‘Low Impact Development’ and ‘Nature-based Solutions’ in mitigating environmental impacts from the perspectives of water and energy balances and biogeochemical cycling at multiple scales.

The discipline of hydrology has long focused on quantifying the water balance, which is frequently used to estimate unknown water fluxes or stores. While technologies for measuring water balance components continue to improve, all components of the balance have substantial uncertainty at the watershed scale. Watershed‐scale evapotranspiration, storage, and groundwater import or export are particularly difficult to measure. Given these uncertainties, analyses based on assumed water balance closure are highly sensitive to uncertainty propagation and errors of omission, where unknown components are assumed negligible. This commentary examines how greater insight may be gained in some cases by keeping the water balance open rather than applying methods that impose water balance closure. An open water balance can facilitate identifying where unknowns such as groundwater import/export are affecting watershed‐scale streamflow. Strategic improvements in monitoring networks can help reduce uncertainties in observable variables and improve our ability to quantify unknown parts of the water balance. Improvements may include greater spatial overlap between measurements of water balance components through coordination between entities responsible for monitoring precipitation, snow, evapotranspiration, groundwater, and streamflow. Measuring quasi‐replicate watersheds can help characterize the range of variability in the water balance, and nested measurements within watersheds can reveal areas of net groundwater import or export. Well‐planned monitoring networks can facilitate progress on critical hydrologic questions about how much water becomes evapotranspiration, how groundwater interacts with surface watersheds at varying spatial and temporal scales, how much humans have altered the water cycle, and how streamflow will respond to future climate change.

Abstract. Permafrost thaw profoundly changes landscapes in the Arctic-boreal region, affecting ecosystem composition, structure, function and services and their hydrological controls. The water balance provides insights into water movement and distribution within a specific area and thus helps understand how different components of the hydrological cycle interact with each other. However, the water balances of small- (<101 km2) and meso-scale basins (101–103 km2) in thawing landscapes remain poorly understood. Here, we conducted an observational study in three small-scale basins (0.1–0.3 km2) of a thawing boreal peatland complex. The three small-scale basins were situated in the headwater portion of Scotty Creek, a meso-scale low-relief basin (drainage area estimated to range between 130–202 km2) near the southern permafrost limit in the Taiga Plains ecozone in northwestern Canada. By measuring water losses (discharge and evapotranspiration [ET]), inputs (rainfall [R] and snow water equivalent [SWE]) and storage change (ΔS), and by calculating runoff (Q), we (1) aimed to quantify the growing season water balances (May–September, 2014–2016) of the three small-scale headwater sub-basins. After (2) comparing monthly sub-basin and corresponding basin water losses through ET and Q, we aimed to (3) assess the long-term (1996–2022) annual basin water balances using publicly available observations of discharge (and thus calculated Q), R and SWE in combination with simulated ET. (1) Growing season water balance residuals (RES) for the sub-basins ranged from −81 to +122 mm. The monthly growing season water balance for the sub-basin for which all water balance components throughout the 3 year study period were recorded exhibited large positive RES for May (+117 to +176 mm), since it included late-winter SWE routinely estimated in late March right before snowmelt. In contrast, lower monthly and negative RES were obtained for June–September (−41 to 0 mm). For two sub-basins, we provide two different drainage area estimates, highlighting the challenges associated with automated terrain analysis using digital elevation models (DEMs) in low-relief landscapes. Drainage areas were similar for one sub-basin, but they exhibited a fivefold difference for the other. This discrepancy was attributed to the high degree of landscape heterogeneity and resulting hydrological connectivity, with implications for Q calculations and RES. (2) Spring freshet contributed 41 % to 100 % (sub-basins) and 50 % to 79 % (basin) of the April–September Q. Spring freshet peaks were comparable, except for the driest year (2014), when the basin Q was more than 10 times lower than in the sub-basins. At both scales, ET was the dominant source of water loss, more than twice Q. (3) Over the long term (1996–2022), the increase in the basin runoff ratio (the ratio of runoff to precipitation) from 1996 to 2012 (0.1 to 0.5) has been attributed to the increased connectivity of wetlands to the drainage network due to permafrost thaw. However, the smaller mean and more variable runoff ratio from 2013 to 2022 may be due to wetland drying and/or changes in precipitation patterns. Overall, we demonstrate how the hydrological responses of rapidly thawing boreal peatland complexes – at both the sub-basin and basin scales – are shaped by complex factors that extend beyond year-to-year changes in precipitation and ET. Long-term hydrological monitoring is crucial to identify and understand potential threshold effects (e.g. changes in land cover and hydrological connectivity) and ecohydrological feedbacks affecting the local (e.g. subsistence activities), regional (e.g. water storage) and global ecosystem services (e.g. carbon storage) provided by thawing boreal peatland complexes.

Integration of the generalized complementary relationship into a lumped hydrological model for improving water balance partitioning: A case study with the Xinanjiang model

Hydrological modelling of a small watershed using MIKE SHE for irrigation planning

Understanding and modelling low-flow regimes in ungauged basins remains a key challenge in hydrology and water resources management under a changing climate and anthropogenic pressures. Low-flow rivers, characterized by reduced streamflow over extended periods, play an essential role in ecosystem functioning and water availability. Often overlooked, they represent a substantial portion of the global river network, spanning a wide range of climatic regions, including arid, semi-arid, temperate, humid tropical, boreal, and alpine environments. The variability observed in these watercourses is shaped by regional physiographic and climatic factors, making their behaviour complex to model, especially in ungauged contexts.To address this limitation, we use SMASH (Spatially-distributed Modelling and ASsimilation for Hydrology), a modular and open-source platform developed at INRAE for distributed hydrological modelling. SMASH supports the combination of process-based conceptual structures and data-driven components such as neural networks, and allows spatially distributed calibration using multi-source hydrometeorological data. It is designed to simulate discharge hydrographs and hydrological states across all grid cells in a catchment, making it particularly suited for representing diverse hydrological responses, including low-flow regimes under varying physiographic and climatic conditions.This work aims to assess the performance of different hydrological model structures and regionalization strategies through large-scale calibration and validation experiments for simulating low-flow regimes. The assessment relies on a 40-year daily dataset covering 248 French catchments representative of diverse hydrometeorological conditions. Each model–regionalization configuration is assessed based on its ability to simulate low-flow dynamics, capture seasonal variability, and preserve overall water balance, with particular attention to its generalization capacity in ungauged contexts. In this framework, artificial neural networks (ANN) are applied to perform parameter regionalization based on physiographic attributes, as part of a data-driven strategy aimed at improving transferability to ungauged basins.Results will highlight the most effective combinations of model structure and regionalization method. This work aims to improve hydrological modelling in ungauged basins through hybrid approaches combining conceptual models with data-driven tools.

Key message Decreasing stand density increases resistance, resilience, and recovery ofQuercus petraeatrees to severe drought (2003), particularly on dry sites, and the effect was independent of tree social status. Context Controlling competition is an advocated strategy to modulate the response of trees to predicted changes in climate. Aims We investigated the effects of stand density (low, medium, high; relative density index 0.20, 0.53, 1.04), social status (dominant, codominant, suppressed), and water balance (dry, mesic, wet; summer water balance - 182, - 126, - 96 mm) on the climate-growth relationships (1997-2012) and resistance (Rt), resilience (Rs), and recovery (Rc) following the 2003 drought. Methods Basal area increments were collected by coring (269 trees) in young stands (28 +/- 7.5 years in 2012) of sessile oak (Quercus petraea) in a French permanent network of silvicultural plots. Results We showed that the climate-growth relationships depend on average site-level water balance with trees highly dependent on spring and summer droughts on dry and mesic sites and not at all on wet sites. Neither stand density nor social status modulated mean response to climate. Decreasing stand density increased Rt, Rs, and Rc particularly on dry sites. The effect was independent of tree social position within the stand. Conclusion Reducing stand density mitigates more the effect of extreme drought events on drier sites than on wet sites.

Evaluation of baseflow modelling structure in monthly water balance models using 443 Australian catchments

Nagavali River plays a major role in supporting agriculture and water supply demands in parts of the Kalahandi, Rayagada and Koraput districts of Odisha and Vizianagaram and Srikakulam districts of Andhra Pradesh. With the increase in demand for water and stress from the climate change, there is a need to understand the water balance of the river basin and suggest suitable measures to improve water resources development and water harvesting. In this study, a semi-distributed model, soil and water assessment tool (SWAT) is used to study the spatio-temporal water balance for the entire basin. The model was calibrated and validated at monthly scale using the observed river discharge at Srikakulam over a period of 15 years from 1985 to 2000. Calibration and Validation results for stream flow are satisfactory on a monthly scale. The seasonal water budget analysis reveals that 70% of the annual rainfall and 74% of the annual runoff occur during the monsoon season. The sensitivity analysis showed that the lateral flow travel time, average slope steepness, curve number, available water capacity of the soil basin are the most sensitive parameters. Further the spatial analysis of the water balance components show that there is considerable difference in the precipitation and streamflow within the basin ranging from 914 to 1319 and 82 to 246 mm respectively. The results from the study provide valuable information for the proposed interlinking projects in the future.

The Soil and Water Assessment Tool (SWAT) model, owing to its robustness and wide applicability, was tested in Mabacan watershed in Laguna, Philippines, with the objective of developing a model for predicting surface runoff or streamflow in ungauged watersheds under conditions of limited hydrologic data. Model parameterization was performed based on the physical characteristics of the watershed and guided by sensitivity analysis. SWAT-predicted monthly surface runoff values for a 7-year period were then compared with manually computed values obtained using water balance and SCS-Curve Number methods. Agreement between runoff values was evaluated using the coefficient of determination (R2) and Nash-Sutcliffe model efficiency (NSE). Correlation analysis between SWAT-simulated monthly runoff and observed monthly rainfall was also performed to further assess the plausibility of simulation results. Sensitivity analysis showed that that the curve number (CN2), soil evaporation compensation factor (ESCO), and soil depth (SOL_Z) are the most sensitive parameters with sensitivity index (I) of 2.92, 0.08, and 0.04, respectively. Comparison between SWAT-predicted surface runoff with those obtained using water balance and SCS-CN methods yielded R2 values of 0.95 and 0.94, and NSE values of 0.70 and 0.63, respectively. The SWAT-simulated runoff series also proved to be well-correlated with observed rainfall series (R2=0.82). Results of this study suggest that the SWAT model is highly applicable for predicting surface runoff or streamflow in ungauged watersheds in the Philippines even with limited hydrologic data and that familiarity with the physical characteristics of the watershed area is essential in enhancing model parameterization and performance.