Endorheic River Basin Research Articles

Effects of hydraulic conductivity on simulating groundwater–land surface interactions over a typical endorheic river basin

Hydraulic conductivity is the key variable in subsurface configuration that is important in modeling groundwater and its interactions with land surface processes. However, an acquisition of quantitatively distributed hydraulic conductivity is challenging due to limitations on data availability, accuracy and resolution, which inhibits an advanced understanding of hydrologic and energy cycles at the basin scale. This study investigated the role of hydraulic conductivity in simulating groundwater-land surface interactions over a typical endorheic river basin in northwestern China. A 3D variably saturated groundwater model coupled to a land surface model was configured to account for interactions between groundwater dynamics and land energy fluxes. Four scenarios of hydraulic conductivity were tested and inter-compared, adopting both homogeneous and heterogeneous parameterization strategies based on Global Soil Dataset for use in Earth System Models (GSDE) and HYdrogeology MaPS (GLHYMPS). The overall objectives are to diagnose effects of hydraulic conductivity on simulating groundwater–land surface interactions, with a particular focus on differences in spatial distributions and statistical characteristics between scenarios, as well as the dynamics and feedback mechanisms between groundwater and land surface energy fluxes. Comprehensive simulations and analyses were conducted to discern spatial distributions and statistical characteristics of key hydrological variables (streamflow, water table depth, latent heat flux, sensible heat flux) under different scenarios. Results showed that the hydraulic conductivity of the saturated zone had a significant control over water table depth, while the combine hydraulic conductivity in the vadose zone and the saturated zone governed the evolution of streamflow. Moreover, the water table critical range—where the land energy budget is highly sensitive to groundwater table—was 0.1 m–5 m and the area might expand approximately 4.5 % after considering the heterogeneity of hydraulic conductivity. Compared to the homogeneous scenario, the spatial heterogeneity of hydraulic conductivity amplified water table depth variability while diminishing land energy fluxes variability.

The evolution of river systems under the influence of climate change and human activities in the endorheic zones during the Holocene

Endorheic river basins and their terminal lakes are highly sensitive to climate change and human activities. Based on chemical and pollen indicators, lake level, and erosion/accumulation rates of rivers, we explore the phasing of the evolution of the river system in the Hexi Corridor during the Holocene. The results suggest that climate change dominated the evolution of the river system during the early-Mid-Holocene. Entering the historical period, humans began to have an impact on runoff, water resources, and lake evolution, and since 1000 BP, anthropogenic perturbations recorded by regional proxies increased and humans dominated the migrations of river . In addition, we discuss the widespread erosion of rivers in the global endorheic zone and the impact of human activities in this context and found the timing of human influence on river evolution is not the same in different regions.

Irrigation impacts on grassland hydrological regimes in an arid endorheic river basin

Irrigation is an important management measure for maintaining grassland health and improving productivity in arid and semi-arid areas; however, it can also impact hydrological regimes. Owing to terrain and water source constraints, suitable grassland areas for irrigation are limited and widely dispersed, making it difficult to accurately extract the area and spatial distribution of irrigated grasslands. In this study, we used the Google Earth Engine platform and optical remote sensing (Sentinel-2) data to classify irrigated grassland and other land-use types in the Bayin River Basin, a typical arid endorheic river basin. The classification results were then combined with measured irrigation information from the Delingha Irrigation Management Bureau and input into the SWAT–MODFLOW coupled model to simulate differences in plant growth status, groundwater recharge, and evapotranspiration between irrigated and non-irrigated (rain-fed) grasslands. The accuracy of grassland extraction and secondary classification of irrigated grasslands based on optical remote sensing images was high. Irrigation effectively increased the leaf area index of grasslands during later stages of the growing season and significantly increased the groundwater recharge depth, evapotranspiration, and total water yield during the irrigation season, as well as the annual depth of groundwater replenishment. These changes stabilised annual evapotranspiration and substantially reduced the impact of annual precipitation.

Impact of increasing vegetation coverage on gaining and losing streams in an arid endorheic river watershed

AbstractTo improve the ecology in northern China, the Chinese government has launched a series of revegetation projects. However, the impact of increasing vegetation coverage on gaining and losing streams remains largely unexplored, particularly in arid endorheic watersheds with frequent exchanges of water and other materials between surface water and groundwater. To address this, we modified the vegetation growth module of the soil and water assessment tool (SWAT) to simulate the vegetation coverage change proposed for the middle and lower reaches of the Bayin River. We then coupled the modified SWAT to a modular finite‐difference groundwater flow model to simulate the surface and subsurface water exchange. Hydrological, meteorological, and hydrogeological observation data were then used to build and calibrate the coupled model for the period 2001–2019. We set two vegetation coverage scenarios (non‐revegetation and vegetation) and two irrigation variants (non‐irrigation and irrigation) to investigate the impact of changes in vegetation coverage and the related irrigation variant on gaining and losing streams. The results showed that the vegetated land area and annual leaf area index both increased significantly between 2019 and 2001. The groundwater discharge to the gaining streams was more strongly affected by revegetation and related irrigation than the groundwater recharge from the losing streams. Irrigation related to revegetation emphasizes the effect of revegetation on streamflow. We suggest that in arid endorheic river basins, because of the related irrigation, revegetation has a substantial impact on groundwater level and streamflow, which is mainly composed of baseflow. Therefore, the irrigation efficiency of newly planted vegetation must be improved to stabilize the groundwater level and related ecological conditions.

Simulating drip irrigation in large-scale and high-resolution ecohydrological models: From emitters to the basin

Drip irrigation is deemed as a solution to the water conflict between agricultural and ecological needs in arid regions, but assessing hydrological impacts of drip irrigation remains challenging. This study developed a new approach to simulating drip irrigation at a basin scale, with the configuration of irrigation system explicitly represented and the emitter-scale wetted soil volume directly modeled, and incorporated the approach into HEIFLOW, a fully distributed and physically based ecohydrological model. The improved model, named HEIFLOW-drip, was used to study the potential implementation of drip irrigation in the Zhangye basin in Northwest China. Due to the recycling effect of irrigation return flow, the basin-scale water-saving efficiency (WSE) of drip irrigation fully implemented in the irrigated farmlands is 16.8%, less than half of its field-scale WSE (36.5%). This discrepancy indicates that accounting the water saved in fields may lead to notable overestimation of the basin-wide water saving by drip irrigation in basins with strong surface water-groundwater interactions. If fully implemented, drip irrigation would significantly alter the regional water balance, increasing the stream outflow by approximately 30%, while causing a decline in groundwater level. This tradeoff can be alleviated by tuning the management parameters of drip irrigation, which can achieve a synergistic effect of increasing the streamflow while preventing dramatic groundwater depletion. Another effective strategy is to implement drip irrigation in part of the basin. Given the intricate impacts of drip irrigation on hydrological processes, the determination of the scale and location of drip irrigation should be thought through at the basin level. Further analyses indicate the importance of proper management of the water saved by drip irrigation, which is crucial to prevent the paradox of irrigation efficiency. The findings of this study have great implications for addressing the complex water-food-ecosystem nexus in arid endorheic river basins.

The influence of nitrogen fertilization on crop production and ecohydrology in an endorheic river basin

Nitrogen fertilization is extensively applied in agricultural activities to boost food production, which changes the local hydrologic and carbon cycles. Quantitative evaluation of nitrogen fertilization on carbon and hydrologic dynamics remains challenging due to the complex linkage between hydrologic and biogeochemical processes during irrigation. To address the challenge, we utilized the Community Land Model 5 (CLM5) with activated crop module (CLM5-Crop) to investigate the response of crop yield, gross primary production (GPP), evapotranspiration (ET), and water use efficiency (WUE) to nitrogen fertilization in the cropland area of the Heihe River Basin (HRB) in northwestern China. Two batches of numerical experiments (irrigation and non-irrigation) with different nitrogen fertilization rates (0, 5, 10, 15, and 20 gN m−2, respectively) during 2000–2012 were designed to gain insights into the feedback of agricultural production and ecohydrological response to fertilization. As a result, the crop yield is significantly promoted by ∼ 120% with the nitrogen fertilization rate increasing from 0 to 20 gN m−2 under irrigation conditions. Unlike the crop yield, a clear peak of GPP is observed at the nitrogen fertilization rate of 5 gN m−2 under irrigation conditions, while higher nitrogen fertilization rates have no further promotion on GPP. ET shows a weak response to the nitrogen fertilization but a good correlation with irrigation. Different crops show different patterns of WUE under different nitrogen fertilization rates. Overall, our study demonstrates that the different influence of nitrogen fertilization on crop yield and hydrologic dynamics in a large endorheic river basin and such a finding should be taken into consideration in the future agricultural activities for the balance between food production and carbon neutrality.

Improving regional climate simulations based on a hybrid data assimilation and machine learning method

Abstract. The energy and water vapor exchange between the land surface and atmospheric boundary layer plays a critical role in regional climate simulations. This paper implemented a hybrid data assimilation and machine learning framework (DA-ML method) into the Weather Research and Forecasting (WRF) model to optimize surface soil and vegetation conditions. The hybrid method can integrate remotely sensed leaf area index (LAI), multi-source soil moisture (SM) observations, and land surface models (LSMs) to accurately describe regional climate and land–atmosphere interactions. The performance of the hybrid method on the regional climate was evaluated in the Heihe River basin (HRB), the second-largest endorheic river basin in Northwest China. The results show that the estimated sensible (H) and latent heat (LE) fluxes from the WRF (DA-ML) model agree well with the large aperture scintillometer (LAS) observations. Compared to the WRF (open loop – OL), the WRF (DA-ML) model improved the estimation of evapotranspiration (ET) and generated a spatial distribution consistent with the ML-based watershed ET (ETMap). The proposed WRF (DA-ML) method effectively reduces air warming and drying biases in simulations, particularly in the oasis region. The estimated air temperature and specific humidity from WRF (DA-ML) agree well with the observations. In addition, this method can simulate more realistic oasis–desert boundaries, including wetting and cooling effects and wind shield effects within the oasis. The oasis–desert interactions can transfer water vapor to the surrounding desert in the lower atmosphere. In contrast, the dry and hot air over the desert is transferred to the oasis from the upper atmosphere. The results show that the integration of LAI and SM will induce water vapor intensification and promote precipitation in the upstream of the HRB, particularly on windward slopes. In general, the proposed WRF (DA-ML) model can improve climate modeling by implementing detailed land characterization information in basins with complex underlying surfaces.

Evaluation of the intrinsic temperature sensitivity of ecosystem respiration in typical ecosystems of an endorheic river basin

Evaluation of the intrinsic temperature sensitivity of ecosystem respiration in typical ecosystems of an endorheic river basin

Land Use Pattern Changes and the Driving Forces in the Shiyang River Basin from 2000 to 2018

Thorough understanding of the evolution processes and drivers behind the formation of and changes in land use and land cover (LULC) is essential for maintaining the balance between humans and fragile nature in arid regions. This quantitative driving analysis provides in-depth insight into the driving mechanisms behind the formation of and changes in LULC through a case study of the Shiyang River Basin in Northwest China. Based on land use, meteorological, topographic, and socioeconomic data from 2000 to 2018 (2000, 2005, 2010, 2015, and 2018), this study employed land use transfer matrices and the GeoDetector model to explore the evolution and driving forces behind the formation of and variations in the LULC patterns. The results demonstrated that anthropic factors mainly drove the spatial distributions of cropland and settlement. The spatial distributions of the forest, grassland, and bare land were determined by the mutual influence of natural and anthropic factors. The LULC patterns exhibited consequential variations throughout the study period. Through the occupation of the surrounding cropland and grassland, urbanization expanded rapidly. The ecological environment had been improved, but there were still considerable areas of degraded land, characterized by the grassland degradation downstream and the forest degradation upstream. Geographical differentiation was the primary driver for the transformation of bare land to grassland. The main driving forces behind urban expansion and forest loss were socioeconomic development and geographical differentiation. The degree of a certain LULC change varied among different levels of its driving factor. This research can provide scientific advice for administrators and policymakers to formulate scientific, rational, and targeted land use planning and policies in the future to achieve the sustainable development of endorheic river basins.

Linking Critical Zone With Watershed Science: The Example of the Heihe River Basin

AbstractWatersheds are the basic unit of Earth's terrestrial systems and are therefore ideal units for the study of critical zones (CZs). However, grand challenges remain regarding the observation, modeling, and management of CZs and watershed systems. We synthesize the progress and breakthroughs associated with watershed science and CZ research in the Heihe River basin (HRB), a large‐scale endorheic river basin with unique mountain cryosphere‐oasis‐desert landscapes and prominent human‐nature competition for water resources. The HRB observation system consists of mountain cryosphere, agricultural oasis, and natural oasis observatories and is promoted by large‐scale comprehensive experiments to achieve multiscale observations. A watershed system model that couples ecohydrological models with socioeconomic models is developed to investigate the complex interactions among water, ecology, and socioeconomics in the HRB. The model is embedded in a decision support system to bridge science and decision‐making and to better serve river basin sustainability. Significant progress and breakthroughs have been made in CZ and watershed process research (e.g., cryospheric hydrological processes, ecological and hydrological interactions, and surface‐groundwater interactions) in the HRB. Nevertheless, observation and modeling of geochemical and geomorphological processes in the CZ have not been well addressed in integrated watershed studies of the HRB. In the future, new observation technologies, agent‐based models, machine learning, and data assimilation will benefit both watershed science and CZ science and help to address complexity and dynamics in the CZ at the river basin scale. Overall, the HRB has successfully demonstrated how an experimental river basin can link CZ science with watershed science.

How does irrigation alter the water, carbon, and nitrogen budgets in a large endorheic river basin?

• Irrigation substantially alters the water, carbon, and nitrogen budgets in the cropland area. • Evapotranspiration as well as surface runoff and baseflow is sharply increased after irrigation. • Water use efficiency of each crop species is significantly enhanced because of irrigation. • The cropland area of the Heihe River Basin switches from being a carbon source to a carbon sink as a result of irrigation. Irrigation can substantially alter the hydrologic and biogeochemical cycles in river basins. However, little attention has been given to the quantitative evaluation of the effects of irrigation on both of these cycles. In this study, we utilized the latest version of the Community Land Model (CLM5) to assess the effects of irrigation on the water, carbon, and nitrogen budgets in a cropland area of a large endorheic river basin (Heihe River Basin, HRB) in northwestern China and further identified the contributions of each type of crop to the abovementioned budgets. The model has been validated against local flux towers, remote sensing products, and reanalysis datasets, demonstrating the capability of CLM5 in capturing the temporal and spatial characteristics of water, carbon, and nitrogen at the basin scale. Two numerical experiments, one with irrigation and one without it, were conducted to explore the relationships between water and carbon/nitrogen budgets for the period from 2000 to 2018. The simulation results showed that the greatest amount of irrigation was applied in the summer (>50 mm month −1 ) and then declined to a low level during the other seasons. Evapotranspiration (ET) in the cropland area sharply increased from 106 mm year −1 to 508 mm year −1 , with an increase in surface runoff and baseflow after irrigation. Both irrigated gross primary production (GPP), net ecosystem exchange (NEE) and water use efficiency (WUE) peaked in the summer, which corresponded to the crop growing season. A shift in NEE from 6 gC m −2 year −1 to –229 gC m −2 year −1 indicated that the cropland area in the HRB switched from being a carbon source to a carbon sink because of irrigation. The corn area presented the largest increase/decrease in GPP/NEE after irrigation, whereas the lowest increase/decrease in GPP/NEE was observed in the cotton area. Total ecosystem carbon (TEC) and total ecosystem nitrogen (TEN) markedly increased as a result of irrigation, with increases of 3.29 × 10 13 gC and 2.96 × 10 12 gN, respectively, throughout the year in the cropland area. Overall, our study demonstrates that irrigation has substantial impacts on water, carbon, and nitrogen budgets in the cropland area of the HRB and provides important insights into the application of CLM5 in an arid basin with multiple crop types.

Structural Characteristics of Endorheic Rivers in the Tarim Basin

Endorheic rivers as landlocked systems with no hydrological connections to marine environments are suffering from water and ecosystem crisis worldwide, yet little is known about their structural characteristics with complex geomorphic and climatic dependence. Based on the river networks identified from 30 m resolution digital elevation models and surface water dynamic information derived from Landsat images, we investigate the hierarchical characteristics of 60 sub-basins in the Tarim Basin, the largest endorheic river basin in China. In the Tarim River basin, endorheic rivers exhibit a self-similarity only in the range of stream-orders 1–4, compared to the range of stream-orders 1–5 observed in exorheic rivers, owning to the limited stream power to maintain the similar aggregation of rivers in the arid regions. Moreover, the Tarim River networks demonstrate lower bifurcation ratio (2.48), length ratio (2.03), fractal dimension (1.38), and drainage density (0.24 km−1) in representative sub-basins, with a significant decay in median values compared with those derived from exohreic rivers at similar scales, suggesting sparser and imperfect developed branching river networks in endorheic basins. Further analysis on the Tarim reveals that endorheic river structure is more related to glacier extent (r = 0.67~0.84), potential evapotranspiration (r = 0.63~0.81), and groundwater type index (r = 0.64~0.73), which is essentially different from the structure of exorheic river represented by the Yellow River largely controlled by surface runoff, precipitation, and vegetation coverage. This study stresses the differences in intrinsic structural characteristics and extrinsic drivers of endorheic and exorheic rivers and highlights the necessity of differentiated strategies for endorheic river management in fragile ecosystems.

Ultrashort-term responses of riparian vegetation restoration to adjacent cycles of ecological water conveyance scheduling in a hyperarid endorheic river basin

The management of ecological water conveyance (EWC) can allow riparian vegetation communities to survive the threat of degradation in hyperarid inland areas and promote the health of groundwater-recharged riparian ecosystems. However, the ultrashort-term effects of periodic EWC scheduling on riparian vegetation remain unclear. This study explored the spatiotemporal differentiation in species structure (herbs, shrubs, and trees), diversity (measured by the Simpson, Shannon–Wiener, Pielou, and Margalef indices), stability (evaluated via Godron fitting distances and abundance-biomass comparison curves), and integrity (proxied by the vegetation-based index of biotic integrity) of vegetation communities in the downstream Heihe River Basin, China. Empirical orthogonal function, Pearson correlation, canonical correspondence analysis (CCA), and partial CCA methods were used to evaluate the effects of dominant habitat environmental factors from the hydrogeographic features, soil physicochemical properties, and anthropogenic impacts. The results showed that the riparian vegetation community diversity, stability, and integrity varied moderately to slightly with hierarchical distance from near wetlands (<200 m; containing mainly herbs) to far desert edges (>800 m; occupied by shrubs/subshrubs). The middle transition zone (200–800 m; occupied mostly by trees/subtrees) had the best diversity and integrity but relatively poor stability. The most significant influencing factors were EWC and soil moisture. The simple diversity, fair-level integrity, and disturbed but not irreversibly damaged stability of the vegetation community were generally improved by 14.82%, 20.33%, and 30.57%, respectively, in the pre-EWC period but worsened in the post-EWC period. The difference in spatially distributed EWC quantities caused more apparent vegetation restoration in high water-supplied subareas where certain biological community instability existed. Therefore, adequate EWC management can be considered a prerequisite for the maintenance of high richness and structural stability in local communities and requires a good balance between interregional vegetation abundance and enhanced environmental tolerance.

Abundant Precipitation in Qilian Mountains Generated from the Recycled Moisture over the Adjacent Arid Hexi Corridor, Northwest China

The observed precipitation was suggestive of abundant precipitation in upstream Qilian mountains and low precipitation in the downstream oasis and desert in an endorheic basin. However, precipitation in mountains generated from the recycled moisture over oasis and desert areas has rarely been studied. The climatological patterns of water vapor from 1980 to 2017 in the Qilian Mountain Region (QMR) and Hexi Corridor Region (HCR) were investigated by the European Centre for Medium-Range Weather Forecasts Interim reanalysis dataset and the Modern-Era Retrospective Analysis for Research and Application, Version 2 reanalysis dataset. The results suggest that the precipitable water content decreases from the adjacent to the mountain areas. There are two channels that transport water vapor from the HCR to the QMR in the low troposphere (surface—600 hPa), suggesting that parts of recycled moisture generated from evapotranspiration over the oasis and desert of the HCR is transported to the QMR, contributing to the abundant precipitation in the QMR. This indicates that the transport mechanism is probably because of the “cold and wet island effect” of the cryosphere in QMR. This is likely one of the essential mechanisms of the water cycle in endorheic river basins, which has rarely been reported.

Recovery of an endorheic lake after a decade of conservation efforts: Mediating the water conflict between agriculture and ecosystems

Widespread water losses in endorheic river basins have triggered ecological disasters. Agricultural irrigation has been one of the major drivers of water losses. It is imperative to address the water conflict between agriculture and ecosystems in arid regions, but significant gaps remain in knowledge of the water-agriculture-ecosystem nexus. The Heihe River Basin, China’s second largest endorheic river basin, lost its terminal lake in the 1990 s due to a rapid decline in river discharge to the downstream area, but the lake has returned quickly since the early 2000 s. A system analysis on the recovery of the lake is performed in this study, fusing field observations, remote sensing data, and ecohydrological model simulations. The major findings show that the waning and waxing of the lake are driven remotely by agricultural irrigation in the midstream area, rather than the climate. A one-decade effort involving diversion restrictions in the middle reach, starting from 2000, has recovered the lake to the condition of the early 1970 s. Under historical climate conditions, saving extra irrigation water may change the recovery trajectory but would not significantly enhance the stability level. The aquifer connected to the lake acts as a buffer, receiving leakage from the lake under normal conditions while discharging water to the lake under dry conditions. The overall water efficiency of water conservation for lake restoration during 2002–2012 is estimated to be only 3.5% due to significant riverbed leakage in the downstream area. Nevertheless, leakage loss is beneficial to the recovery of groundwater and vegetation downstream. Whether the diversion restriction needs modification depends on the tradeoff between ecological services provided by the lake and its related ecosystems and economic benefit produced by the midstream agriculture. This study provides insight into agricultural water management and ecological conservation in endorheic river basins in a fast-changing global environment.

Declining glaciers endanger sustainable development of the oases along the Aksu-Tarim River (Central Asia)

ABSTRACT Tarim River basin is the largest endorheic river basin in China. Due to the extremely arid climate the water supply solely depends on water originating from the glacierised mountains with about 75% stemming from the transboundary Aksu River. The water demand is linked to anthropogenic (specifically agriculture) and natural ecosystems, both competing for water. Ongoing climate change significantly impacts the cryosphere. The mass balance of the glaciers in Aksu River basin was clearly negative since 1975. The discharge of the Aksu headwaters has been increasing over the last decades mainly due to the glacier contribution. The average glacier melt contribution to total runoff is 30–37% with an estimated glacier imbalance contribution of 8–16%. Modelling using future climate scenarios indicate a glacier area loss of at least 50% until 2100. River discharge will first increase concomitant with glacier shrinkage until about 2050, but likely decline thereafter. The irrigated area doubled in the Aksu region between the early 1990s and 2020, causing at least a doubling of water demand. The current water surplus is comparable to the glacial runoff. Hence, even if the water demand will not grow further in the future a significant water shortage can be expected with declining glacial runoff. However, with the further expansion of irrigated agriculture and related industries, the water demand is expected to even further increase. Both improved discharge projections and planning of efficient and sustainable water use are necessary for further socioeconomic development in the region along with the preservation of natural ecosystems.

Evapotranspiration partitioning for multiple ecosystems within a dryland watershed: Seasonal variations and controlling factors

Partitioning of evapotranspiration (ET) into evaporation (E) and transpiration (T) is challenging but important to better understand the mechanisms of water loss from the ground surface to the atmosphere, especially in semiarid and arid regions. In this study, the underlying water use efficiency (uWUE) method is compared with hydrometric and remote sensing-based ET partitioning methods. The uWUE method is considered to be a promising method for partitioning ET, and can also be used to obtain temporal continuous data with fine spatial representation. Thus, the uWUE method was used to partition ET in six typical ecosystems in the Heihe River basin (HRB), which is the second largest endorheic river basin in western China. During 2008–2016, the daily average contributions of T to ET (T/ET) were 0.53, 0.52, 0.59, 0.37, 0.56, and 0.59 in the alpine meadow, Qinghai spruce, maize, desert, Tamarix, and Populus euphratica–Tamarix ecosystems, respectively. The T/ET ratio exhibited obvious seasonal variations in alpine meadow, maize, Tamarix, and Populus euphratica–Tamarix ecosystems. The ET partitioning results were related to air temperature in all ecosystems, especially in areas with sufficient precipitation or irrigation water supply. The vapor pressure deficit was also a main controlling factor in the upper- and middle-reach ecosystems, especially in the Qinghai spruce ecosystem. Groundwater and/or soil moisture made high relative contributions (RCs, %) to the T/ET ratio of the riparian forest in the lower reaches. Additionally, the leaf area index also made a high RC to the T/ET ratio of deciduous vegetation. Determining the quantitative contribution of T to ET in the HRB is beneficial for water resource management to guide the rational allocation and efficient use of water resources.

Novel hybrid coupling of ecohydrology and socioeconomy at river basin scale: A watershed system model for the Heihe River basin

Watershed system models are important tools for understanding complex watershed systems and integrated river basin management. However, there are very few models that can represent the coevolution of water, ecology and socioeconomic system at the river basin scale. In this study, a watershed system model was developed using the case of the Heihe River basin (HRB), a typical endorheic river basin in Northwest China. The model is mainly composed of an integrated ecohydrological model, a socioeconomic model, and two interface models. It has been embedded into a decision support system using the surrogate modeling technique. The watershed system model proved to be reliable in simulating multiple eco-hydrological processes, and showed good abilities in exploring the trajectories and interactions of water, ecology and socioeconomy. This study advances our understanding of the water-land-air-plant-human nexus at the watershed scale, and has great implications for sustainable developments in endorheic river basins.

Accounting for field-scale heterogeneity in the ecohydrological modeling of large arid river basins: Strategies and relevance

It remains a great challenge to represent field-scale (e.g., meters to tens of meters) heterogeneity in the ecohydrological modeling of large river basins due to the tradeoff between spatial resolution and computational cost. This study improved an existing ecohydrological model, HEIFLOW, by introducing the subgrid structure of land cover, multilayer soil water simulation and accurate spatial coverage of irrigation within the grid. These improvements enable the model to provide reliable simulations over a wide spectrum of spatial scales, from the field scale to the large-basin scale (i.e., 104 to 105 km2). The new model was implemented in the Heihe River Basin, the second largest endorheic river basin in China, with a grid size of 1 km by 1 km for a modeling domain of approximately 90,589 km2. The major study findings include the following. First, in arid areas with sparse vegetation, ignoring the subgrid characteristics of land surfaces will lead to significant errors that may be further propagated when the modeling results are used to support management or scaled up for larger-scale climate modeling. Second, the multilayer soil structure can improve ecohydrological simulations in terms of temporal variations, and it is necessary to separate a thin surface layer from the soil zone in arid areas. In the case study, a single-layer soil structure would introduce greater than 10% error in simulating the annual maximum leaf area index (LAI). Third, considering the accurate spatial coverage of irrigation within the grid cell is critical for successful simulations of ecohydrological processes in arid areas. In the case study, accurate spatial coverage would lead to −31%, +46%, and +13% changes in the simulated average soil evaporation, transpiration and LAI, respectively, over the entire area with irrigation. Overall, this study provides a unique perspective to address the scale issue in ecohydrological modeling and reveals the importance of field-scale heterogeneity to the management of water resources and ecosystems based on ecohydrological modeling.

Baseflow dynamics and multivariate analysis using bivariate and multiple wavelet coherence in an alpine endorheic river basin (Northwest China)

Baseflow is a component of streamflow derived from shallow and deep subsurface flows that is concurrently controlled by multiple factors. Rational estimation of baseflow is critical for understanding its spatiotemporal dynamics and influencing factors within a river basin. To address this, different filtering parameters were applied to separate the baseflow of the Heihe River Basin (HRB) in Northwest China using digital filtering methods. Moreover, using bivariate and multivariate wavelet coherences, multivariate relationships between baseflow and meteorological factors/large-scale circulation indices were identified for several factors, which explained most of the variations. Results showed annual average baseflow was 10.3–91.1 mm and that the baseflow index (BFI) varied between 0.50 and 0.72 (average: 0.62). This indicates that 62% of long-term streamflow likely originates from groundwater discharge and other delayed sources. Positive/negative Spearman correlation coefficients between baseflow and extreme climate indices were more significant at upstream (Yingluoxia, Liyuanbao-and Wafangcheng) stations in comparison with midstream (Suyukou, Shunhua) and downstream (Yangyangchi) stations. Correlation for the BFI was relatively weaker than for baseflow. Furthermore, bivariate wavelet coherences revealed that precipitation (six stations) and the Atlantic Multidecadal Oscillation (four stations) were the individual factors that best explained baseflow variations. Multiple wavelet coherence demonstrated that all meteorological factors/large-scale circulation indices had the highest percentage of the numbers of power significant at the 95% significance level that could best explain baseflow variations. However, the average power of wavelet coherence was not increased. Differences likely attributable to consideration of additional variables were diminished by collinearity effects among factors. Furthermore, baseflow at the midstream Zhengyxia and downstream Yangyangchi stations had significant positive and negative correlation with population and effective irrigation area, respectively. The findings indicate that development of regional hydrometeorological models should primarily consider the impact of climate change in the upstream HRB, whereas the effects of both climate change and human activities should be considered in the midstream and downstream HRB.