Changes in evapotranspiration and its response to control variables are crucial for understanding water balance and climate change in high-altitude areas. Environmental changes will inevitably disturb regional water cycles and water balance, especially in the high-altitude alpine regions of the Qilian Mountains. To better understand the variation of evapotranspiration at different altitudes in the high-altitude region of the Qilian Mountains and the applicability of the model and its response to environmental factors, we measured the variation of actual evapotranspiration at three altitude gradients using meteorological stations and automatic observation of continuous data with a weighing-type micro-lysimeter at three altitude gradients of 3797 m, 4250 m, and 4303 m in the Shaliu River basin of the Qilian Mountains during the growing season from June 2020 to October 2022 in our research. Using ten models to calculate the variation of reference evapotranspiration, and fitting them to the actual evapotranspiration, we selected the most suitable model. The results showed that the cumulative total evapotranspiration during the growing season in our study period was 1974.556 mm, 2203.066 mm, and 2201.393 mm, respectively, with intra-annual fluctuations consistent across the three elevation gradients. The value of evapotranspiration in August showed the highest at the monthly scale of 4.809 mm·day -1 and a bimodal variation at the daily scale with peaks at 10:00 and 15:00. The model of Dalton simulations showed the best results with the lowest analysis of residuals (RA), root mean square error (RMSE), and percentage error (PE), which had values of 3.291 mm·day -1, 3.994 mm·day -1, and 0.692%, and the values of R 2 between simulated and measured values of 0.622, 0.609, and 0.420. Water balance results showed that a portion of evapotranspiration in the study area originated from deep soil moisture. Partial Least Squares Regression (PLSR) analysis and enhanced regression tree model results indicated that precipitation was the most important variable, with Variable Importance in Projection (VIP) scores of 2.079 and a relative contribution to evapotranspiration of 52.6%. Overall, moisture conditions and precipitation were important factors limiting evapotranspiration variation in our research area. Our findings have implications for future climate change conditions. This conclusion is important for future water budget details in alpine mountains under climate change.

The knowledge of soil moisture spatio-temporal variability is highly relevant for water resources management. This is especially significant in highly demanding areas of arid and semiarid regions where sensitive ecosystems, agriculture and recreational areas are competing for water resources. This study reports an analysis of the spatial-temporal variability of soil moisture data for a small to medium-scale network in a coastal wetland of southwestern Spain. Measurements were taken from 5 sites located in the Doñana National Park, with an extension of approx. 25 × 3 km over the time-period of one hydrological year from September 2017 to September 2018. Absolute soil moisture was separated in two components: the temporal mean (time-invariant term) and the deviations from the mean or anomalies (time-variant term). The time-invariant component was generally the main contributor to the total spatial variance of absolute soil moisture data and it was mostly controlled by the groundwater levels in the area. Nevertheless, the time variant terms have a huge effect on soil moisture variability in very dry states at all 5 sites. Time-dependent patterns were found between average soil moisture state and the variance and covariance of both the time-variant and time-invariant components of soil moisture, which could be used for the up and downscaling of soil moisture from satellite data. Those patterns were only affected by heavy rainfalls giving rise to hysteretic behavior. This study shows that even though groundwater level is a time-variant variable, it significantly affects soil moisture’s time-variant and time-invariant terms.

Stable isotopic methods in hydroclimate monitoring are powerful for improving water resources management, but applications are limited, especially in semi-arid regions where such management is needed most. In this study, we show that we can address shortcomings related to lack of a seasonal signal using stable water isotopic signatures measured over the eastern San Francisco Bay Area of California during two contrasting events. We use hydrometric data from a gauged watershed in the study area and isotopic signatures of rain sampled at more than 20 locations during two contrasting storm events (Winter Storm Olive in February, 2023 and a warmer atmospheric river event in March 2023), and apply a solute transport model with a travel-time approach to examine predicted watershed responses and potential water tracing applications. The observed range in δ 18O in the rain samples is similar for both storms, about 5‰. However, the distributions do not overlap – the mean air temperature during Olive was about 2 0C, and the mean δ 18O of the rain samples is -12‰, while the AR event had a mean temperature of about 9 0C and a mean δ 18O of -6‰, close to the long-term average δ 18O measured in local precipitation. In the model results, event size exerts a strong control on the relative amounts of runoff vs pre-event water in the stream, while uncertainty in stream hydrograph separation is related to the degree of contrast between precipitation/runoff and pre-event water. Key to flood prediction, adaptation and mitigation, especially in coastal urban areas, is knowledge of the contributing water sources and timing of flows in streams and other features susceptible to flooding. The strong contrast in stable isotopes between these two events over the same area, illustrates the potential to use stable isotope signatures to track the transport and mixing of event water through natural and engineered watersheds.

Aquifer-Stream Interactions (ASIs) play a critical role in effective groundwater management, yet their complex dynamics remain poorly understood in channelized lowland perennial streams. This study presents a multi-scale, multi-technique investigation of ASIs along two streams in Shelby County, Tennessee, overlying the confined Memphis aquifer. The goal is to define a suitable methodology for characterizing ASIs in this specific hydrological setting, serving as a starting point for developing a standardized approach. The methodology includes an initial evaluation of various field techniques, followed by extensive surveys using potentiomanometers, electromagnetic induction (EMI), vertical temperature profilers (VTPs), and complementary techniques such as seepage meters, bank tests, and well-data analyses. Results reveal distinct hydrogeomorphic behaviors across and along the streams, challenging the ASI homogeneity notions typically assumed in groundwater models. Nonconnah Creek exhibited streambed colmation and negligible hydraulic gradients, resulting in disconnection from the aquifer during low flows, except for a 300-m losing reach with high downward gradients, potentially degrading the Memphis aquifer. In contrast, the Loosahatchie River displays relatively homogeneous streambed properties and small, upward hydraulic gradients, suggesting uniform ASIs along the surveyed reaches. EMI proved highly effective for mapping streambed sediments quickly, while potentiomanometers accurately measure small head differences critical for understanding ASI dynamics. VTPs were less practical due to extended data-collection times and vulnerability to flooding. This study emphasizes the importance of site-specific, multi-scale investigations using diverse techniques to accurately characterize ASIs in lowland streams, highlighting the confounding influences of geological formations, anthropogenic alterations, and depositional processes on groundwater-surface water interactions. The findings contribute to refining local water balances, informing groundwater management strategies, and underscoring the need for incorporating local-scale field data into regional groundwater models. The proposed methodology serves as a foundation for developing a standardized approach for characterizing ASIs in lowland channelized perennial streams, adaptable for similar stream systems worldwide.

Hydrology has a long history due to its early origin, but it is still considered young due to lack of a solid scientific foundation as a natural science. To lay a solid foundation of hydrology, field experimentation is crucial for investigating hydrological processes and revealing hydrological mechanisms. Professor Wei-Zu Gu (1932–2022) was an internationally renowned scientist in the field of hydrology and is recognized as the greatest pioneer of experimental hydrology and isotope hydrology in China. He created the Hydrohill experimental catchment, which serves as both a great public works for experimental hydrology and a valuable legacy for future researchers to conduct hydrological experiments. This legacy represents an innovative infrastructure that bridges the gap between natural watershed experiments and artificial physical models. The Hydrohill is an intensively-instrumented experimental catchment, allowing for comprehensive measurement of elements of the hydrologic cycle and their tracing indicators in a sophisticated manner. To provide an in-depth understanding of the Hydrohill, this paper presents its short history, experimental objectives, site description (including location, construction, and instrumentation), site conditions (such as soil, hydrological and meteorological properties), and contributions to hydrologic science. We pay our respects to Professor Gu for his hard work in creating the Hydrohill for experimental hydrology and enhancing our understanding of hydrological processes and mechanisms. Finally, we hope that with healthy operation at Chuzhou Scientific Hydrology Laboratory (CSHL) along with support from Professor Gu’s friends, CSHL will enable the continued growth of the Hydrohill so that it can address some unsolved problems in hydrology.

not-yet-known not-yet-known not-yet-known unknown Land surface models (LSMs) are used to simulate water and energy fluxes between the land surface and atmosphere. These simulations are useful for water resources management, drought and flood prediction, and numerical climate/weather prediction. However, the usefulness of LSMs are dependent by their ability to reproduce states and fluxes realistically. Accurate measurements of water storage are useful to calibrate and validate LSMs outputs. Geological Weighing Lysimeters (GWLs) are instruments that can provide field-scale estimates of integrated total water storage within a soil profile. We use field estimates of total water storage and subsurface storage to critically evaluate two different land surface models: the Modélisation Environnementale communautaire - Surface Hydrology (MESH) which uses the Canadian Land Surface Scheme (CLASS), and the Structure for Unifying Multiple Modeling Alternatives: (SUMMA). These models have differences in how the processes and properties of the land surface are represented. We attempted to parameterize each model in an equivalent manner, to minimize model differences. Both models were able to reproduce observations of total water storage and subsurface storage reasonably well. However, there were inconsistencies in the simulated timing of snowmelt; depth of soil freezing; total evapotranspiration; partitioning of evaporation between soil evaporation and evaporation of intercepted water; and soil drainage. No one model emerged as better overall, though each model had specific strengths and weaknesses that we describe. Insights from this study can be used to improve model physics and performance.

Soil water is a crucial factor for the growth of vegetation and sustainable development in water-limited areas. After large-scale vegetation restoration on the Chinese Loess Plateau, understanding the relationship between vegetation and deep soil moisture has become a crucial focus in current research. In this study, we selected artificial forest ( Pinus tabulaeformis, Robinia pseudoacacia, and Platycladus orientalis), apple orchard, secondary forest and farmland as the research objects, and native grassland as the control, using soil drilling techniques, we systematically monitored the soil water content of 0-10 m soil layer over two hydrological years, and explore the effects of different vegetation types on soil water deficiency. The results showed that: (1) The deep soil water various significantly among different vegetation types, which indicating the depth of the influence of vegetation on soil water has reached 10 m. (2) The mean deficit size values of P. tabulaeformis (0.14), R. pseudoacacia (0.17), P. orientalis (0.07), apple orchard (0.15) and secondary forest (0.10) and farmland (0.27) were positive in 0–1 m, indicating that surface soil water had accumulated during more than half of the sampling periods. In the 2–10 m soil layer, the mean deficit size was negative in all vegetation types except in farmland, leading to soil desiccation. The deficit size was found to fluctuate with soil depth. (3) Soil water deficit degree was affected by a combination of soil properties and vegetation growth. Altitude, soil bulk density and canopy density had a significant impact on soil water deficit. Our results indicate that the current afforestation model could lead to the deficiency of deep soil water. Therefore, in planning future vegetation allocation and management, it is imperative to make reasonable vegetation structure according to the available local soil and water resources.

River estuaries are characterized by mixing processes between marine water masses and freshwater inflows depleted in heavier stable isotopes. Therefore, they often show a linear correlation between salinity and water isotopes (δ 18O and δ 2H values). In this study we evaluated spatial and seasonal isotope dynamics along three estuarine lagoon transects, located at the northern German Baltic Sea coast. The data show strong seasonality of isotope values, even at locations located furthest from the river mouths. They further reveal a positive and linear salinity-isotope correlation in spring, but hyperbolic and partially reverse correlations in summers. We conclude, that additional physical processes such as evaporation from the shallow lagoons, partially overprint the two-phase mixing correlation during summers. Understanding those water isotope and salinity dynamics are crucial in context of ecological studies. For example, when interpreting oxygen and hydrogen isotope data in aquatic organisms, that depend on ambient estuarine waters.

Dried soil layer (DSL) is a common phenomenon forms by soil moisture deficiency in the Chinese Loess Plateau (CLP). In order to reveal the temporal stability and elimination degree of DSLs, we obtained the 19 occasions data of soil water content (SWC) at 15 observation sites in a typical slope-gully unit. The available soil moisture (ASM) and DSL were estimated by representative sites which determined through the temporal stability method, and then assessed the reliability of simulation equations between representative locations condition and mean conditions of the study area. Results show that: (1) The temporal dynamics of DSL were characterized with somewhat complexity. The ASM within the DSL (DSL-ASM), ASM within the sandwiched dried soil layer (SDSL-ASM) and quantitative index (QI) varied within the range of 2.75%–3.11%, 2.98%–4.22% and 0.254–0.356, respectively. (2) The possibility of development and recovery for DSL and SDSL in deep layers were less than that in surface layers. The maximum depth of DSL (DSLMD) was significantly and negatively related to the SDRD of DSL-ASM, the maximum depth of SDSL (SDSLMD) was negatively related to the SDRD of SDSL-ASM. (3) The prediction results of ASM above 300 cm depth were more accurate than other layers ( R 2=0.89). The DSL-ASM has a more prediction accurate than SDSL-ASM and QI. On the analysis of the time stability characteristics of ASM and dried soil layers, A2 and C3 can better represent the mean conditions of ASM at the depth of three and four soil layers, respectively. C2, A1 and A1 can better represent the average levels of DSL-ASM, SDSL-ASM and QI, respectively ( R 2=0.43, 0.14 and 0.18). (4) The restoration degrees of DSLs mainly showed no elimination and slight elimination, the DSLs cannot be completely eliminated within a short time. Scientific regulation of soil moisture can alleviate the formation and development of DSLs a certain extent, and provide the possibility for DSLs restoration.

The Yarlung Zangbo River Basin (YZRB), located in the Qinghai-Tibetan Plateau, has been significantly impacted by global warming and greening. Serving as an indicator of coupled vegetation growth and climate variation, the spatiotemporal land surface temperature (LST) has undergone substantial changes in recent decades. In this study, we evaluated the components of the water and energy cycle from 1980 to 2015 using the VIC model, a widely recognized and applied distributed hydrological model, to obtain continuous 35-year daily LST data. The results demonstrated that the VIC model exhibited high adaptability in the YZRB. Then, the fluctuation of LST was examined, and the influence of environmental elements on LST was identified. Our modeling indicated that climate factors were increasing, while human activities remained stable in the YZRB. In YZRB, the greening was witnessed while LST showed an increasing trend. By distinguishing the impacts of climate and human activities on LST, LST was mainly affected by climate with contribution rate at 70.36% from 1980 to 1995. After 1995, LST was mainly affected by human activities, and its contribution rate was 55%. Grassland with medium cover showed the potential of a cooling influence. Among all environmental factors, albedo showed a negative and delayed effect on LST. Temperature, precipitation, and evapotranspiration were positively correlated with LST and displayed relatively synchronous changes. Soil moisture and NDVI were detected as leading positive changes in LST. Our study contributes to clarifying the mechanisms influencing LST in high-altitude and high-latitude regions under global greening, providing fundamental insights for socio-economic development in alpine mountainous regions.

The increased drought risk, stemming from global warming, has intensified ecological issues and consequently, positioned ecological drought as a significant research topic within the field of eco-hydrology. Nevertheless, due to limitations in the precision of spatiotemporal data, there remains a divergence of opinions regarding the primary water resources for vegetation growth in the northwest region of China, whether it be precipitation or groundwater. Consequently, this study endeavors to construct meteorological drought index, groundwater drought index, and ecological drought index, utilizing precipitation, groundwater storage anomaly, and ecological water deficit, respectively. The maximum correlation coefficient and residual analysis methods were used to analyze the joint and relative impacts of meteorological drought and groundwater drought on ecological drought. The primary findings can be summarized as follows: (1) The area dominated by the joint impact of precipitation and groundwater on the ecological drought variation accounts for about 60%, mainly distributed in arid and semi-arid regions. (2) In spring, summer, autumn, and winter, the average contribution of the joint impact of precipitation and groundwater to the increase in ecological drought variation is between 0.26 and 0.43. (3) In contrast to precipitation variation, ecological drought induced by groundwater scarcity predominantly impacts regions like southern Shaanxi, southeastern Gansu, and southern Qinghai. These areas represent between 12.7% and 21.8% of the total area.

Changes in groundwater recharge in areas where irrigated agriculture has increased despite low precipitation and surface water allocation are a big concern. The increasing water demand has raised global drought intensities by 10-500%, worsening hydrological drought. Agriculture accounts for 69% of total groundwater abstraction worldwide, and South America expects to face unprecedented hydrological drought conditions within the next 30 years with uncertain impacts (UNESCO 2022, Wada 2016, Wanders & Wada 2015, Siebert et al. 2015). This study compares coupled and non-coupled versions of PCR-GLOBWB2.0 with MODFLOW regarding model selection and scenario comparison. Natural and human scenarios are presented to understand the effects of groundwater depletion and drought recovery. The scenario comparison evaluation of groundwater fluxes, drought characteristics, and recovery reveals changes in the water cycle due to anthropogenic impacts. This is valuable for identifying drought vulnerability in regions where water management is critical for human consumption and ecosystems.