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THE REMARKABLE RISE OF PROCESS HYDROLOGY IN CHINA Gu was at the leading edge of process hydrology in China during its remarkable ascent in the past several decades. Through the 1980s and 1990s, Gu authored a number of papers in the Chinese and English literature (Gu, [4]; Gu, [5]; Gu, [6]; Gu, [7]; Gu, [8]; Gu, [9], [10] - summarized in an important review paper in Gu et al., [11]) that showed - in many cases for the first time - the controls on the spatial and temporal evolution of soil water, groundwater and streamflow from the catchment. WEI-ZU GU AND HYDROHILL Gu was a research professor at the Nanjing Institute of Hydrology and Water Resources when he set up the Chuzhou Hydrology Laboratory and now-famous Hydrohill catchment. Following the USGS delegation to China, the USGS team reciprocated, and Gu came to the United States about a year later and toured the Mattole watershed, the Panola watershed in Georgia and other USGS research sites. [Extracted from the article]

The flow duration curve (FDC) is the cumulative distribution function, which represents the relationship between the frequency and magnitude of streamflow, and the precipitation duration curves (PDC) follows the same principle. Nowadays, the correlation between the shape of PDC and FDCs, their respective physical control factors including human activities, and their fitting conditions in unmeasured catchments across China have not been fully understood. In this paper, daily precipitation and streamflow data with 30 years records from 224 hydrological stations in the middle and lower Yangtze River basin were chosen to fit PDC and FDCs through gamma distribution. Framework was proposed for modelling FDCs to analyse the relationship, similarity, regional patterns and response mechanism of fitting parameters between PDC and FDCs, dividing the streamflow time series into fast and base flow and stations into three categories in consideration of human activities to attribute the shapes of PDC and FDCs to catchment meteorological and geographical characteristics and physical processes under natural conditions. Results indicate that the parameters of PDC and certain FDCs (TFDC, FFDC, SFDC) share similar spatial patterns but vary for the different duration and interactions of the processes. The climate and catchment characteristics such as extreme properties of precipitation, base flow index (BFI), Pmax*αp and concentration ratio index (CIM) will influence the shape of PDC and FDCs. The relationship between BFI and SFDC/TFDC can be better reflected in "Regulated watersheds", while CIM/Pmax*αp and PDC/FFDC in "watersheds in the mainstream" are more related. This paper provides a deeper understanding and more accurate way of the application of these parameters in describing and predicting hydrological processes and estimation of PDC and FDCs in unmeasured catchments, and can be applied to more future research about processes based on catchment rainfall‐runoff responses and physical controls. [ABSTRACT FROM AUTHOR]

Accurately estimated reference evapotranspiration (ET0) is essential to regional water management. The FAO recommends coupling the Penman–Monteith (P‐M) model with the Ångström–Prescott (A‐P) formula as the standard method for ET0 estimation with missing Rs measurements. However, its application is usually restricted by the two fundamental coefficients (a and b) of the A‐P formula. This paper proposes a new method for estimating ET0 with missing Rs by combining machine learning with physical‐based P‐M models (PM‐ET0). The benchmark values of the A‐P coefficients were first determined at the daily, monthly, and yearly scales, and further evaluated in Rs and ET0 estimates at 80 national Rs measuring stations. Then, three empirical models and four machine‐learning methods were evaluated in estimating the A‐P coefficients. Machine learning methods were also used to estimate ET0 (ML‐ET0) to compare with the PM‐ET0. Finally, the optimal estimation method was used to estimate the A‐P coefficients for the 839 regular weather stations for ET0 estimation without Rs measurement for China. The results demonstrated a descending trend for coefficient a from northwest to southeast China, with larger values in cold seasons. However, coefficient b showed the opposite distribution as the coefficient a. The FAO has recommended a larger a but a smaller b for southeast China, which produced the region's largest Rs and ET0 estimation errors. Additionally, the A‐P coefficients calibrated at the daily scale obtained the best estimation accuracy for both Rs and ET0, and slightly outperformed the monthly and yearly coefficients without significant difference in most cases. The machine learning methods outperformed the empirical methods for estimating the A‐P coefficients, especially for the sites with extreme values. Further, ML‐ET0 outperformed the PM‐ET0 with yearly A‐P coefficients but underperformed those with daily and monthly ones. This study indicates an exciting potential for combining machine learning with physical models for estimating ET0. However, we found that using the A‐P coefficients with finer time scales is unnecessary to deal with the missing Rs measurements. [ABSTRACT FROM AUTHOR]

In frequency analysis of annual maximum flood series (AMFS), it is mechanism‐based and significant to incorporate the information of the underlying "ordinary" daily streamflow events for improving accuracy of flood risk estimation and appropriate management. In previous studies related to flood nonstationarity, the classical norming constants method (C‐NCM) has been used to derive statistical parameters of annual maximum flood distribution from daily streamflow distribution. However, C‐NCM does not flexibly consider the feasible range of the scale parameter of annual maximum flood distributions, which can result in the inability to provide sufficient and reliable models to fit AMFS. This paper aims to investigate the potential of two alternative norming constants methods in fitting annual maximum floods, namely the Hall's norming constants method (H‐NCM) and the Fisher and Tippetts' norming constants method (FT‐NCM) respectively. A comparative study of these three methods was carried out using hydrological streamflow series of 77 stations in the Yangtze and Yellow River basins, China. The results found that H‐NCM outperforms both C‐NCM and FT‐NCM for the stations with relatively low‐skewness coefficient of AMFS. Therefore, H‐NCM is recommended to prioritize for practical applications when considering daily streamflow rather than small samples of flood events. Furthermore, if the skewness coefficient exceeds Cs,Gumbel≈1.14, the overall modelling performance of these three methods is significantly deteriorated. This can provide some reference for the applicability of the norming constants method in flood frequency analysis. [ABSTRACT FROM AUTHOR]

Groundwater exploitations in semi‐arid regions result in significant declines of groundwater levels and streamflow and even trigger seasonal streamflow drying up, which may bring adverse impacts on riparian ecosystems. It is critical to explore future streamflow changes caused by variable climate scenarios and groundwater exploitation intensities; however, a comprehensive assessment of the streamflow drying up frequency remains lacking. Furthermore, the thresholds of controlling factors beyond which streamflow drying up occurs have rarely been explored. Here we develop an integrated surface water‐groundwater model for the Taoer River Basin (TRB), a typical semi‐arid watershed in northeast China. We apply the model to multiple future scenarios to reveal the frequency of potential streamflow drying up and explore the thresholds of controlling factors of streamflow drying up based on a decision tree method. We find that the groundwater level in the lower region of the TRB (LTRB) is hard to recover to the riverbed elevation and the streamflow drying up frequency is still high unless the groundwater exploitation is reduced by 50%. However, this leads to a competition between the ecological and economic objectives. Our results also show that the streamflow drying up frequency in the LTRB is very sensitive to the upstream water inflow that can be regulated through the reservoir operation or inter‐basin water diversion, indicating the importance of a basin scale strategic management. The findings of this paper can help policy makers sustainably exploit water resources in semi‐arid regions. [ABSTRACT FROM AUTHOR]

The frequency and magnitude of extreme meteorological or hydrological events such as floods and droughts in China have been influenced by global climate change. The water problem due to increasing frequency and magnitude of extreme events in the humid areas has gained great attention in recent years. However, the main challenge in the evaluation of climate change impact on extreme events is that large uncertainty could exist. Therefore, this paper first aims to model possible impacts of climate change on regional extreme precipitation (indicated by 24-h design rainfall depth) at seven rainfall gauge stations in the Qiantang River Basin, East China. The Long Ashton Research Station-Weather Generator is adopted to downscale the global projections obtained from general circulation models (GCMs) to regional climate data at site scale. The weather generator is also checked for its performance through three approaches, namely Kolmogorov-Smirnov test, comparison of L-moment statistics and 24-h design rainfall depths. Future 24-h design rainfall depths at seven stations are estimated using Pearson Type III distribution and L-moment approach. Second, uncertainty caused by three GCMs under various greenhouse gas emission scenarios for the future periods 2020s (2011-2030), 2055s (2046-2065) and 2090s (2080-2099) is investigated. The final results show that 24-h design rainfall depth increases in most stations under the three GCMs and emission scenarios. However, there are large uncertainties involved in the estimations of 24-h design rainfall depths at seven stations because of GCM, emission scenario and other uncertainty sources. At Hangzhou Station, a relative change of −16% to 113% can be observed in 100y design rainfall depths. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A statistically based runoff-yield model is proposed in this paper. The model considers spatial heterogeneities of rainfall, soil infiltration capacity and soil water storage capacity that are main factors controlling runoff-yield process. It assumes that the spatial variation of rainfall intensity at each time step can be characterized by a probability density function, which is estimated by matching the hyetograph through goodness-of-fit measure, whereas the spatial heterogeneities of soil infiltration capacity and soil water storage capacity are described by parabola-type functions. Surface runoff is calculated according to infiltration excess mechanism; the statistical distribution of surface runoff rate can be deduced with the joint distribution of rainfall intensity and soil infiltration rate, thus obtaining a quasi-analytical solution for surface runoff. Based on saturation excess mechanism, the groundwater flow (flows below the ground are collectively referred to as groundwater flow) is calculated by infiltration and the probability distribution of soil water storage capacity. Consequently, the total runoff is composed of infiltration excess and saturation excess runoff components. As an example, this model is applied to flood event simulation in Dongwan catchment, a semi-humid region and a tributary of Yellow River in China. It indicates that the proposed runoff-yield model could achieve acceptable accuracy. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Water temperature is an important habitat factor in river ecosystems that exhibits the characteristics of continuous change. Dam construction disrupts the continuity of river water temperature and reset it, thus exerting sharp rise/decrease on the characteristics of water temperature change. The effect of a dam on river continuity is directly related to the dam size. To explain this relationship, two rivers in China were selected: one river without reservoirs and one river with cascade reservoirs. Through the analysis of the longitudinal change of water temperature in free‐flowing rivers, we found that water temperature changes continuously and steadily in the longitudinal direction. Based on this, a temperature trend hypothesis in river was proposed, and the discontinuity of the water temperature in the reservoir section was evaluated. The results are as follows: (1) In mixed reservoirs, river water temperature remained as continuous as free‐flowing rivers. However, the river water temperature had a large discontinuity in the stratified reservoir. (2) Water residence time was used as an indicator of the continuity of reservoir water temperature. (3) Selective withdrawal of stratified reservoirs in January could not remove the discontinuity caused by itself, but it worked in June. [ABSTRACT FROM AUTHOR]

The Heihe River Basin (HRB) is an inland watershed in northwest China with a total area of approximately 130,000 km2, stretching from the Qilian Mountains in the south to the oases and agricultural fields in the middle and further to the Gobi desert in the north bordering Mongolia. As part of a major ecohydrological research initiative to provide a stronger scientific underpinning for sustainable water management in arid ecosystems, a regional-scale integrated ecological and hydrological model is being developed, incorporating the knowledge based on the results of environmental isotope tracer analysis and the multiscale observation datasets. The first step in the model development effort is to construct and calibrate a groundwater flow model for the middle and lower HRB where the oases and vegetation along the Heihe river corridor are highly dependent on groundwater. In this study, the software tool 'Arc Hydro Groundwater' is used to build and visualize a hydrogeological data model for the HRB that links all relevant spatiotemporal hydrogeological data in a unified geodatabase within the ArcGIS environment. From the conceptual model, a regional-scale groundwater flow model has been developed using MODFLOW-2005. Critical considerations in developing the flow model include the representation of mountainous terrains and fluvial valleys by individual model layers, treatment of aquifer heterogeneities across multiple scales and selection of proper observation data and boundary conditions for model calibration. This paper discusses these issues in the context of the Heihe River Basin, but the results and insights from this study will have important implications for other large, regional groundwater modelling studies, especially in arid and semiarid inland river basins. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The effects of climate change and population growth in recent decades are leading us to consider their combined and potentially extreme consequences, particularly regarding hydrological processes, which can be modeled using a generalized extreme value (GEV) distribution. Most of the GEV models were based on a stationary assumption for hydrological processes, in contrast to the nonstationary reality due to climate change and human activities. In this paper, we present the nonstationary generalized extreme value (NSGEV) distribution and use it to investigate the risk of Niangziguan Springs discharge decreasing to zero. Rather than assuming the location, scale, and shape parameters to be constant as one might do for a stationary GEV distribution analysis, the NSGEV approach can reflect the dynamic processes by defining the GEV parameters　as functions of time. Because most of the GEV model is designed to evaluate maxima (e.g. flooding, represented by positive numbers), and spring discharge cessation is a ‛minima', we deduced an NSGEV model for minima by applying opposite numbers, i.e. negative instead of positive numbers. The results of the model application to Niangziguan Springs showed that the probability of zero discharge at Niangziguan Springs will be 1/80 in 2025, and 1/10 in 2030. After 2025, the rate of decrease in spring discharge will accelerate, and the probability that Niangziguan Springs will cease flowing will dramatically increase. The NSGEV model is a robust method for analysing karst spring discharge. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Water resources are the most critical factors to ecology and society in arid basins, such as Kaidu River basin. Isotope technique was convenient to trace this process and reveal the influence from the environment. In this paper, we try to investigate the temporal and spatial characteristics in stable isotope (18O and 2H) of surface water and groundwater in Kaidu River. Through the water stable isotope composition measurement, spatial and temporal characteristics of deuterium (δ2H) and oxygen 18 (δ18O) were analysed. It is revealed that (1) comparing the stream water line with the groundwater line and local meteorological water line of Urumqi City, it is found that the contribution of precipitation to surface water in stream runoff is the main source, whereas the surface water is the main source of groundwater. Groundwater is mainly drainage of surface runoff in the river; (2) in the main stream of Kaidu River, the spatial variability of river water showed a 'heavier-lighter-heavier' change along with the main stream for δ18O, and temporal variability showed higher in summer and lower in winter; (3) the δ18O and δ2H values of groundwater samples ranged from −11.36 to −7.97‰ and −73.45 to −60.05‰, respectively. There is an increasing trend of isotopic values along the groundwater flow path. The seasonal fluctuation of δ18O is not clear in most samples. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Soil and vadose zone profiles are used as an archive of changes in groundwater recharge and water quality following changes in land use in an area of the Loess Plateau of China. A typical rain-fed loess-terrace agriculture region in Hequan, Guyuan, is taken as an example, and multiple tracers (chloride mass balance, stable isotopes, tritium and water chemistry) are used to examine groundwater recharge mechanisms and to evaluate soil water chloride as an archive for recharge rate and water quality. Results show that groundwater recharge beneath natural uncultivated grassland, used as a baseline, is about 94-100 mm year−1 and that the time it takes for annual precipitation to reach water table through the thick unsaturated zone is from decades to hundreds of years (tritium free). This recharge rate is 2-3 orders of magnitude more than in the other semiarid areas with similar annual rainfall but with deep-rooted vegetation and relatively high temperature. Most of the water that eventually becomes recharge originally infiltrated in the summer months. The conversion from native grassland to winter wheat has reduced groundwater recharge by 42-50% (50-55 mm year−1 for recharge), and the conversion from winter wheat to alfalfa resulted in a significant chloride accumulation in the upper soil zone, which terminated deep drainage. The paper also evaluates the time lag between potential recharge and actual recharge to aquifer and between increase in solute concentration in soil moisture and that in the aquifer following land-use change due to the deep unsaturated zone. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Quantitative evaluation of the effect of climate variability and human activities on runoff is of great importance for water resources planning and management in terms of maintaining the ecosystem integrity and sustaining the society development. In this paper, hydro-climatic data from four catchments (i.e. Luanhe River catchment, Chaohe River catchment, Hutuo River catchment and Zhanghe River catchment) in the Haihe River basin from 1957 to 2000 were used to quantitatively attribute the hydrological response (i.e. runoff) to climate change and human activities separately. To separate the attributes, the temporal trends of annual precipitation, potential evapotranspiration (PET) and runoff during 1957-2000 were first explored by the Mann-Kendall test. Despite that only Hutuo River catchment was dominated by a significant negative trend in annual precipitation, all four catchments presented significant negative trend in annual runoff varying from −0.859 (Chaohe River) to −1.996 mm a−1 (Zhanghe River). Change points in 1977 and 1979 are detected by precipitation-runoff double cumulative curves method and Pettitt's test for Zhanghe River and the other three rivers, respectively, and are adopted to divide data set into two study periods as the pre-change period and post-change period. Three methods including hydrological model method, hydrological sensitivity analysis method and climate elasticity method were calibrated with the hydro-climatic data during the pre-change period. Then, hydrological runoff response to climate variability and human activities was quantitatively evaluated with the help of the three methods and based on the assumption that climate and human activities are the only drivers for streamflow and are independent of each other. Similar estimates of anthropogenic and climatic effects on runoff for catchments considered can be obtained from the three methods. We found that human activities were the main driving factors for the decline in annual runoff in Luanhe River catchment, Chaohe River catchment and Zhanghe River catchment, accounting for over 50% of runoff reduction. However, climate variability should be responsible for the decrease in annual runoff in the Hutuo River catchment. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Climate change may significantly affect the hydrological cycle and water resource management, especially in arid and semi-arid regions. In this paper, output from the Providing Regional Climates for Impacts Studies (PRECIS) regional climate model were used in conjunction with the Soil and Water Assessment Tool (SWAT) to analyse the effects of climate change on streamflow of the Xiying and Zamu rivers in the Shiyang River basin, an important arid region in northwest China. After SWAT model calibration and validation, streamflow in the Shiyang River Basin was simulated using the PRECIS climate model data for greenhouse gas emission scenarios A2 (high emission rate) and B2 (low emission rate) developed by Intergovernmental Panel on Climate Change. Monthly streamflow and hydrological extremes were compared for present-day years (1961-1990), the 2020s (2011-2040), 2050s (2041-2070) and 2080s (2071-2100). The results show that mean monthly streamflow in Shiyang River Basin generally increased in the 2020s, 2050s and 2080s between 0.7-6.1% at the Zamu gauging station and 0.1-4.8% at the Xiying gauging station. The monthly minimum streamflow increased persistently, but the maximum monthly streamflows increased in the 2020s and slightly decreased in the 2050s and 2080s. This study provides valuable information for guiding future water resource management in the Shiyang River Basin and other arid and semi-arid regions in China. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The retreat of mountain glaciers and ice caps has dominated the rise in global sea level and is likely to remain an import component of eustatic sea-level rise in the 21st century. Mountain glaciers are critical in supplying freshwater to populations inhabiting the valleys downstream who heavily rely on glacier runoff, such as arid and semi-arid regions of western China. Owing to recent climate warming and the consequent rapid retreat of many glaciers, it is essential to evaluate the long-term change in glacier melt water production, especially when considering the glacier area change. This paper describes the structure, principles and parameters of a modified monthly degree-day model considering glacier area variation. Water balances in different elevation bands are calculated with full consideration of the monthly precipitation gradient and air temperature lapse rate. The degree-day factors for ice and snow are tuned by comparing simulated variables to observation data for the same period, such as mass balance, equilibrium line altitude and glacier runoff depth. The glacier area-volume scaling factor is calibrated with the observed glacier area change monitored by remote sensing data of seven sub-basins of the Tarim interior basin. Based on meteorological data, the glacier area, mass balance and runoff are estimated. The model can be used to evaluate the long-term changes of melt water in all glacierized basins of western China, especially for those with limited observation data. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The Xiaolangdi Dam, completed in 2000, is second in scale in China to the Three Gorges Project. It has generated remarkable economic and social benefits but with profound impacts to the riverine and regional environments. This paper reports field monitoring of riparian groundwater in the Kouma section of the Yellow River to illustrate the interactions between dam-regulated river flow and riparian groundwater. The results show that the hydrological condition in riparian zones downstream from the dam has changed from a typical wet-dry cycle to a condition of semi-permanent dryness, resulting in degradation of the typical attributes and functions of the wetland ecosystem. Hydrological processes in the riparian zone have changed from a complex multiple flooding regime to a simple regime of dominant groundwater drainage towards the river, which only reverses temporarily during the water and sediment regulation period of the dam. Data on groundwater level and groundwater quality show that there are two key points, at ca 200 and 400 m from the river bank, which distinguish zones with different sensitivity to changes of river flow and indicate different interactions between river water and groundwater. The shallow groundwater quality also is negatively affected by the intensive agricultural development that has occurred since the dam was completed. Ecological restoration needs to be carried out to construct a protective natural riparian zone within ca 200 m from the river, this being an ecotone, which is key to the protection of both riparian groundwater and the river. The riparian zone from 200 to 400 m also should be treated as a transitional zone. In addition, ecologically sensitive agriculture and ecotourism organized by local communities would be beneficial in the area beyond 400 m. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Influences of climatic change on the components of global hydrological cycle, including runoff and evapotranspiration are significant in the mid- and high-latitude basins. In this paper, the effect of climatic change on annual runoff is evaluated in a large basin-Songhua River basin which is located in the northeast of China. A method based on Budyko-type equation is applied to separate the contributions of climatic factors to changes in annual runoff from 1960 to 2008, which are computed by multiplying their partial derivatives by the slopes of trends in climate factors. Furthermore, annual runoff changes are predicted under IPCC SRES A2 and B2 scenarios with projections from five GCMs. The results showed that contribution of annual precipitation to annual runoff change was more significant than that of annual potential evapotranspiration in the Songhua River basin; and the factors contributing to annual potential evapotranspiration change were ranked as temperature, wind speed, vapour pressure, and sunshine duration. In the 2020s, 2050s, and 2080s, changes in annual runoff estimated with the GCM projections exhibited noticeable difference and ranged from − 8·4 to − 16·8 mm a−1 (−5·77 to − 11·53% of mean annual runoff). Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Global warming poses a great threat to wetland ecological stability and water quality improvement. In this paper, we sampled six types of wetlands representing different kinds of land utilisation around the TaiHu Lake Basin in southeast China. An outdoor computerized microcosm was set up in May 2008 to simulate climate scenarios of ambient temperature (control) and a warmed ambient temperature (+5 °C) using a novel minute-scale daily and seasonal temperature manipulation technology. The 18-month incubation indicated that warming impaired the ecological sink of sediment (fixing P from porewater) while strengthening the role of the ecological source, moreover, the rate of P release from sediment into porewater (19-113% for total phosphorus) was much stronger than from top sediment into overlying water. Warming enhanced the activity of neutral and alkaline phosphatases for P-deficient wetland sediments and neutral phosphatases for P-rich sediments, at the 14th month of incubation. A significant increase in total phospholipid fatty acids (PLFAs) occurred under warming in sediments with relatively low levels of PLFAs, but bacterial abundance in the sediment biomass tended to decrease (18%) along with an increase (4·5%) of the fungi-to-bacteria ratio. The variations of carbon consumption and availability indicated by measured dissolved organic carbon in sediment may have contributed to P release from sediments to water. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

When hydrology model parameters are determined, a traditional data assimilation method (such as Kalman filter) and a hydrology model can estimate the root zone soil water with uncertain state variables (such as initial soil water content). The simulated result can be quite good. However, when a key soil hydraulic property, such as the saturated hydraulic conductivity, is overestimated or underestimated, the traditional soil water assimilation process will produce a persistent bias in its predictions. In this paper, we present and demonstrate a new multi-scale assimilation method by combining the direct insertion assimilation method, particle swarm optimisation (PSO) algorithm and Richards equation. We study the possibility of estimating root zone soil water with a multi-scale assimilation method by using observed in situ data from the Wudaogou experiment station, Huaihe River Basin, China. The results indicate there is a persistent bias between simulated and observed values when the direct insertion assimilation surface soil water content is used to estimate root zone soil water contents. Using a multi-scale assimilation method (PSO algorithm and direct insertion assimilation) and an assumed bottom boundary condition, the results show some obvious improvement, but the root mean square error is still relatively large. When the bottom boundary condition is similar to the actual situation, the multi-scale assimilation method can well represent the root zone soil water content. The results indicate that the method is useful in estimating root zone soil water when available soil water data are limited to the surface layer and the initial soil water content even when the soil hydraulic conductivities are uncertain. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The article presents a study that compares data using several evapotranspiration methods, including the models of Katerji-Perrier (KP), Todorovic (TD), and Priestley-Taylor (PT). The paper aims to compare the models and uses data obtained from coniferous trees of Changbai Mountains in China. According to the authors, the most effective values are obtained using the KP method while 30% overestimation of evapotranspiration is recorded by the TD method. Moreover, the authors add that the results of PT methods are site dependent and effective in large time-scale evapotranspiration. Lastly, the possibility of measuring evapotranspiration of Korean pine mixed forest is also proven in the study.

Some previous studies have shown that drying-up of the lower Yellow River resulted from decreasing precipitation and excessive industrial and agricultural consumption of water from the middle and downstream regions of the Yellow River. On the basis of average air temperature, precipitation, and pan evaporation data from nearly 80 gauging stations in the Yellow River basin, the monotonic trends of major climate variables over the past several decades are analysed. The analysis was mainly made for 12 months and the annual means. The isograms for annual and typical months are given in the paper. The result shows that the average temperature in the study area exhibits an increasing trend, mainly because of the increase of temperature in December, January and February. The largest trend is shown in December and the smallest is in August. There are 65 of 77 stations exhibiting a downward trend for annual precipitation. In all seasons except summer, there is a similar trend in the upstream region of the Yellow River, south of latitude 35°N. It is interesting to note that the pan evaporation has decreased in most areas of the Yellow River basin during the past several decades. April and July showed the greatest magnitude of slope, and the area from Sanmenxia to Huayuankou as well as the Yiluo River basin exhibited the strongest declining trend. The conclusion is that the decreasing pan evaporation results from complex changes of air temperature, relative humidity, solar radiation, and wind speed, and both climate change and human activities have affected the flow regime of the Yellow River during the past several decades. [ABSTRACT FROM AUTHOR]

Potential evapotranspiration (PET) is a key input to hydrological models. Its estimation has often been via the Penman--Monteith (P--M) equation, most recently in the form of an estimate of reference evapotranspiration (RET) as recommended by FAO-56. In this paper the Shuttleworth--Wallace (S--W) model is implemented to estimate PET directly in a form that recognizes vegetation diversity and temporal change without reference to experimental measurements and without calibration. The threshold values of vegetation parameters are drawn from the literature based on the International Geosphere--Biosphere Programme land cover classification. The spatial and temporal variation of the LAI of vegetation is derived from the composite NOAA-AVHRR normalized difference vegetation index (NDVI) using a method based on the SiB2 model, and the Climate Research Unit database is used to provide the required meteorological data. All these data inputs are publicly and globally available. Consequently, the implementation of the S--W model developed in this study is applicable at the global scale, an essential requirement if it is to be applied in data-poor or ungauged large basins. A comparison is made between the FAO-56 method and the S--W model when applied to the Yellow River basin for the whole of the last century. The resulting estimates of RET and PET and their association with vegetation types and leaf area index (LAI) are examined over the whole basin both annual and monthly and at six specific points. The effect of NDVI on the PET estimate is further evaluated by replacing the monthly NDVI product with the 10-day product. Multiple regression relationships between monthly PET, RET, LAI, and climatic variables are explored for categories of vegetation types. The estimated RET is a good climatic index that adequately reflects the temporal change and spatial distribution of climate over the basin, but the PET estimated using the S--W model not only reflects the changes in climate, but also the vegetation distribution and the development of vegetation in response to climate. Although good statistical relationships can be established between PET, RET and/or climatic variables, applying these relationships likely will result in large errors because of the strong non-linearity and scatter between the PET and the LAI of vegetation. It is concluded that use of the implementation of the S--W model described in this study results in a physically sound estimate of PET that accounts for changing land surface conditions. [ABSTRACT FROM AUTHOR]

Groundwater is often a critical source of water for municipal, industrial and agricultural uses, especially in arid and semi-arid environments. Songnen Plain, located in the central part of northeast China, is such a region, it being an important productive base of commodity grain in this country. In the past two decades, groundwater quality in the region, especially salinization, has deteriorated under natural changes and human activities, and has become a crucial factor restricting sustainable ecoenvironmental and socio-economic development. In this paper, The Taoer River catchment, situated in the middle of the region, was selected as the study area for the groundwater quality evolution study using hydrochemistry and stable isotopes to obtain a better understanding of the system. Fifty-two groundwater samples were collected with systematic design during the low-water and high-water periods in 2003. A series of comprehensive quality data interpretations, e.g. statistics, ratios of ions and Piper diagrams, together with stable isotope data, have been used to gain an insight into the spatial and temporal variations and evolution laws of groundwater hydrochemistry. The following main hydrochemical processes were identified as controlling the water quality of the groundwater system: weathering--dissolution, evaporation--condensation, ion-exchange reactions and groundwater salinization. This latter process, salinization, is the most important process and is caused by the leaching of superficial or near-surface salts from the saline--alkaline soil into shallow groundwater. [ABSTRACT FROM AUTHOR]

Instream flows are essential determinants of channel morphology, riparian and aquatic flora and fauna, water quality estuarine inflow and stream load transport. The ecological and environmental instream flow requirements (EEIFR) should be estimated to make the exploitation and utilization of water resources in a highly efficient and sustainable way and maintain the river ecosystem good health. As the largest tributary of the Yellow River, the Wei River is the 'Mother River' of Guanzhong region in Shaanxi province. It plays a great role in the development of West China and the health of the ecosystem of the Yellow River. The objective of this study is to estimate the EEIFR for improving the Wei River's ecological and environmental condition and develop the river healthily. Concerning the main ecological and environmental functions of the Wei River in Shaanxi Province, the EEIFR for each section of the Wei River including minimum instream flow requirements (IFR) for aquicolous biotopes maintenance, IFR for channel seepage, channel evaporation, stream self-purification and sediment transportation were estimated in this paper. The methods to estimate the instream flow requirements for stream self-purification and instream flow requirements for sediment transportation were proposed. The temporal scale of typical years include the year with the probability 25% of occurrence (high-flow year), the year with the probability 50% (normal-flow year) and the year with the probability 75% (low-flow year). The results show that the EEIFR for the Wei River mainly include instream flow requirements for self-purification and sediment transportation in each typical year. From high-flow year to low-flow year, the annual EEIFR for each reach decrease, except those for the reach from Linjiacun to Weijiabao, and from Linjiacun at the upper reaches to Huaxian at the lower reaches, and the annual reach EEIFR decrease in a sequence. [ABSTRACT FROM AUTHOR]

Sensible heat flux greatly influences the Indian monsoon. In this study, we calculated sensible heat flux time-series for 12 sites over the western Tibetan Plateau using Price and Dunne's formula and adjusting the stability function. The time-series were derived from the field observations from the GEWEX Asian Monsoon Experiment (GAME)/Tibet programme under the Global Energy and Water Cycle Experiment (GEWEX). This paper demonstrates that monthly sensible heat fluxes show strong correlations with corresponding precipitation, and that the correlation coefficients increase with precipitation amount. The preceding winter and spring solid precipitation (snowfall and resulting snowpack) can also influence sensible heat flux in May, but the situation is complex. The correlations between heat flux and snowfall at Amdo, Naqu, and Lhasa are negative, but they are positive at Gaize (also known as Gerze) and Dingri. There is a significant relationship between how the variations from the mean calculated heat fluxes at Amdo differ from those at Rikaze, or Dingri, Cuona and Longzi, and their respective June-September precipitation amounts. This phenomenon may result from changes in circulation. When the sensible heat fluxes are above average north of the influence of the Indian monsoon and below average to the south, the summer monsoon circulation develops early and with greater intensity and precipitation, and vice versa. [ABSTRACT FROM AUTHOR]

This paper deals with hydrological research in regard to the water resources crisis in the vulnerable areas found in the northern part of China. This area includes three main river basins, namely the basins of the Yellow (Huang) River, the Hai River and the Huai River. Several water problems are becoming very severe. Among them, two are the most critical: the Yellow River has been drained dry in the main course of its lower reaches and along its major tributaries, and the groundwater table has rapidly declined in the floodplains of the three rivers'' downstream areas. To counter the problems, particularly the critical issues mentioned above, hydrological research, which serves as the basis of water development and management, has been carried out in the last two decades. This paper addresses three basic scientific problems in North China, namely: (a) water consumption and the capacity for saving water; (b) the changes in hydrological processes and water resources caused by natural change and human activities; and (c) the ability to supply water resources and water safety in terms of both quantity and quality within a changing environment. However, opportunities and challenges for ameliorating the problems exist, and new ideas and methodology to solve the problems have been proposed, such as the interface process study on the interactions in the soil–root interface, the plant–atmosphere interface, the soil–atmosphere interface, and the interface of soil water and groundwater. In order to manage water resources in a sustainable manner, the study of water resources'' renewal ability as affected by natural change and human activity is addressed from the viewpoint of both water quantity and quality, and their integration. To reduce the vulnerability of water resources in regional water management, a paradigm of sustainable water resources utilization is also proposed, using water–heat balance, water–salt balance, water–sediment balance, and water supply–demand balance. This approach may help reveal the basic problems and point to possible approaches to solving the water problems in North China in the 21st century. Copyright © 2004 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Forest canopies are the functional interface between ecosystem and the atmospheric nitrogen (N) deposition. However, the sources of spatio‐temporal variability and intra‐event variation of N deposition via throughfall (TF) remain ambiguous. Here, we analysed TF samples for concentrations and fluxes of ammonium (NH4+) and nitrate (NO3−) using an array of 20 fixed‐position collectors on an event and within event basis throughout the 2019 growing season in a Chinese pine plantation, Northern China. Results showed that the volume weighted mean concentrations of NH4+ and NO3− in TF were significantly higher than those in bulk precipitation (BP) during the study period. A canopy budget model indicated that canopy uptake was more dominant than dry deposition for NH4+, while the reverse was true for NO3−. This caused a comparable TF NH4+ flux but an enhanced TF NO3− fluxes compared to BP. The intra‐event trend and magnitude of TF NH4+ and NO3− concentrations were influenced by the antecedent dry period and rainfall intensity, and indicated that biological nitrification may occur in the canopies. In addition, due to the larger of canopy NH4+ uptake that counteracted the amplifying effect of dry deposition, the spatial variability of TF NH4+ concentration was significantly lower than that of TF NO3− concentration, and exhibited more temporal persistence. No relationship was found between the spatial distribution of TF NH4+ concentration and the TF amount, the distance to the nearest trunk, nor the canopy cover, whereas the TF NO3− concentration was significantly related to the TF amount and the canopy cover. These findings can enhance our understanding of N processes within canopies, and have important implications for evaluating the impacts of N deposition in forest ecosystems. [ABSTRACT FROM AUTHOR]

Land use significantly affects the rate and amount of groundwater recharge. To understand these patterns, stable isotope analyses (δ2H and δ18O) and hydrochemical methods were employed to study the recharge and evolution of groundwater. MixSIAR Bayesian mixture modelling and statistical regression techniques were utilized to investigate the influence of land‐use type on groundwater recharge and to calculate the contribution of various water sources to different groundwater types. These findings are important for effective management and conservation of groundwater resources. The groundwater in all studied areas displayed a magnesium bicarbonate type with notable cation exchange, and also demonstrated the dissolution of carbonate and weathering of silicate. Results from the Bayesian mixture model indicate that 69.3% of the groundwater in the Loess Plateau region was recharged by precipitation, while 86.9% of the groundwater in the loess hilly‐gully region was recharged from the surface water. Groundwater recharge was more rapid in the loess hilly‐gully region than in the Loess Plateau and was less sensitive to the land use type in the former. Dryland‐moderate coverage grassland was found to be most conducive to groundwater infiltration, whereas monotypic dryland and high‐coverage grassland inhibited infiltration to some degree. To maintain balanced soil erosion control, an increase the area of moderate‐coverage grasslands in the Loess Plateau region is recommended. [ABSTRACT FROM AUTHOR]

Understanding the spatiotemporal nitrate retention in streambed is essential for developing management practices in reducing nitrate enrichment. However，the process of nitrate change in the profile of streambed at an elevated nitrate across a watershed scale is still not sufficiently investigated. In this study, we used a combination of hydraulic and hydro‐geochemistry methods to quantify total nitrate retention in streambeds of an agriculture‐intensive watershed in Central China. To conduct surface and groundwater measurements, we collected 1440 water samples for nitrate analysis from 40 shallow drilled wells during the dry and wet seasons from 2018 to 2020. The results showed a clear spatiotemporal variation of nitrate retention in streambed in the watershed. Spatially, nitrate retention in the midstream and downstream reaches was higher than that of the upstream reach. The lowest point of nitrate retention in downstream both in dry and wet seasons was at the depth of 0.75 m. While the lowest nitrate retention was found in midstream and upstream reaches, both in the dry and wet seasons at the depth between 1.5 and 2.5 m. Temporally, nitrate retention was higher in the wet season (1.56 μg N m−2d−1) than in dry season (1.41 μg N m−2d−1). DO min at 3 mg/L was found to the nitrate retention zero threshold in up and midstream. Water change fluxes and nitrate retention both have positive and negative relationship at watershed scale. Nitrate retention at the watershed scale was strongly affected by streambed lithology, precipitation, surface water ‐ groundwater exchange, and human activities. Those findings can provide reference for nitrate removal in international important agricultural areas. [ABSTRACT FROM AUTHOR]

Pressure on river water resources may increase continuously under global climate change and the increasing intensity of human activity, and the identification of river water sources is needed to maintain water resource sustainability. In this study, river water sources were studied using hydrochemical tracers (i.e. the major ions) and end‐member mixing analysis (EMMA) during two hydrological years, in the subtropical monsoon basin of the Xiangjiang River in the south‐central China. Various water types including precipitation, river water, and mountain creek water, were collected in the basin at a high frequency. The results of EMMA showed that the average contributions from the direct input of precipitation, near‐surface runoff (represented by mountain creek water), and groundwater (represented by river water sampled during the base flow period) were 28 ± 24%, 49 ± 24% and 23 ± 16%, respectively. The contributions from these water sources to the river water varied greatly within the year, which may indicate a shift in the major flow pathways from direct precipitation input and near‐surface flow pathways in the spring rainy months (e.g. from March to May), to a deeper flow pathway in the dry or rainless months (e.g. January, February, July and December). The shift of the major flow pathways may be due to the factors such as the seasonality of precipitation, evapotranspiration and soil wetness conditions. Groundwater and near‐subsurface runoff dominated the river water and the direct input of precipitation was negligible during the dry or rainless months, and consequently, the river water chemistry was less diluted and more influenced by water‐soil/rock interactions and anthropogenic pollution during these months. The creek water contribution to the river water was relatively high in summer, especially in June and July 2021, which may indicate the substantial inter‐seasonal carryover of precipitation that contributed to the river water. With the projected increased frequency of extreme weather events such as seasonal drought, streamflow in the subtropical monsoon regions may experience greater recharge by deeper flow pathways, with the enhanced role of base flow in regulating streamflow, and there may be an increased risk to the water resource security under the condition of the decreased river dilution capacity and increased anthropogenic pollution. [ABSTRACT FROM AUTHOR]

Under the urbanization context, land use/land cover (LULC) changes rapidly which affects both the probability of urban flood disaster and its consequence. In order to explore the impact of LULC changes on rainstorm flood disaster risk in the future, a framework of rainstorm flood disaster risk assessment is constructed from natural‐social perspective through LULC simulation based on the patch‐generating land use simulation (PLUS) model in this study. Specifically, the spatial LULC under three future development scenarios (inertia development scenario, economic priority scenario and sustainable development scenario) for the year 2030 is simulated, and then assessment model of rainstorm flood disaster risk is constructed through obtaining inundation depth and social vulnerability based on simulated LULC. Jingdezhen City in China is selected as an example. The results show that proposed risk assessment framework based on PLUS model can be used to detect the risk changes in different development scenarios. The risk of urban rainstorm flood disaster under economic priority scenario is most severe and risk under the sustainable development scenario is the lowest under the same return period rainstorm. Under the sustainable development scenario, green land including forest land, grassland and open water increases, which reduces the surface runoff and decrease the risk of rainstorm flood disaster effectively, however the reduction is limited with the greater rainstorm return period. Therefore, adaptive measures should be implemented together with land use planning to prevent and control urban flood risk. This study provides a framework to predict the future risk of urban rainstorm flood under different development scenarios, which can support for flood disaster risk prevention and urban sustainable development. [ABSTRACT FROM AUTHOR]

Small hydropower projects play a significant role in supplying clean energy and promoting economic development all around the world. The calculation of a river's ecological flow, that is, not only the quantity but also the quality of flowing water, in the watershed where small hydropower projects are located play an important role in scientifically guiding the discharge of small hydropower stations in the upper reaches of the river. By combining Gini coefficient with hydrological variation diagnosis, this study provided a novel approach to test the randomness of hydrological series and assess the evenness of runoff distribution. Furthermore, this study is the first attempt to calculate ecological flow for two separate periods (flood and low‐water periods), and thus is able to better reflect annual runoff variations. In the meantime, the Tennant method was improved by introducing of monthly runoff coefficient and median substitution, and integrating four other hydrological methods (empirical method, Qp method, annual distribution method and minimum monthly average measured runoff method) into the comprehensive calculation and analysis. The above methods were applied to the ecological base flow calculation for a small hydropower project in Dongjiang River basin of China. Through comprehensive and comparative analysis, better applicability was demonstrated by the improved Tennant method and the annual distribution method. The recommended values of ecological base flow for each month were derived at the range of 407–431 m3/s during the flood period and 179–239 m3/s during the low‐water period. This study demonstrated that the method could meet the ecological flow calculation requirements for river ecosystem and provide scientific basis for the delineation of ecological flow thresholds, and thereby ensure the healthy and sustainable development of small hydropower and the healthy river aquatic ecosystem. [ABSTRACT FROM AUTHOR]

This paper studies the coherent modes of multi-scale variability of precipitation over the headwater catchments in the Pearl River basin in South China. Long-term (1952-2000) daily precipitation data spatially averaged for 16 catchments in the basin are studied. Wavelet transform analysis is performed to capture the fluctuation embedded in the time series at different temporal timescales ranging from 6 days to 8.4 years. The catchment clusters of the coherent modes are delineated using the principal component analysis on the wavelet spectra of precipitation. The results suggest that as much as 98% of the precipitation variability is explained by only two coherent modes: high small-scale mode and high seasonal mode. The results also indicate that a large majority of the catchments (i.e., 15 out of 16) exhibit consistent mode feature on multi-scale variability throughout three sub-periods studied (1952-1968, 1969-1984, and 1985-2000). The underlying effects of the coherent modes on the regional flood and drought tendency are also discussed. [ABSTRACT FROM AUTHOR]

Insight into the rainfall‐soil moisture (SM) response to land cover is critical for soil hydrological process modelling and management. In this study, five typical land‐cover types (forest, shrub, grass, crop and bare land) and four rainfall patterns (heavy, intermediate, light and continuous rains) were selected to assess the effects of SM response characteristics on the Loess Plateau of China. We monitored SM at five depths on each land‐cover type at 1‐h intervals over the growing season of 2019. The results showed that rainfall patterns and land‐cover types together determined the SM response process and infiltration efficiency. A minimum accumulated rainfall amount of 5 mm was the threshold required to trigger a 10‐cm SM response. Rain events with a higher intensity and smaller total rainfall amount triggered a quick surface SM response, while larger amounts could percolate deeper and faster. Land‐cover change significantly altered the rainfall‐SM response dynamics and rainwater utilization efficiency after 20 years of ecological construction. Revegetation sites (mean values of forest, shrub and grass) increased the soil wetting depth by 14.7%, shortened the SM response time by 27.3%, and accelerated the SM wetting front velocity by 67.2%, which promoted a 35.2% rainfall transformation rate (RTR) across the 1‐m profile over all rainfall events (R1–13). Moreover, planted forest showed the highest RTR of R1–13 and the maximal increase in soil water storage, which did not aggravate the soil water deficit across the 1‐m profile over the growing season. Therefore, we present evidence that planted forests, instead of shrubs, may be beneficial for water conservation if precipitation is greater than 550 mm. The findings of this study prove the role of revegetation on rainwater infiltration capacity and efficiency and can help improve the management of afforestation in arid and semiarid regions. [ABSTRACT FROM AUTHOR]

Information on both the magnitude and relative contributions of inter‐gully and gully areas to the sediment yield from small catchments in the Loess Plateau of China represents an important requirement for understanding catchment sediment dynamics, assessing the sensitivity of catchment sediment budgets to environmental change and developing effective soil and water conservation strategies. Contemporary catchment monitoring programmes can provide such information, but there is also a need to consider past behaviour. Where long‐term records are lacking, attention has been directed to the potential for using the sediment deposits behind check dams to reconstruct the sediment response of the upstream catchment. This contribution reports a study of the 0.181 km2 Nianyangou catchment located in the hilly region of the Chinese Loess Plateau, in Shaanxi Province, China, which has extended a previous study aimed at using individual flood couplets preserved in the check dam deposits to reconstruct the record of annual sediment yield, to include the use of sediment source fingerprinting techniques to document changes in the annual sediment contribution from inter‐gully and gully areas for the period 1960–1990. The sediment response of the catchment provided evidence of two change points in 1964 (Period 1–Period 2) and 1983 (Period 2–Period 3), which reflect the introduction of soil and water conservation measures in the mid 1960s through to the 1970s and a change in the land tenure system in 1983. The annual sediment yields from inter‐gully and gully areas follow the trend P1 > P3 > P2, whereas the equivalent information on the relative contribution of inter‐gully and gully areas follow different trends P3 > P1 > P2 and P2 > P1 > P3, respectively. [ABSTRACT FROM AUTHOR]

For many urban woodlands, knowledge of how water is cycled and partitioned by plants is important for improving water management strategies, particularly during warm and wet periods with heavy, convective events in subtropical monsoon regions which are characterized by hot and humid summers. Still, most studies on these processes have been conducted in rural, none subtropical areas. Useful tools for tracing water movement during different precipitation events are stable water isotopes. So far, very few studies have been carried out in urbanized woodlands where the natural hydrological cycle is affected by strong anthropogenic influences. We conducted an intense spatio‐temporal sampling campaign of water isotopes, combined with relevant atmospheric parameters, climatic and hydrometric data across an evergreen broad‐leaved woodland near Chengdu, SW China, to assess event‐based changes in water cycling and partitioning during the warm and wet summer of 2020. Based on event‐scale precipitation samples, soil and plant water samples, we found similar temporal variations in the water isotopes of precipitation, soil, and plants, with a gradual enrichment in the order of precipitation – soil water – plant water. However, the responses of soil and plant water to precipitation were delayed. From the dynamics of soil water content and soil water isotopes, evaporative fractionation and precipitation signatures were determined as the critical factors in influencing water storage and compositions through the soil profile. Only precipitation events >5 mm recharged the soil profile, with a ~ 24 h time lag in the response time of soil water (0–50 cm) to effective events. The water isotopes in the branches and leaves of the plants reflected their water sources primarily from 0 to 50 cm soil layers and water losses through evapotranspiration, respectively. These isotopic signals described the transfer and exchange of water in the soil–plant‐atmosphere continuum and therefore improved our understanding of water cycling and partitioning in an urban woodland in a subtropical monsoon humid region, which may have important implications for urban green space and ecosystem services in other regions. [ABSTRACT FROM AUTHOR]

Precipitation is the only source of groundwater recharge in the Chinese loess region. However, rainfall has been the main driver of geological disasters and engineering challenges in China, such as landslides, slope collapse and sinking of foundations. The roles of preferential flow and piston flow in moisture transfer in the thick loess vadose zone remain controversial. This study used a 100‐m thick loess shaft to monitor soil moisture content and soil temperature over 5 years, whereas climate indices were monitored on the Earth surface. The soil water characteristic curves and hydraulic conductivity function of the loess and the palaeosol collected in the profile were measured in a laboratory. The soil profile moisture transfer process was simulated based on the climatic boundary conditions and measured hydraulic parameters. The monitored and simulated results suggest that loess moisture transfer is dominated by piston flow. The thick loess vadose zone above the groundwater level could be divided into two sub‐zones, namely the active and steady zones. The active zone has a thickness of ~7 m and shows a direct response to climate change, with soil moisture content regulated by rainfall infiltration, evaporation and transpiration and soil moisture moving as transient flow. Although preferential flow can move in the active zone along joints, fissures and holes when water accumulates in the lower ground during heavy rainfall, piston flow remains the main mode of water movement. The steady zone has a thickness of <7 m and is above the groundwater level. Soil moisture content and soil temperature in the steady zone show temporal stability and soil moisture moves as a steady flow. Piston flow is the only mode of water movement in the steady zone. Since percolation velocity in the steady zone is estimated to be 73.3 mm/year, 1330 years of rainfall infiltration through the 97.5 m thick vadose zone would be required to recharge groundwater. [ABSTRACT FROM AUTHOR]

Widespread clearing of natural forests for tea plantations have profoundly altered the local water balance and energy budget. However, studies of comprehensive effects of land‐use conversion on energy and hydrological cycling are still rare. In this study, we used 5 years (2014–2018) of eddy covariance (EC) data, modelled estimates, and hydrological monitoring of different functional layers (canopy, litter and soil layers) to summarize the seasonal and inter‐annual variations and differences of evapotranspiration and water balance components in tea plantations (Camellia sinensis) and natural bamboo forests (Phyllostachysedulis). Results suggested the bamboo forest had higher actual evapotranspiration (ETa) (835 ± 5 mm) than the tea plantation (730 ± 19 mm), owing to differences in canopy conditions and plant physiology. Evident differences in ETa components were found between the tea plantation site and the bamboo forest site. The proportion of transpiration (Etr) was higher at the bamboo site (0.71–0.76) than the tea plantation site (0.60–0.65). Additionally, the soil evaporation ratio (0.10–0.13) was lower at the bamboo forest site than the tea plantation site (0.21–0.24) due to vegetation structure differences altering the radiation energy partitioning between the two sites. Moreover, the tea plantations were more susceptible to drought stress with low water storage in soil and simple rhizome‐root system. The comprehensive water holding capacity of canopy, litter, and soil layers of bamboo forest was 1.0–1.8 times of that in tea plantation. Compared with bamboo forest, pruning and losing of soil with intensive management activities enhanced drainage at the tea plantation site. This study provides insights into land‐use conversion of subtropical forest and managed tea plantation that have important implications on regional energy and water budgets. [ABSTRACT FROM AUTHOR]

Climate change and anthropogenic factors have significantly impacted hydrological regimes, especially in semiarid regions. Using the trend‐free pre‐whitening Mann–Kendall (TFPW‐MK) test and linear regression, trends of precipitation, temperature, runoff, and water balances from different spatiotemporal scales were analysed during the past sixty years in the Upper Yongding River Basin (UYRB). Based on the Choudhury‐Yang equation, the contributions of climate change and human activities to runoff variation were further quantified in five mountainous catchments, and the actual effects of climate change and human activities on the water balance in the whole UYRB were also discussed. The results suggest that the UYRB has become much warmer and drier. The lower annual precipitation and intensity in summer were directly attributed to runoff reductions. Intra‐annual scale analyses of meteorological, hydrological and biological responses helped the comprehensive interpretation of the runoff trends. Human activities, including agricultural irrigation, were the main causes for runoff reduction, especially in the middle and lower reaches, and the contribution of human activities increased gradually in recent decades. The water balance changed drastically temporally, with the precipitation being converted less to runoff and more to evaporated fractions, which was also due to the second order effects of climate change and vegetation improvement. It is suggested that low water‐consumption plant species and the related water‐saving techniques in agriculture and afforestation should be introduced for the sustainable development in the similar regions. [ABSTRACT FROM AUTHOR]

Budyko's framework has been widely used to study basin‐scale water balance. In this study, we focus on the extended application of Fu's equation (one formulation of the Budyko‐type curves) to 371 large irrigation districts in China over a period of 2010–2017. Water balance method was used to estimate actual evapotranspiration ET in the irrigated areas. Considering the contribution of shallow groundwater to ET, the water availability in the Budyko framework defined as equivalent precipitation Pe for irrigation areas is the sum of irrigation water I, precipitation P and groundwater evaporation ETgw. Results showed that the relationships between evapotranspiration ET, water availability Pe and energy supply ET0 can be accurately described by the Budyko's curves. The optimal values of Budyko parameter ω for each irrigation district was obtained with multi‐annual data using least square method. The Fu's equation performed better in humid and semi‐humid regions than arid and semi‐arid regions. The comparison between ∂ET/∂Pe and ∂ET/∂ET0 confirmed the relative effect of water availability and energy supply on ET according to the variation of climatic conditions. Normalized difference vegetation index (NDVI) and soil properties (denoted by the proportion of clay and sand) were selected to develop empirical equation for parameter ω using multiple linear regression analysis method. This study shows that the Budyko framework can be extended to irrigation areas and provides useful information on evapotranspiration to assist in water management in irrigation areas. [ABSTRACT FROM AUTHOR]

Budyko's framework has been widely used to study basin‐scale water balance. In this study, we focus on the extended application of Fu's equation (one formulation of the Budyko‐type curves) to 371 large irrigation districts in China over a period of 2010–2017. Water balance method was used to estimate actual evapotranspiration ET in the irrigated areas. Considering the contribution of shallow groundwater to ET, the water availability in the Budyko framework defined as equivalent precipitation Pe for irrigation areas is the sum of irrigation water I, precipitation P and groundwater evaporation ETgw. Results showed that the relationships between evapotranspiration ET, water availability Pe and energy supply ET0 can be accurately described by the Budyko's curves. The optimal values of Budyko parameter ω for each irrigation district was obtained with multi‐annual data using least square method. The Fu's equation performed better in humid and semi‐humid regions than arid and semi‐arid regions. The comparison between ∂ET/∂Pe and ∂ET/∂ET0 confirmed the relative effect of water availability and energy supply on ET according to the variation of climatic conditions. Normalized difference vegetation index (NDVI) and soil properties (denoted by the proportion of clay and sand) were selected to develop empirical equation for parameter ω using multiple linear regression analysis method. This study shows that the Budyko framework can be extended to irrigation areas and provides useful information on evapotranspiration to assist in water management in irrigation areas. [ABSTRACT FROM AUTHOR]

Located along the northeastern edge of the Qinghai‐Tibet Plateau in China, the Qilian Mountains are an important ecological barrier in Northwest China. Therefore, it is of great significance to study the plant water sources in subalpine habitats for understanding the ecological and hydrological processes in the Qilian Mountains. Here, based on the samples of precipitation, xylem water, soil water, river water, and spring water collected during May–October 2019 from subalpine habitats on the northern slope of the Qilian Mountains and the measured hydrogen and oxygen stable isotope values (δ2H and δ18O, respectively), the present study gained further insight into plant water sources in this region using the IsoSource model. Water absorption characteristics of shrubs were similar in subalpine habitats. Specifically, plants absorbed water primarily from the topsoil layer (0–30 cm) and rarely from the deep soil layers. Nevertheless, in the dry and growing seasons with high water demand, subalpine shrubs competed for water resources at similar depths. As a result of this competition, the utilization rate of deep soil water improved in some shrubs, expanding the differences in water sources of subalpine shrubs within the same habitat. In addition, in the dry and growing seasons, the water sources of the same subalpine shrub varied across different habitats. Compared with other subalpine shrubs, Salix cupularis Rehder, Salix oritrepha Schneid., Potentilla fruticosa L., Salix sclerophylla Anderss., Rhododendron anthopogonoides Maxim., and Rhododendron przewalskii Maxim. could alter their water use strategies with variations in water conditions, exhibiting a greater drought tolerance. These findings provide valuable information for understanding the subalpine region's eco‐hydrological processes and contribute to the selection of suitable species for the restoration of the subalpine ecological environment under the background of global change. [ABSTRACT FROM AUTHOR]

River runoff and suspended sediment play a critical role in the chemical, physical, biological processes of Earth's ecosystems. We assessed the temporal changes in runoff and sediment load at both the annual and event scales in the Gushanchuan watershed in the middle reaches of the Yellow River. A total of 330 events, from 1955 to 2018, were selected to evaluate the relationships between rainfall, runoff, and suspended sediment load at the Gaoshiya gauging station. Significant reductions were examined in the maximum runoff peaks, suspended sediment concentration, total runoff, suspended sediment yield, and ratio of event runoff to annual total, whereas the ratio of event sediment yield to annual total remained relatively stable. Hysteresis analysis suggested that the complex loops, anti‐clockwise, and figure‐eight loops accounted for 93.7% of the total events in the watershed. High positive correlations were observed between the flood‐event sediment yield, runoff volume, maximum runoff peaks, and total rainfall amount. The flow‐sediment relationship can be well fitted by the proportional model, and was weaker in 1997–2018 than those in the periods of 1955–1979 and 1980–1996. Runoff during the rising limb of the event significantly contributed to the total runoff and sediment yield. Land use changes and various soil conservation practices were responsible for the decrease in the flood frequency and magnitude. The results of this study provide useful insights into the relationships between rainfall, runoff, and suspended sediment dynamics, and their responses to soil conservation practices. [ABSTRACT FROM AUTHOR]

The magnitude and frequency of regional extreme precipitation events may have variability under climate change. This study investigates the time-space variability and statistical probability characteristics of extreme precipitation under climate change in the Haihe River Basin. Hydrological alteration diagnosis methods are implemented to detect the occurrence time, style and degree of alteration such as trend and jump in the extreme precipitation series, and stationarity and serial independence are tested prior to frequency analysis. Then, the historical extreme precipitation frequency and spatio-temporal variations analyses are conducted via generalized extreme value and generalized Pareto distributions. Furthermore, the occurrence frequency of extreme precipitation events in future is analysed on the basis of the Fourth Assessment Report of the Intergovermental Panel on Climate Change multi-mode climate models under different greenhouse gases emission scenarios (SRES-A2, A1B and B1). Results indicate that (1) in the past, alteration of extreme precipitation mainly occurred in the area north of 38°N. Decreasing trends of extreme precipitation are detected at most stations, whereas jump alteration is not obvious at most stations. (2) Spatial variation of estimated extreme precipitation under different return periods shows similarity. Bounded by the Taihang Mountain-Yan Mountain, extreme rainfall in the Haihe River Basin gradually reduces from the southeast to the northwest, which is consistent with the geographical features of the Haihe River Basin. (3) In the future, extreme precipitation with return period 5-20 years accounts for a significant portion of the total occurrence times. The frequency of extreme precipitation events has an increase trend under A1B and A2 scenarios. The total occurrence times of extreme precipitation under A1B senario are not more than that under B1 senario until the 2030s. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Rainfall prediction is of vital importance in water resources management. Accurate long‐term rainfall prediction remains an open and challenging problem. Machine learning techniques, as an increasingly popular approach, provide an attractive alternative to traditional methods. The main objective of this study was to improve the prediction accuracy of machine learning‐based methods for monthly rainfall, and to improve the understanding of the role of large‐scale climatic variables and local meteorological variables in rainfall prediction. One regression model autoregressive integrated moving average model (ARIMA) and five state‐of‐the‐art machine learning algorithms, including artificial neural networks, support vector machine, random forest (RF), gradient boosting regression, and dual‐stage attention‐based recurrent neural network, were implemented for monthly rainfall prediction over 25 stations in the East China region. The results showed that the ML models outperformed ARIMA model, and RF relatively outperformed other models. Local meteorological variables, humidity, and sunshine duration, were the most important predictors in improving prediction accuracy. 4‐month lagged Western North Pacific Monsoon had higher importance than other large‐scale climatic variables. The overall output of rainfall prediction was scalable and could be readily generalized to other regions. [ABSTRACT FROM AUTHOR]

We present a runoff reconstruction for the Bailong River based on the composite chronology developed from four sampling sites of Pinus tabulaeformis in the China north–south transition zone. The runoff reconstruction, spanning 1601–2013 CE, was developed by calibrating tree‐ring data with the instrumental runoff record. Runoff reconstruction accounted for 44.3% of the actual runoff variance during the common period 1958–2010 and provided a long‐term perspective on hydrological change in the China north–south transition zone. In the past 413 years, high‐ and low‐runoff years accounted for 15.50% and 15.98%, respectively. Of all the 17 extreme hydrological events, 14 of them are extremely high‐flow years, and 17th century was the wettest period during the past 413 years. The preliminary analysis results show that there is a relationship between our runoff reconstruction and the Atlantic multidecadal oscillation at multidecadal scale. Since the 1990s, runoff in the China north–south transition zone has also seen a significant decrease as a result of dry trends of the source region. [ABSTRACT FROM AUTHOR]

Rainfall prediction is of vital importance in water resources management. Accurate long‐term rainfall prediction remains an open and challenging problem. Machine learning techniques, as an increasingly popular approach, provide an attractive alternative to traditional methods. The main objective of this study was to improve the prediction accuracy of machine learning‐based methods for monthly rainfall, and to improve the understanding of the role of large‐scale climatic variables and local meteorological variables in rainfall prediction. One regression model autoregressive integrated moving average model (ARIMA) and five state‐of‐the‐art machine learning algorithms, including artificial neural networks, support vector machine, random forest (RF), gradient boosting regression, and dual‐stage attention‐based recurrent neural network, were implemented for monthly rainfall prediction over 25 stations in the East China region. The results showed that the ML models outperformed ARIMA model, and RF relatively outperformed other models. Local meteorological variables, humidity, and sunshine duration, were the most important predictors in improving prediction accuracy. 4‐month lagged Western North Pacific Monsoon had higher importance than other large‐scale climatic variables. The overall output of rainfall prediction was scalable and could be readily generalized to other regions. [ABSTRACT FROM AUTHOR]

We present a runoff reconstruction for the Bailong River based on the composite chronology developed from four sampling sites of Pinus tabulaeformis in the China north–south transition zone. The runoff reconstruction, spanning 1601–2013 CE, was developed by calibrating tree‐ring data with the instrumental runoff record. Runoff reconstruction accounted for 44.3% of the actual runoff variance during the common period 1958–2010 and provided a long‐term perspective on hydrological change in the China north–south transition zone. In the past 413 years, high‐ and low‐runoff years accounted for 15.50% and 15.98%, respectively. Of all the 17 extreme hydrological events, 14 of them are extremely high‐flow years, and 17th century was the wettest period during the past 413 years. The preliminary analysis results show that there is a relationship between our runoff reconstruction and the Atlantic multidecadal oscillation at multidecadal scale. Since the 1990s, runoff in the China north–south transition zone has also seen a significant decrease as a result of dry trends of the source region. [ABSTRACT FROM AUTHOR]

Investigating the changes in streamflow regimes in response to various influencing factors contributes to our understanding of the mechanisms of hydrological processes in different watersheds and to water resource management strategies. This study examined streamflow regime changes by applying the indicators of hydrologic alteration method and eco‐flow metrics to daily runoff data (1965–2016) from the Sandu, Hulu and Dali Rivers on the Chinese Loess Plateau, and then determined their responses to terracing, afforestation and damming. The Budyko water balance equation and the double mass curve method were used to separate the impacts of climate change and human activities on the mean discharge changes. The results showed that the terraced and dammed watersheds exhibited significant decreases in annual runoff. All hydrologic metrics indicated that the highest degree of hydrologic alteration was in the Sandu River watershed (terraced), where the monthly and extreme flows reduced significantly. In contrast, the annual eco‐deficit increased significantly, indicating the highest reduction in streamflow among the three watersheds. The regulation of dams and reservoirs in the Dali River watershed has altered the flow regime, and obvious decreases in the maximum flow and slight increases in the minimum flow and baseflow indices were observed. In the Hulu River watershed (afforested), the monthly flow and extreme flows decreased slightly and were categorized as low‐degree alteration, indicating that the long‐term delayed effects of afforestation on hydrological processes. The magnitude of the eco‐flow metrics varied with the alteration of annual precipitation. Climate change contributed 67.47% to the runoff reduction in the Hulu River watershed, while human activities played predominant roles in reducing runoff in the Sandu and Dali River watersheds. The findings revealed distinct patterns and causes of streamflow regime alteration due to different conservation measures, emphasizing the need to optimize the spatial allocation of measures to control soil erosion and utilize water resources on the Loess Plateau. [ABSTRACT FROM AUTHOR]

Knowledge of exchanges of energy and water over terrestrial surfaces is the first step towards understand the ecohydrological mechanisms, particularly in water‐limited ecosystems in dryland environments. However, patterns of energy exchange and evapotranspiration (ET) are not well understood in the oasis‐desert ecotone, which plays an important role in protecting oases against the threat of desertification in arid regions of northwestern China. Here, the continuous measurements of surface energy fluxes were made using eddy covariance in conjunction with auxiliary measurements for 2 years (2014–2015) in an oasis‐desert ecotone mainly covered by phreatophyte shrubs Haloxylon ammodendron, Nitraria tangutorum/sphaerocarpa, and Calligonum mongolicum in arid northwestern China. Based on the collated data for 2 years, statistical analysis on a 30‐min time scale indicated that approximately 50% of daytime net radiation (Rn) in the ecotone was dissipated as H on average, and one‐third of Rn was consumed by soil heat flux (G). Only 9% of Rn was consumed for latent heat flux (λE), which peaked in summer (21% in 2014 and 16% in 2015), corresponding to the highest rainfall season. Daily mean ET was approximately 1 mm days−1 during the growing season of the shrub species. Accumulated annual ET was 195 and 181 mm in 2014 and 2015, respectively, exceeding the corresponding precipitation (P) by approximately 87 and 77 mm, indicating that groundwater may be another important source of water for ET in the ecotone aside from rainfall. Results within provide valuable insights into the mechanisms responsible for sustaining energy and water balance in the ecotone, a potentially groundwater‐dependent ecosystem. These results also offer a foremost ecohydrological implication for water and land resources management and ecotone conservation, such as avoiding heavy groundwater pumping for extensive agricultural irrigation use to sustain groundwater availability for these shrub species in the ecotone. [ABSTRACT FROM AUTHOR]