Upper Yangtze River Basin Research Articles

Characteristics of runoff and sediment load during flood events in the Upper Yangtze River, China

Understanding the change in sediment load at a flood-event scale helps to enhance channel regulation and optimize reservoir operation. This is particularly true in the Upper Yangtze River basin where a few flood events contribute most of the annual sediment load. This paper collected field data from representative hydrological stations and applied statistical analysis to investigate the change in runoff and sediment load during flood events in the Upper Yangtze River, including both the mainstream (Jinsha River) and its major tributaries (Min, Tuo, Jialing, and Wu Rivers). The results show that flood events contribute more sediment load than runoff volume to the annual statistics. A general decreasing trend occurs in sediment load in all river basins, which is closely related to soil conservation efforts and dam construction. The series of sediment load modulus of each stations present abrupt change. The variations in sediment load modulus in low flow are more significantly than those in high flow at different abrupt-change period, indicating high flows during flood event have been found to still carry the same amount of sediment as their predecessors in the Upper Yangtze River. An “abnormal” increase in sediment load in recent years (2013–2020) has been observed at the Fushun station in Tuo River, and three factors, i.e., earthquake, high-intensity rainstorm and local scouring, were identified to be responsible for such increase. The findings of our study are helpful for better management of basin-scale sediment resources and regulation of reservoirs in the Upper Yangtze River.

A hybrid hydrologic modelling framework with data-driven and conceptual reservoir operation schemes for reservoir impact assessment and predictions

Reservoirs have been built worldwide to address the water-related issues. To fully understand their potential impacts on the hydrologic regime, explicitly parameterizing reservoir operation in hydrologic models is often required. In this study, two data-driven reservoir operation schemes based on extreme gradient boosting (XGBoost) and artificial neural network (ANN) are respectively developed to predict the reservoir release and storage in hydrologic models for reservoirs with historic in-situ inflow, storage, release data. Then, a hybrid modelling framework is proposed by coupling a high-resolution (3 km) hydrologic model with (1) the developed data-driven reservoir operation schemes and (2) a calibration-free conceptual reservoir operation scheme designed for data-scarce reservoirs. This allows quantitative assessment of the cumulative impacts of dam operation on the hydrologic regime under different reservoir data availability. The framework is applied to the Upper Yangtze River Basin (UYRB) in China that is one of the most regulated river basins across the country due to extensive reservoir construction. Results indicate that both data-driven reservoir operation schemes can well reconstruct the reservoir releases and storage in the UYRB (daily NSE of ∼ 0.9), and the XGBoost performs slightly better than ANN. By coupling reservoir operation, the model shows a remarkably improved performance in reconstructing the daily streamflow of the basin. The major reservoirs in the UYRB can redistribute excessive water in the wet season to the dry season and attenuate the high and low flows, leading to enhanced water security along the river. Our approach provides a practical framework for reservoir impact assessment and predictions.

Convergence Analysis of the Overall Benefits of Returning Farmland into Forest in the Upper Yangtze River Basin, China

Returning farmland to the forest (RFTF) in the upper Yangtze River basin is a strategic measure to promote the sustainable development of ecological protection. The convergence analysis of the overall benefits of the upper Yangtze River basin provides a basis for a scientific and reasonable understanding of the improvement in the comprehensive benefits of RFTF in the basin. It provides policy suggestions for basin governance and sustainable development. Therefore, the convergence analysis and its spatial effect decomposition are carried out based on the entropy weight method to calculate the overall benefits of RFTF in the upper Yangtze River basin. The results indicate that there is no σ convergence in the overall benefits of RFTF in the upper Yangtze River basin, but there is significant absolute β convergence and significant conditional β convergence, and the overall development trend is stable. After thoroughly considering certain economic and social factors, especially the traffic infrastructure and the intensity of economic activities, the convergence rate of the overall benefits of RFTF in the upper reaches of the Yangtze River is accelerated. From the perspective of the spacing effect, the overall benefits of RFTF in this region and surrounding areas in the previous period significantly affected this region. As a result, sustainable development of the upper Yangtze River basin needs to include coordinated regional action to ensure ecological benefits, to improve transportation infrastructure, to scientifically recognize economic activities, and to guard against food security risks.

Spatio-temporal variability of terrestrial water storage in the Yangtze River Basin: Response to climate changes

The Yangtze River Basin (YRB) is an important region for China's economic development. However, it has a complex terrain layout, most of which is affected by monsoon weather, and the geographical and temporal distribution of water resources is severely unbalanced. Therefore, the detailed analysis of spatio-temporal water mass changes is helpful to the development and rational utilization of water resources in the YRB. In this study, the variation of terrestrial water storage (TWS) is monitored by Gravity Recovery and Climate Experiment (GRACE) satellite gravity. We find that the University of Texas Center for Space Research (CSR) solution shows a notable difference with the Jet Propulsion Laboratory (JPL) in space, but the general trend is consistent in time series. Then the GRACE inferred water mass variation reveals that the YRB has experienced several drought and flood events over the past two decades. Global Land Data Assimilation System (GLDAS) results are similar to GRACE. Furthermore, the overall precipitation trend tends to be stable in space, but it is greatly influenced by the strong El Niño-Southern Oscillation (ENSO), which is the response to global climate change. The upper YRB is less affected by ENSO and shows a more stable water storage signal with respect to the lower YRB.

Future changes in annual runoff and hydroclimatic extremes in the upper Yangtze River Basin

The upper Yangtze River Basin (UYRB) provides almost half of the total runoff volume of the entire Yangtze River and is of great importance for the population living in the upstream and downstream regions. The recent streamflow decline and more frequent hydroclimatic extremes have raised concerns about the water sustainability and water security of the UYRB in a changing climate. Here we examine the hydrological changes in the UYRB during the historical period (1961–2020) and the future period (2021–2070) using a physically-based hydrological model. Based on the simulation results, the future changes in water resources and hydroclimatic extremes in the UYRB were analyzed. The results show that the mean annual runoff volume in the UYRB during the future period will further decrease by 20.4 (SSP1-2.6), 17.7 (SSP2-4.5) and 22.0 (SSP5-8.5) billion m3 compared with the historical mean value. The agricultural drought severity will increase in all sub-regions, which tends to be more severe than the meteorological droughts, and Wujiang is going to experience the largest increase in drought severity. Averaged across the UYRB, the unprecedented agricultural droughts and pluvials will occur more than once per decade on average in the future, indicating that the most extreme events in the historical period would become the new normal in the future. The design floods with different return periods at all major hydrological stations in the UYRB are projected to increase, and in the future there will be more frequent drought-pluvial seesaws at the major hydrological stations. These findings highlight the importance of adaptive water resources management to tackle the elevated risk of water shortage and more frequent and severe hydroclimatic extremes in the future UYRB.

How to measure the ecological compensation threshold in the upper Yangtze River basin, China? An approach for coupling InVEST and grey water footprint

Defining a reasonable and feasible watershed ecological compensation threshold is the key to protecting watershed ecological functions and maintaining the sustainable utilization of watershed ecosystems. However, many studies have obtained compensation values that are too high to promote the implementation of ecological compensation policies. This study chose the upper reaches of the Yangtze River as the research area, taking water resources closer to people’s daily needs as the evaluation object. Based on the InVEST (Integrated Valuation of Ecosystem Services and Trade-offs) model and grey water footprint method, the ecological compensation threshold model for water resources was established. Combined with the eco-compensation priority sequence coefficient identification of protected areas and beneficiary areas and allowed for the measurement of the watershed ecological compensation value in 2015 and 2020. Finally, compare the advantages and disadvantages of different ecological compensation calculation methods, compare the gap between different watershed ecological compensation standards and the theoretical threshold globally. The results showed that from 2015 to 2020, the value of the water content in the upper reaches of the Yangtze River increased, while the value of the grey water footprint decreased. The classified watershed ecological compensation beneficiary areas were mainly concentrated in the central-eastern and southern parts of the upper Yangtze River, while the ecological compensation protected areas were concentrated in the western and northwestern parts. The mean absolute values of the watershed ecological compensation thresholds for each prefecture-level city and state ranged from 0.43 to 24.63 billion CNY in 2015 and from 0.67 to 41.60 billion CNY in 2020, which were close to the actual values. Among the different land-use types, the water conservation service value per unit area of shrubs was the highest. The lower limit value of watershed ecological compensation calculated using the grey water footprint method was closer to the amount of compensation in practice than was the commonly used opportunity cost method. The findings of the study can help improve the watershed ecological compensation mechanism in the upper Yangtze River region, promote win–win cooperation among transboundary watershed areas, and form a harmonious and stable green development model.

How can an ecological compensation threshold be determined? A discriminant model integrating the minimum data approach and the most appropriate land use scenarios

Ecological compensation has become very common worldwide due to the imbalance in the development of modern society, economy, and the environment and the increased pressure on ecosystem carrying capacity. Nonetheless, the various approaches for quantifying ecological compensation standards differ significantly. Thus, the process for determining a reasonable ecological compensation threshold is important to understand. To ensure the maximization of ecosystem service supply and economic benefit incentives for farmers, this paper constructs a discriminant model of an ecological compensation threshold based on the minimum data approach and the most appropriate land use scenario to define the ecological compensation threshold of the Sloping Land Conversion Program in the upper Yangtze River basin. The results show that with an increase in the compensation price, the proportion of farmers participating in returning farmland to forests and grassland increases, and water conservation increases. However, the discriminant curve first increases to a certain threshold point and then decreases, after which the ecoefficiency rate obtained from the compensation decreases. The ecological compensation thresholds for Chongqing, Sichuan, Yunnan, Guizhou, Hubei, Qinghai, Gansu, Tibet and Shaanxi provinces are 17.74 yuan/m3, 13.79 yuan/m3, 19.1 yuan/m3, 17.79 yuan/m3, 15.28 yuan/m3, 45.14 yuan/m3, 17.23 yuan/m3, 25.2 yuan/m3 and 22.36 yuan/m3, respectively. This research examines ecological compensation standards in different watersheds throughout the world and discusses the relationship between ecological compensation and water conservation. The discriminant model of the ecological compensation threshold can provide a new reference for the implementation and management of ecological compensation policies.

Future changes in precipitation over the upper Yangtze River basin based on bias correction spatial downscaling of models from CMIP6

Global climate change will change the temporal and spatial distribution of precipitation, as well as the intensity and frequency of extreme precipitation. The Yangtze River basin is one of the world’s largest basins, and understanding the future precipitation changes should be vital to flood control, water resources supply, and hydropower electricity generation in this basin. In this study, projected future characteristics of precipitation are analyzed in the upper Yangtze river basin (UYRB). To this end, based on the observed data from national meteorological stations, the bias correction spatial downscaling (BCSD) of five models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) is carried out. Then, based on the results of multi model ensemble (MME), we find that, relative to the historical period (1988–2014), the mean annual precipitation in the whole UYRB during 2015–2064 increases by 4.23%, 1.11%, 1.24% under SSP1-2.6, SSP2-4.5, SSP5-8.5, respectively, and it increases more in the long term (2040-2064) than that in the near term (2015–2039). Under SSP1–2.6, the precipitation will increase more significantly, which means lower emission of aerosols and greenhouse gases may increase the risk of flood disaster in the future over the UYRB. Interdecadal precipitation variability is more intense than interannual precipitation variability. Future precipitation changes in four seasons are spatially heterogeneous under three scenarios. Three extreme precipitation indices, including R95p, Rx1day and R10 mm, generally increase in the UYRB. R95p and Rx1day increase more in the WR and YBYCR basins with relatively high mean annual precipitation than that in other three sub-basins. R10 mm changes slightly in all sub-basins. The results reveal that the lower region of the UYRB may face greater risk of extreme precipitation. This study provides a timely updated finding about future changes in precipitation in the UYRB based on more accurate climate projections and ground-based observation.

Spatio-Temporal Variability in Hydroclimate over the Upper Yangtze River Basin, China

As global warming produces dramatic climate changes, water management is facing increasingly serious challenges. Given to the process of climate change and its complex effects on watershed hydrology, this paper investigates the spatial and temporal variation characteristics of major climatic factors (i.e., precipitation and temperature) over the upper Yangtze River basin (UYRB), China. The statistical analyses are based on annual and seasonal scales during 1951–2020 with a recorded period of seven decades. The Mann–Kendall nonparametric test and R/S analysis are used to record the temporal trends (past and future) of climate variables; the Pettitt test, standard normal homogeneity test and Buishand test are used to detect the homogeneity in climate series. The sensitivities of the streamflow to climatic parameters are assessed at the watershed scale, especially considering the Three Gorges Dam’s (TGD) effect on changing runoff. The results of the study indicate that the annual precipitation of 29 out of 34 series indicate homogeneity, while 31 out of 34 annual mean temperature series show heterogeneity, with jump points around 1997 in the mean temperature of 20 sites. Detectable changes in precipitation were not observed during 1951–2020; however, the temperature increased significantly in the whole basin on annual and seasonal scales, except for several stations in the eastern part. The magnitude of increase in air temperature in high altitudes (Tibet Plateau) is higher than that in low altitudes (Sichuan Plain) over the last seven decades, and future temperatures continue to sharply increase in high altitudes. The TGD plays an important role in explaining the seasonal variations in streamflow at Yichang station, with streamflow experiencing a sharp increase in winter and spring (dry season) and a decrease in summer and autumn (rainy season) compared to the pre-TGD period. The streamflow variation at an annual scale is mainly regulated by climate fluctuation (variation in precipitation). During the last seven decades, increasing air temperature and decreases in rainfall and runoff signify reduced water resources availability, and the climate tends to be warmer and drier over the basin. The sensitivity of the streamflow to watershed precipitation is higher than that to temperature, with variation in annual rainfall explaining 71% of annual runoff variability.

Projected impacts of climate change on major dams in the Upper Yangtze River Basin

Projected impacts of climate change on major dams in the Upper Yangtze River Basin

Unprecedented sedimentation in response to emerging cascade reservoirs in the upper Yangtze River Basin

Anthropogenic interferences to large river basins are triggering unprecedented changes to earth surface processes. The upper reach of the Yangtze River Basin (UYRB) is a typical example, where a large number of mega dams and reservoirs (storage capacity >108 m3) have been, being, or will be constructed in clusters especially in the 21st Century. Limited studies have quantitatively demonstrated the impact of these reservoirs in trapping sediment; the spatial and temporal patterns of the responses of sediment flux and sedimentation have yet to be elucidated. We develop formulas to estimate basin-wide reservoir trapping efficiency typically for mega cascade reservoirs which better reflect the reality than traditional formulas. Results show that the sediment load at Yichang (downstream of the Three Gorges Reservoir or TGR) dropped below 1×108 t, smaller than the amount trapped in reservoirs upstream (1.6×108 t) for the first time in 2003, and it further decreased to 2% of the historical value since 2006. The overturn of sedimentation-flux relation also occurred at Pingshan (outlet of the Jinsha River Basin) at 2014, after the mega dams Xiluodu and Xiangjiaba were put into operation. The total annual sedimentation in UYRB increased from 3×108 t in 2001–2010 to 4.7×108 t in 2011–2015, meaning that 92% of the sediment was trapped; no considerable increase will be made in the future (after 2021) despite new reservoirs still being constructed. After 2016, the Jinsha River Basin became the most important trapper while the most unimportant contributor to the incoming sediment to the TGR where the annual sedimentation was apparently relieved (from 1.4×108 t before 2015 to 0.7×108 t after 2016). Serious channel erosion is expected to happen due to the unprecedented sediment shortage downstream of the cascade reservoirs, especially at Panzhihua, Pinsghan and Yichang.

A Framework for Methodological Options to Assess Climatic and Anthropogenic Influences on Streamflow

Climate change and human activities are having increasing impacts on the global water cycle, particularly on streamflow. Current methods for quantifying these impacts are numerous and have their merits and limitations. There is a lack of a guide to help researchers select one or more appropriate methods for attribution analysis. In this study, hydrological modeling, statistical analysis, and conceptual approaches were used jointly to develop a methodological options framework consisting of three modules, to guide researchers in selecting appropriate methods and assessing climatic and anthropogenic contributions to streamflow changes. To evaluate its effectiveness, a case study in the Upper Yangtze River Basin (UYRB) of China was conducted. The results suggest that the SWAT-based method is the best approach to quantify the influences of climate change and human activities on streamflow in the UYRB. The comprehensive assessment indicates that climate change is the dominant cause of streamflow changes in the UYRB, and the contribution of climate change, indirect human activities, and direct human activities to streamflow changes is about 7:1:2. The proposed framework is efficient and valuable in assisting researchers to find appropriate methods for attribution analysis of streamflow changes, which can help to understand the water cycle in changing environments.

Possible Linkages of Hydrological Variables to Ocean–Atmosphere Signals and Sunspot Activity in the Upstream Yangtze River Basin

Profiling the hydrological response of watershed precipitation and streamflow to large-scale circulation patterns and astronomical factors provides novel information into the scientific management and prediction of regional water resources. Possible contacts of El Niño–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), sunspot activity to precipitation and streamflow in the upper Yangtze River basin (UYRB) were investigated in this work. Monthly precipitation and streamflow were utilized as well as contemporaneous same-scale teleconnections time series spanning a total of 70 years from 1951 to 2020 in precipitation and 121 years from 1900 to 2020 in streamflow. The principal component analysis (PCA) method was applied so as to characterize the dominant variability patterns over UYRB precipitation time series, with the temporal variability of first two modes explaining more than 80% of total variance. Long-term evolutionary pattern and periodic variation characteristics of precipitation and streamflow are explored by applying continuous wavelet transform (CWT), cross-wavelet transform (XWT) and wavelet coherence (WTC), analyzing multi-scale correlation between hydrological variables and teleconnections in the time-frequency domain. The results manifest that ENSO exhibits multiple interannual period resonance with precipitation and streamflow, while correlations are unstable in time and phase. PDO and sunspot effects on precipitation and streamflow at interannual scales vary with time-frequency domains, yet significant differences are exhibited in their effects at interdecadal scales. PDO exhibits a steady negative correlation with streamflow on interdecadal scales of approximately 10 years, while the effect of sunspot on streamflow exhibits extremely steady positive correlation on longer interdecadal scales of approximately 36 years. Analysis reveals that both PDO and sunspot have significantly stronger effects on streamflow variability than precipitation, which might be associated with the high spatiotemporal variability of precipitation.

Heavy Mineral Assemblage Variation in Late Cenozoic Sediments from the Middle Yangtze River Basin: Insights into Basin Sediment Provenance and Evolution of the Three Gorges Valley

The Three Gorges valley is one of the two key capture points of the evolution of the Yangtze River, yet the formation of this valley—from the pre-Miocene to the late Pleistocene—remains uncertain. The Jianghan Basin, a late Mesozoic–Cenozoic basin located just downstream of the Three Gorges valley, is a crucial area for understanding the formation of the valley. In this study, we used heavy mineral assemblages to trace the provenance of Pliocene–Pleistocene sediments obtained from the 300-m-depth Zhoulao drillcore in the Jianghan Basin. Results show that heavy mineral concentrations, compositions, and species display a clear change at a depth of 110 m in the studied core, consistent with the change in values of magnetic indexes and trace-element geochemical indicators. The heavy mineral assemblage deposited below a depth of 110 m (lower section of the core) comprises zircon, epidote, leucoxene, rutile, anatase, pyrite, and titanite, whereas that deposited above 110 m (upper section) consists of hornblende, pyroxene, garnet, hematite-limonite, and magnetite. In addition, the heavy mineral assemblage of the upper section is similar to that of the modern surface fluvial sediments of the Yangtze, which indicates that materials of the upper core section of the Jianghan Basin were sourced from the upper Yangtze River Basin, west of the Three Gorges. After incision of the Three Gorges valley, sediments from the upper Yangtze were transported to the Jianghan Basin and deposited. Combining the results of this study with the known paleomagnetic chronology of the Jianghan Basin, we propose that the Three Gorges valley was incised at ca. 1.1 Ma.

The Development and Application of a GIS-Based Tool to Assess Forest Landscape Restoration Effects on Water Conservation Capacity

In forest landscape restoration, one of the key objectives is to improve the water conservation capacity of the deforested land. A rapid, accurate assessment of the effects of the restoration measures on the water conservation capacity of targeted forests can help forest managers to identify the best practices for forest restoration. However, the traditional assessment tools of forest water conservation function lack a description of forest growth, and are featured by complex computation, which fails to evaluate the effects of forest restoration on the regional forest water conservation capacity in an efficient way. To address this issue, through combining the forest restoration evaluation model (equivalent recovery area, ERA), classic forest water storage capacity estimation (total water storage capacity), this study has taken advantage of ENVI/IDL, ArcGIS Engine/C#.Net to develop the Forest and Water Assessment Tool (FWAT) for assessing the changes of the regional forest landscape and the associated forest water conservation capacity in various forest restoration scenarios. This tool has been successfully applied in the Upper Zagunao watershed, a large forested watershed in the Upper Yangtze River basin. According to the assessment, the forest water conservation capacity of the study watershed consistently increased from about 1580.76 t/hm2 in 2010 to a projected 2014.34 t/hm2 by natural restoration, and 2124.18 t/hm2 by artificial restoration by 2030. The artificial restoration measures yield a better effect on forest water conservation function than natural restoration. By 2030, the forest water conservation capacity of artificial restoration scenario is expected to be about 7% higher than that of natural restoration scenario. The FWAT as an efficient tool to assess the effects of forest restoration measures on regional forest water conservation capacity can provide scientific support for the design of forest restoration and management strategies worldwide.

A novel impoundment framework for a mega reservoir system in the upper Yangtze River basin

The joint and optimal impoundment operation of cascade reservoirs can dramatically boost the efficiency of water resource utilization. However, most existing techniques fail to conquer the curse of dimensionality in mega multi-objective reservoir system. To overcome this obstacle, this study proposes a novel framework that integrates aggregation-decomposition (AGDP), parameterization simulation optimization (PSO), and the parallel progressive optimization algorithm (PPOA). In detail, it involves three main steps: (1) reservoir grouping and application of AGDP in the same group; (2) derivation of the initial impoundment solution by using the non-dominated sorting genetic algorithm-II to solve the PSO model; and (3) further improvement of the impoundment policy via PPOA. The proposed framework is tested on a mega reservoir system in the upper Yangtze River basin. Results demonstrate that our hybrid method can generate a series of impoundment policies to adapt to different flood event scenarios. Compared to the conventional operating rule, the optimal policy can increase impoundment efficiency from 89.50% to 94.21%, increase hydropower generation by 6.63 billion kWh/year (3.26%) and reduce CO2 emissions by 5.21 billion kg/year while maintaining the flood control risk at a low level. These findings verify the applicability and effectiveness of the novel framework in high-dimensional multi-objective impoundment, and also highlight the substantial potential benefits of sustainable water resources.

Variation of runoff and sediment inflows to the Three Gorges Reservoir: Impact of upstream cascade reservoirs

The construction of cascade reservoirs upstream of the Three Gorges Reservoir (TGR) has been greatly intensified in the 21st century. While research attention is dedicated to the downstream effects of the TGR, few studies have been focused on the radical change of upstream inflows to the TGR. In this study, we reveal the impact of upstream cascade reservoirs on the annual runoff and sediment load inflows to the TGR based on the data in 1956–2016. The measured sediment load at control hydrological stations shows a drastic decrease (59%) while the runoff only had a slight decrease (6%). The similarity of these characteristics for that measured at Yichang indicates the leading effects of TGR inflows on its outflow. We develop a macroscopic quantification method utilizing double mass curves to separate the contribution from climate effects and human activities. Results show that precipitation (climate factors) dominated the runoff change, while human activities were the main driving factors for sediment load reduction in the studied area. In the four sub-basins upstream of the TGR, the sediment retention induced by large reservoirs was the major reason for sediment load reduction, especially in the Jinsha River Basin and the Jialing River Basin, which contributed 70% (or 347 Mt/yr) of the total sediment retention in the whole upper Yangtze River Basin (YRB) since 2015, much larger than that of the TGR (19% or 94 Mt/yr). As the total storage capacity of the cascade reservoirs increased to more than twice the TGR’s capacity in the 2010s, the regression relationship between runoff and sediment load broke down. The unprecedented situation in the upper YRB may have profound impacts on the morphology and ecology of the TGR as well as the downstream Yangtze River including its delta.

Tracking the Atmospheric–Terrestrial Water Cycle over the Tibetan Plateau Based on ERA5 and GRACE

AbstractIn this study, the net water flux (precipitation minus evaporation) over the Tibetan Plateau (TP) and its 12 drainage basins is estimated using ERA5. The terrestrial branch of the water cycle is investigated using the total water storage anomalies (TWSAs) derived from GRACE (Gravity Recovery and Climate Experiment) data and daily streamflow records collected in Zhimenda and Tangnaihai (two hydrological stations located in the upper Yangtze River Basin and upper Yellow River Basin). This work provides a preliminary assessment of discrepancies between model-derived and space-based observations in the atmospheric–terrestrial water cycle over the TP and its drainage basins. The results show that the net water fluxes occurring over the TP and the scale of its drainage basins are closely tied to local dynamics and physical processes and to large-scale circulation and atmospheric water vapor. ERA5 maintains the atmospheric water balance over the TP. ERA5-derived net water flux anomalies constitute a major component of the water cycle and correspond to GRACE-derived TWSAs. The water budget–based approach with the ERA5 and ITSG-Grace2018 datasets constrains the atmospheric–terrestrial water cycle over the TP and its drainage basins. Both the ERA5- and GRACE-derived estimates contain consistent long- and short-term variations over the TP. Discrepancies are evident at the drainage basin, while the ratio of signal to noise in both the ERA5 and GRACE datasets might cause discrepancies between estimates over relatively small or arid basins. Nevertheless, the observed good correspondence between ERA5- and GRACE-derived atmospheric–terrestrial water cycles over the TP highlights the potential value of the rational application of water resource information.

Detection and attribution of reference evapotranspiration change (1951–2020) in the Upper Yangtze River Basin of China

AbstractReference evapotranspiration (ET0) indicates atmospheric evaporating capability over a hypothetical reference surface. ET0 is an important hydrological and meteorological variable to reflect climate change. This is particularly true for the Upper Yangtze River Basin (UYRB), which is vulnerable and sensitive to changing environment. This study aims to provide a newer and longer description of ET0 change and the causes at basin and subbasin scales in the UYRB. Based on the observed data from 1951 to 2020, ET0 in the entire UYRB and subbasins is estimated using the Penman–Montieth method with local calibration. The spatial–temporal characteristics in ET0 change are identified from time-series analysis. Our results show that ET0 increases significantly by 3.3 mm/year in the entire UYRB. Stations with significant increases in annual ET0 are concentrated in the central part of the UYRB, where the mean annual ET0 is low. We further propose an improved method to assess the causes of ET0 change. Our results suggest that the relative humidity decrease has the most dominant effect, causing 4.69 mm/year of ET0 increase. Temperature increase tends to cause 1.26 mm/year of ET0 increase. Sunshine duration decrease and wind speed decrease contribute to 1.96 and 0.48 mm/year of ET0 decrease.

Urbanization Significantly Affects Pan-Evaporation Trends in Large River Basins of China Mainland

The causes of the pan-evaporation decline have been debated, and few researches have been carried out on the possible effect of local land use and land cover change on the regional pan-observation data series. In this paper, the urbanization effect on the estimate of pan-evaporation trends over 1961–2017 was examined for the data series of 331 urban stations, applying a previously developed dataset of the reference stations, in seven large river basins of the China mainland. The trends of pan-evaporation difference series (transformed to anomaly percentage) between urban stations and reference stations were negative and statistically significant in all of the basins, indicating that urbanization significantly reduced the pan-evaporation. The urbanization-induced trend in the whole study region was −2.54%/decade for the urban stations. Except for the Yellow River Basin and the upper Yangtze River Basin, the urbanization effects in the other five large river basins of the country are all significant, with the mid and low reaches of the Yangtze River and the Songhua River registering the largest urbanization effects of −4.08%/decade and −4.06%/decade, respectively. Since the trends of regional average series for reference stations across half of the river basins are not statistically significant, the urbanization effect is a dominant factor for the observed decline in pan-evaporation. This finding would deepen our understanding of the regional and basin-wide change in pan-evaporation observed over the last decades.