Vegetation plays an important role in regulating terrestrial water and energy fluxes. Recent research reported that China is the largest contributor to the global net increase in green foliage. Such ...large‐scale vegetation greenness change would inevitably affect the water budget across China. In this study, we evaluated the effects of vegetation greenness change on evapotranspiration (ET) and water yield in China from 1982 to 2014 with a process‐based conceptual hydrologic model and a detrending experiment of vegetation index. On the national scale, the change in vegetation greenness was dominated by an increasing trend (greening), which has led to an increase in ET and a decline in water yield from 1982 to 2014. The contribution of vegetation greenness change to the trend of annual ET and water yield is 0.25 mm/yr and −0.16 mm/yr, respectively. Spatially, the relative change in ET and runoff caused by vegetation greenness change is significantly greater in water‐limited areas than in energy‐limited areas. Vegetation greening exacerbated water scarcity in water‐limited areas. Therefore, the ecological restoration policy should fully consider the negative effect of vegetation greening on water yield in water‐limited regions and balance the conflicting water demands between the ecosystem and human beings.
Key Points
We evaluated the effects of vegetation greenness change on annual ET and water yield across China from 1982 to 2014
On the national scale, the change in vegetation greenness has led to an increase in ET and a decline in water yield
The water‐reducing effect caused by vegetation greening is more pronounced in water‐limited areas than in energy‐limited areas
The performance of a nonlinear formulation of the complementary principle for evaporation estimation was investigated in 241 catchments with different climate conditions in the eastern monsoon region ...of China. Evaporation (
Ea) calculated by the water balance equation was used as the reference.
Ea estimated by the calibrated nonlinear formulation was generally in good agreement with the water balance results, especially in relatively dry catchments. The single parameter in the nonlinear formulation, namely
αe as a weak analog of the alpha parameter of Priestley and Taylor (), tended to exhibit larger values in warmer and humid near‐coastal areas, but smaller values in colder, drier environments inland, with a significant dependency on the aridity index (AI). The nonlinear formulation combined with the equation relating the one parameter and AI provides a promising method to estimate regional
Ea with standard and routinely measured meteorological data.
Key Points
Ea estimated by the calibrated nonlinear formulation was in good agreement with the water balance results
The single parameter in the nonlinear formulation was shown to depend significantly on the aridity index (AI)
The nonlinear formulation combined with the equation relating the one parameter and AI provides a promising method to estimate Ea
•Pre-whitening cannot really improve trend identification when using the MK test.•Series’ trend magnitudes greatly influence trend identification of series.•EMD method can be an effective alternative ...for trend identification of series.•EMD method can identify the specific shape of the analyzed series’ trend.
Trend identification is an important issue in hydrological time series analysis, but it is also a difficult task due to the diverse performances of methods. This paper mainly investigated the performances between the Mann–Kendall (MK) test and the empirical mode decomposition (EMD) method for trend identification of series. Analyses of both synthetic and observed series indicate the better performance of EMD compared with the other. The results show that pre-whitening cannot really improve trend identification when using the MK test, but cause wrong results sometimes. It can be due to the good correlation of trend, so pre-whitening would weaken trend’s magnitude. If the trend of the analyzed series has small magnitude, it cannot be accurately identified by the MK test, because the trend would be submerged too severely by other components of series to accurately identify trend. When the analyzed series has short length, its trend cannot be accurately identified by the MK test. However, the EMD method can eliminate the influences of trends’ magnitude and series’ length, so it has more effective power for trend identification. As a result, it is suggested that series’ trend can be directly identified by the MK test but need not do pre-whitening; moreover, the influences of trends’ magnitude should be carefully considered for trend identification. Comparatively, the EMD method can adaptively determine the specific shape of the nonlinear and non-stationary trend of series by considering statistical significance, so it can be an effective alternative for trend identification of hydrological time series.
•A multi-objective calibration (MOC) was used to calibrate hydrological models.•The MOC was constructed using GRACE total water storage change (TWSC) and observed runoff.•The MOC improved model ...performance compared with the calibration scheme using only runoff.•The simulation improvement was more significant in dry catchments than in wet catchments.•Hydrologic model calibrated using GRACE data alone cannot obtain accurate runoff.
Hydrological model parameters are typically calibrated by observed streamflow data. This calibration strategy is questioned when the simulated hydrological variables of interest are not limited to streamflow. Well-performed streamflow simulations do not guarantee the reliable reproduction of other hydrological variables. One of the reasons is that hydrological model parameters are not reasonably identified. The Gravity Recovery and Climate Experiment (GRACE)-derived total water storage change (TWSC) data provide an opportunity to constrain hydrological model parameterizations in combination with streamflow observations. In this study, a multi-objective calibration scheme based on GRACE-derived TWSC and streamflow observations was compared with the traditional single-objective calibration scheme based on only streamflow simulations. Two hydrological models were employed on 22 catchments in China with different climatic conditions. The model evaluations were performed using observed streamflows, GRACE-derived TWSC, and actual evapotranspiration (ET) estimates from flux towers and from the water balance approach. Results showed that the multi-objective calibration scheme provided more reliable TWSC and ET simulations without significant deterioration in the accuracy of streamflow simulations than the single-objective calibration. The improvement in TWSC and ET simulations was more significant in relatively dry catchments than in relatively wet catchments. In addition, hydrological models calibrated using GRACE-derived TWSC data alone cannot obtain accurate runoff simulations in ungauged catchments. This study highlights the importance of including additional constraints in addition to streamflow observations to improve performances of hydrological models.
The headwater catchments of the Yellow River Basin are of great importance for the whole basin in terms of water resources, and streamflow from these catchments has decreased in the last decades. The ...concept of climate elasticity was used to assess the impacts of climate and land surface change on the streamflow. Results show that for the period 1960-2000 the elasticity of streamflow in relation to precipitation and potential evapotranspiration are 2.10 and -1.04, respectively, indicating that streamflow is more sensitive to precipitation than to potential evapotranspiration. However, land use change played a more important role than climate in reducing streamflow in the 1990s. It is estimated that land use change is responsible for more than 70% of the streamflow reduction in the 1990s, while climate change contributed to less than 30% of the reduction. The precipitation elasticity appears to have an inverse relationship with the runoff coefficient but a positive relationship with the aridity index, showing that the drier the catchment, the more sensitive the streamflow with respect to precipitation change.
•First comprehensive study on climate change impact on runoff in Brahmaputra upstream.•It is suitable to use rainfall–runoff modeling for climate change impact study in Tibetan Plateau.•Mean annual ...future precipitation and runoff are projected to increase by most of GCMs.•The middle region shows considerable runoff increase especially in wet season.
The Tibetan Plateau (TP) is the “water tower of Asia” and strongly influences both the hydrology and climate of southern and eastern Asia. Exploring the impact of climate change on the runoff of TP rivers is critical to improve water resources management. However, thorough studies on the runoff response to climate change are seldom conducted on large TP river systems. To complement the current body of work, this study uses two rainfall–runoff models (SIMHYD and GR4J) to simulate the monthly and annual runoff across the Yarlung Tsangpo River (YTR) basin in the southeastern TP (i.e., upstream of the Brahmaputra) under historical (1962–2002) and future (up to approximately 2030) climate conditions. The future climate series are obtained by using 20 Global Climate Models (GCMs) outputs from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) to reflect a 1°C increase in the global average surface air temperature. The two rainfall–runoff models successfully simulate the historical runoff for the eight catchments in the YTR basins with median monthly runoff Nash–Sutcliffe Efficiencies (NSE) of 0.86 for SIMHYD and 0.83 for GR4J. The mean annual future precipitation and runoff across the region are projected to increase by most of the modeling results. The mean annual precipitation changes obtained from the 20 GCMs are −15%, 7% and 16% for the 10th percentile, median and 90th percentile of GCM outputs, respectively, and the corresponding changes in the simulated mean annual runoffs are −24%, 13% and 29% for the SIMHYD model outputs and −22%, 11% and 26% for the GR4J model outputs. The projected increase in the runoff at the median percentile mainly occurs in the middle reaches of the YTR and its two tributaries, the Lhasa River and the Nyangqu River, with a 12% increase in annual runoff that mainly occurs in the wet season from May to September. The present work is the first comprehensive study on the hydrological response to climate change covering the entire upstream area of the Brahmaputra, and the results found in this study are not only helpful for local water resource management but also for the lower reaches of the Brahmaputra.
Abstract
Despite the increasing Siberian river discharge, the sensitivity of streamflow to climate forcing/permafrost thawing is poorly quantified. Based on the Budyko framework and superposition ...principles, we detected and attributed the changes in streamflow regimes for the three great Siberian rivers (Ob, Yenisei, and Lena) during 1936–2019. Over the past 84 years, streamflow of Ob, Yenisei and Lena has increased by ∼7.7%, 7.4% and 22.0%, respectively. Intensified precipitation induced by a warming climate is a major contributor to increased annual streamflow. However, winter streamflow appears to be particularly sensitive to temperature. Whilst rising temperature can reduce streamflow via evapotranspiration, it can enhance groundwater discharge to rivers due to permafrost thawing. Currently, every 1 °C rise in temperature likely leads to 6.1%–10.5% increase in groundwater discharge, depending on the permafrost condition. For permafrost-developed basins, the contribution to increased streamflow from thawing permafrost will continue to increase in the context of global warming.
The concentrations of heavy metals (mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), copper (Cu) and arsenic (As)) in surface water and sediments were investigated in two adjacent drinking water ...reservoirs (Hongfeng and Baihua Reservoirs) on the Yunnan-Guizhou Plateau in Southwest China. Possible pollution sources were identified by spatial and statistical analyses. For both reservoirs, Cd was most likely from industrial activities, and As was from lithogenic sources. For the Hongfeng Reservoir, Pb, Cr and Cu might have originated from mixed sources (traffic pollution and residual effect of former industrial practices), and the sources of Hg included the inflows, which were different for the North (industrial activities) and South (lithogenic origin) Lakes, and atmospheric deposition resulting from coal combustion. For the Baihua Reservoir, the Hg, Cr and Cu were primarily derived from industrial activities, and the Pb originated from traffic pollution. The Hg in the Baihua Reservoir might also have been associated with coal combustion pollution. An analysis of ecological risk using sediment quality guidelines showed that there were moderate toxicological risks for sediment-dwelling organisms in both reservoirs, mainly from Hg and Cr. Ecological risk analysis using the Hakanson index suggested that there was a potential moderate to very high ecological risk to humans from fish in both reservoirs, mainly because of elevated levels of Hg and Cd. The upstream Hongfeng Reservoir acts as a buffer, but remains an important source of Cd, Cu and Pb and a moderately important source of Cr, for the downstream Baihua Reservoir. This study provides a replicable method for assessing aquatic ecosystem health in adjacent plateau reservoirs.
► Recharge source to the Ejina Basin is surface water and desert groundwater. ► River infiltration and desert front recharge are the main recharge mechanisms. ► Desert recharge rate to the Ejina ...Basin has significantly decreased.
Groundwater plays a dominant role in the eco-environmental protection of arid/semi-arid areas. Understanding sources and mechanisms of groundwater recharge in the Ejina Basin, an arid inland river basin in northwest China, is important for water resource planning in this ecologically sensitive area. In this study, 90 water samples were collected from rainfall, rivers and lakes, and springs and pumping wells in 2009. Analysis of the aquifer system and hydrological conditions, together with hydrogeochemical and isotope techniques were used to investigate groundwater sources and their associated recharge processes. Our results show that shallow phreatic and deep confined groundwater differ greatly in their compositions, with a distinct spatial heterogeneity of phreatic groundwater TDS (from 365mg/L to 5833mg/L), which increase along the shallow groundwater flow paths. Groundwater chemical evolution is mainly controlled by rock dominance and the evaporation-crystallization process, and the dominant anion species change systematically from HCO3 to SO4 to Cl, and the dissolved ions within the groundwater system from Na- and K-rich minerals and sulfate phases also contribute significantly to the groundwater composition. The stable isotope levels (δ18O and δ2H) of the surface water and the shallow phreatic groundwater confirm that the Heihe River and Badain Jaran Desert groundwater are the main sources recharging the phreatic aquifer in the Ejina Basin. Thus, river infiltration and desert front recharge should be considered as the two main recharge mechanisms of the Ejina aquifer. However, recharge from the Badain Jaran Desert aquifer to the Ejina Basin has occurred at a lower rate due to aridification since the middle Holocene. For this reason, the sustainable improvement of the ecological environment should be based on the shallow groundwater recharge of the phreatic aquifer in the Ejina Delta, which mainly takes place via seepage through the riverbed and direct infiltration during periods of environmental flow control.
Delineating the boundaries of numerous watersheds is essential for hydrological analysis in the big data era. Various techniques, such as ArcGIS, have been developed to delineate watershed ...boundaries. However, existing techniques suffer from two laborious manual processes: (a) Determine flow accumulation thresholds through trial and error to derive digital stream networks; (b) Manually relocate watershed outlets to the correct stream networks. Here, we proposed an automatic outlet relocation (AOR) algorithm for rapid watershed delineation without manual processing. The AOR algorithm relocates outlets to correct stream networks by analyzing flow accumulation gradients along the flow directions. An experiment using the AOR algorithm in 1,398 watersheds across the contiguous United States showed that 94.1% of the watersheds were correctly delineated, compared to 81.3% accuracy of the ArcGIS relocation algorithm (Snap Pour Point). In addition, the AOR algorithm took 58 min to delineate the 1,398 watersheds on a 90 m‐resolution flow direction, while the ArcGIS algorithm required 586 min to perform the same task. Thus, the AOR algorithm offers an opportunity to automatically delineate a large number of watersheds for hydrological simulations in the big data era. Software with a graphical user interface for the AOR algorithm is freely available on GitHub (https://github.com/xiejx5/watershed_delineation).
Key Points
An automatic outlet relocation (AOR) algorithm was developed to circumvent the manual processes in ArcGIS watershed delineation
An experiment in 1,398 watersheds showed AOR had 94.1% accuracy, compared to ArcGIS’s 81.3% accuracy
AOR provides a tool to automatically delineate a large number of watersheds for hydrological simulations in the big data era
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