Abstract
This study assessed the potential impact of climate change on spatiotemporal distribution of blue and green water resources in the Beijiang River basin, southern China, using the SWAT hydrological model and CORDEX-EAS regional climate models (RCMs). The outputs of three RCMs (namely RMOH, RMPI and RNCC) under two emission scenarios (RCP 2.6 and RCP 8.5) were bias-corrected by using the quantile delta mapping method for the control (1975–2004), near future (2021–2050) and far future (2061–2090) periods. Driven by the corrected climate variables, future blue and green water were assessed by using the SWAT model calibrated with streamflow data. The results indicated that the green water flow in the northwest basin predicted by RMOH under RCP 2.6 would increase up to about 2.7% in 2061–2090; RNCC suggests that most of the basin would experience the most significant reduction in green water storage (over 30%) under RCP 8.5 in the far future. For blue water, the ensemble mean of three RCMs indicates a slight decreasing trend in the northern part of the basin and an increasing one in the southern part under RCP 8.5 in the far future. Our findings could help watershed managers evaluate future hydrological risks and design appropriate adaptation strategies.
HIGHLIGHTS
Impact of climate change on blue and green water resources is assessed.
The Beijiang River basin, southern China, is used as a study case.
SWAT model and CORDEX-EAS regional climate models are used.
The study is helpful for water resources management under climate change.
Graphical Abstract
INTRODUCTION
Global climate change caused by the continuous rise of anthropogenic greenhouse gas emissions has had a notable impact on the hydrological cycle, subsequently affecting the regional available water resources (Hoegh-Guldberg et al. 2019; Padhiary et al. 2020). In some regions, such impacts may lead to ecological degradation and water-scarcity crisis (Zuo et al. 2015; Afshar et al. 2018). Presently, about one-third of the world's population dwells in water-scarce regions, and their lifestyles, distribution and numbers have to be changed to adapt to the water storage due to climate change (Chakilu et al. 2020). By 2035, the proportion of water used for energy and food will increase significantly and this will put tremendous pressure on the entire human civilization in coping with such a difficult situation (Veettil & Mishra 2016). Thus, quantification of water resources in a changing environment is important to provide guidance for adaptation strategies to achieve sustainable and efficient water use.
Previously, a large number of water resources evaluation works have explored the climate-related influences on available water resources presented as streamflow (Arnell et al. 2003; Islam et al. 2005, 2007; Gosling et al. 2011), lake/reservoir water storage (Sahoo et al. 2013) and groundwater recharge (Loáiciga et al. 2000). Falkenmark & Rockström (2006) opened a new angle in water management by dividing water resources into green and blue components and formalized their definitions. Green water denotes the non-runoff portion from precipitation which is deposited impermanently in the soil media and finally goes back to the atmosphere by evapotranspiration. Blue water is the part of water collected in surface or underground water bodies, like lakes, rivers and aquifers, which is utilizable directly by industry, agriculture and municipalities (Falkenmark & Rockström 2006). Obviously, the conventional water resources evaluation focusing on available water resources only considers the blue water but ignores green water. In fact, green water is also essential as it occupies over 80% of the total water resources in some arid regions and it supports the growth of crop plants and the healthy development of grassland and forest ecosystems (Schuol et al. 2008b; Li et al. 2018). Compared with the traditional evaluation method, the blue/green water concept-based climate change-related assessment is more reasonable for supporting sustainable and balanced water resources management (Rockström et al. 2009; Badou et al. 2018; Pandey et al. 2019; Cooper et al. 2022).
In general, a typical approach to appreciate future spatiotemporal changes of basin-scale blue and green water is to utilize future climate data to drive hydrological models. The Soil and Water Assessment Tool (SWAT) is a widely used sophisticated hydrological model to provide quantitative outputs of necessary water cycle components to enable computation of blue and green water distributions (Yuan et al. 2019a). Global circulation models (GCMs) have traditionally been utilized as the primary means for climate projections under a variety of greenhouse gas emission scenarios over long time series (Pandey et al. 2017). These global-scale climate projections are available from the Intergovernmental Panel on Climate Change (IPCC)'s Coupled Model Intercomparison Project (CMIP) with some experiment designs like concentration pathways (RCPs) (Taylor et al. 2012). Yuan et al. (2019b) used SWAT and five CMIP5 GCMs under the RCP 4.5 scenario to investigate the variations of blue and green water flow/storage in the Yangtze River source region, China, for the period of 2021–2050. Farsani et al. (2019) assessed the spatiotemporal alterations of green and blue water amounts in the Bazoft watershed, Iran, by using SWAT and 18 emission scenarios from both CMIP3 and CMIP5 GCMs. Naderi (2020) used 22 CMIP5 GCMs to run SWAT for quantifying blue and green water amounts for the Dorudzan Dam watershed, Iran, considering climatic change influences.
It is noted that most of the above-mentioned studies have used climate data from multi-GCM ensembles to run the SWAT hydrological model to examine the climate impacts on blue and green water for a specific basin of interest. However, the recently popular CMIP5 GCMs are usually coarse (∼100–300 km), and not suitable for capturing the detailed features of the basin-scale hydrological processes. Regional climate models (RCMs) are more advantageous because they produce climate variables that are more physically consistent and have higher spatial resolutions (Rummukainen 2016). The World Climate Research Programme (WCRP) Coordinated Regional Downscaling Experiment (CORDEX) initiative was launched to offer synchronized groups of regional downscaled projections for the whole world. As part of the CORDEX framework, the CORDEX-East Asia (CORDEX-EAS) initiative produces ensemble regional climate projections for East Asia, covering the study case in South China, at horizontal resolutions of around 50 km (EAS-44) and 25 km (EAS-22), based on dynamical and statistical downscaling models forced by CMIP5 GCMs. Gu et al. (2018b) evaluated regional climate simulations of five CORDEX-EAS RCMs and found that these RCMs could well depict the major climatological variabilities of temperature and precipitation over China. Currently, some studies have emerged to use the CORDEX-EAS RCMs for the application of climate change impact on extreme flow over the Yangtze River Basin Gu et al. (2018a), operational performance of reservoir network over the Poyang Lake Basin (Dong et al. 2019), and water and energy security for the Three Gorges Reservoir (Hu et al. 2020). However, research works that focus on the impact of climate change on blue and green water at a large watershed scale using high-resolution CORDEX-EAS RCM projections are still scarce.
Thus, this study aims to use CORDEX-EAS RCM projections to drive the SWAT model to evaluate future spatiotemporal alterations of blue and green water resources over the Beijiang River basin, southern China. Firstly, we calibrate the SWAT model using the observed discharge station in the lower reaches of the Beijiang River basin, and calculate the areal information of blue and green water using the hydrological outputs of SWAT at each sub-basin. Then, three CORDEX-EAS RCMs are bias-corrected to produce future climate scenarios that can be incorporated in the hydrological simulations. Lastly, the spatiotemporal changes of the blue and green water arising from climate change in various stages of the 21st century are examined.
MATERIALS AND METHODS
Study area
Beijiang River basin map and locations of hydrometeorological stations.
Blue and green water estimated by the hydrological model
In this work, SWAT is adopted to describe the hydrological processes and calculate the basin-scale blue and green water quantities. The model is physically based and one of the most commonly used hydrological models that can simulate the future hydro-climate changes on water resources at a regional scale (Faramarzi et al. 2015; Dai & Qin 2019; Zhang et al. 2021). Details about this model can be found in Arnold et al. (1998). The data needed to build SWAT consist of digital elevation model (DEM), land-use and soil maps, climate and streamflow data. A 90-m resolution DEM with data range of [0, 1,871] m (Figure 1) is collected from the NASA Shuttle Radar Topographic Mission (SRTM) project (http://srtm.csi.cgiar.org). The land-use and soil map were obtained from databases of USGS Global Land Cover Characterization (GLCC; Brown et al. 1999) and Food and Agriculture Organization of the United Nations (FAO; Batjes 1997), respectively. The land use of the Beijiang River consisted of six main types, namely water, grassland, shrubland, forest, cropland and construction land (see Supplementary Figure S1). The two-layer soil physical property for the local SWAT soil database was estimated by the soil-plant-air-water (SPAW) tool (https://www.ars.usda.gov). The climatic data at daily scale (i.e. precipitation, air temperature, solar radiation, relative humidity and wind speed) for the period of 1975–2017 from seven stations (Figure 1 and Supplementary Table S1) were gathered from China Meteorological Data Service Center (http://data.cma.cn). The monthly streamflow data observed at the Shijiao station in the downstream of Beijiang River (Figure 1) were collected from Global Runoff Data Centre (GRDC; https://www.bafg.de) for 1981–1986 and the publications of Pan et al. (2017) and Li (2015) for 1987–2000 and 2001–2010, respectively.





SWAT parameters after calibration
Parametera . | Calibration typeb . | Initial range . | Best calibrated value . | |
---|---|---|---|---|
Min . | Max . | |||
CN2 | r | −0.2 | 0.5 | 0.237 |
SOL_AWC | r | −0.5 | 0.2 | −0.00125 |
ESCO | v | 0 | 1 | 0.172 |
EPCO | v | 0 | 1 | 0.0435 |
SURLAG | v | 0.1 | 15 | 6.32 |
OV_N | r | −0.2 | 0 | −0.0301 |
SOL_K | r | −0.8 | 0.8 | −0.607 |
SLSUBBSN | r | 0 | 0.2 | 0.127 |
ALPHA_BF | v | 0 | 1 | 0.399 |
GW_DELAY | v | 30 | 450 | 31.5 |
GWQMN | a | −1,000 | 1,000 | −85.0 |
GW_REVAP | a | 0 | 0.2 | 0.0227 |
REVAPMN | a | 1 | 10 | 1.28 |
CH_N2 | v | 0 | 0.3 | 0.00195 |
CH_K2 | v | 50 | 130 | 108 |
RCHRG_DP | a | −0.05 | 0.05 | 0.0327 |
ALPHA_BNK | v | 0 | 1 | 0.182 |
CANMX | v | 0 | 100 | 4.15 |
CH_N1 | v | 0 | 1 | 0.734 |
Parametera . | Calibration typeb . | Initial range . | Best calibrated value . | |
---|---|---|---|---|
Min . | Max . | |||
CN2 | r | −0.2 | 0.5 | 0.237 |
SOL_AWC | r | −0.5 | 0.2 | −0.00125 |
ESCO | v | 0 | 1 | 0.172 |
EPCO | v | 0 | 1 | 0.0435 |
SURLAG | v | 0.1 | 15 | 6.32 |
OV_N | r | −0.2 | 0 | −0.0301 |
SOL_K | r | −0.8 | 0.8 | −0.607 |
SLSUBBSN | r | 0 | 0.2 | 0.127 |
ALPHA_BF | v | 0 | 1 | 0.399 |
GW_DELAY | v | 30 | 450 | 31.5 |
GWQMN | a | −1,000 | 1,000 | −85.0 |
GW_REVAP | a | 0 | 0.2 | 0.0227 |
REVAPMN | a | 1 | 10 | 1.28 |
CH_N2 | v | 0 | 0.3 | 0.00195 |
CH_K2 | v | 50 | 130 | 108 |
RCHRG_DP | a | −0.05 | 0.05 | 0.0327 |
ALPHA_BNK | v | 0 | 1 | 0.182 |
CANMX | v | 0 | 100 | 4.15 |
CH_N1 | v | 0 | 1 | 0.734 |
Note:aThe detailed description and explanation of parameters can be referred to in Faramarzi et al. (2015).
bThe symbols of a, v and r denote an absolute increase, a replacement and a relative change to the default parameter value, respectively.
After model validation, the HRU outputs of SWAT (i.e. WYLD, GW_RCHG, GW_Q, ET and SW) can provide necessary data to compute blue and green water amounts at the sub-basins (Rodrigues et al. 2014). The schematic diagram of blue and green water components in the SWAT model can be referred to in Supplementary Material (Zhao et al. 2016). The blue water is calculated by summating the water yield (WYLD) and groundwater storage. The former refers to the amount contributed from HRU through the main channel and the latter denotes the net amount between aquifer water recharge (GW_RCHG) and groundwater input to the channel (GW_Q) (Veettil & Mishra 2016). Green water could be split into two components, with one being the green water flow as equivalent to the actual evapotranspiration (ET) from the HRU, and the other one being the green water storage estimated as the soil water content (SW) (Schuol et al. 2008a).
Climate change scenarios and bias correction
In this study, the RCMs accustomed to drive the calibrated SWAT model are based on the CORDEX-EAS outputs downscaled from three CMIP5 GCMs. We choose the latest version of RCM originally developed by the Max Planck Institute for Meteorology and nowadays maintained by the Climate Service Center Germany (REMO2015) (Jacob 2001; Pietikäinen et al. 2018). The CORDEX-EAS domain REMO2015 simulation has a spatial resolution of 25 km×25 km and covers the historical range from 1951 to 2005 and the future one from 2006 to 2100. The ensemble consists of one REMO2015 RCM driven by three different GCMs, i.e. MOHC-HadGEM2-ES GCM (RMOH), MPI-M-MPI-ESM-LR (RMPI) and NCC-NorESM1-M (RNCC) (as shown in Table 2). The three RCM simulations offer climate variables required by SWAT and can be downloaded from the CORDEX-SEA website (http://www.cordex.org/domains/region-7-east-asia) at a daily time scale. We consider emission scenarios of RCP 2.6 and RCP 8.5 and use the RCMs simulations for control period (1975–2004) and projections for two future windows (near: 2021–2050; far: 2061–2090).
List of RCMs used in this study
Acronyma . | RCM . | Driving GCM . |
---|---|---|
RMOH | REMO2015b | MOHC-HadGEM2-ES from Met Office Hadley Centrec |
RMPI | REMO2015 | MPI-M-MPI-ESM-LR from Max Planck Instituted |
RNCC | REMO2015 | NCC-NorESM1-M from Norwegian Climate Centree |
Acronyma . | RCM . | Driving GCM . |
---|---|---|
RMOH | REMO2015b | MOHC-HadGEM2-ES from Met Office Hadley Centrec |
RMPI | REMO2015 | MPI-M-MPI-ESM-LR from Max Planck Instituted |
RNCC | REMO2015 | NCC-NorESM1-M from Norwegian Climate Centree |
aRMOH, RMPI and RNCC denote that the RCM REMO2015 is driven by MOHC-HadGEM2-ES, MPI-M-MPI-ESM-LR and NCC-NorESM1-M GCM, respectively.
bREMO2015 RCM was originally developed at the Max Planck Institute for Meteorology, and nowadays further developed and maintained by German Institute for Climate Services (Jacob 2001).
cThe details on MOHC-HadGEM2-ES can be found in https://www.metoffice.gov.uk/weather/climate/met-office-hadley-centre/index.
dThe details on MPI-M-MPI-ESM-LR can be found in https://www.mpg.de/institutes.
eThe details on NCC-NorESM1-M can be found in https://portal.enes.org/models/earthsystem-models/ncc-1.
Although an RCM can downscale climate variables to finer resolution, its outputs also suffer from considerable systematic biases compared with observation (Ngai et al. 2017). We, therefore, adopt the quantile delta mapping (QDM) method to eliminate these systematic biases before using the outputs of RCMs for further analysis. Teutschbein & Seibert (2012) reviewed a few commonly used bias-correction methods and concluded that the QDM method achieved the best correction result. The QDM method is conducted to adjust the biases in the cumulative distribution function (CDF) of RCM outputs in connection with the observed distribution (Piani et al. 2010).
















RESULTS
SWAT model calibration and validation
Comparison between the observed and simulated monthly streamflow for (a) the calibration period from 1981 to 1998 and (b) the validation period from 1999 to 2010.
Comparison between the observed and simulated monthly streamflow for (a) the calibration period from 1981 to 1998 and (b) the validation period from 1999 to 2010.
Bias correction of climate variables
Comparison of the CDFs of observed and RCM-simulated climate variables, including (a) precipitation (Pr), (b) maximum temperature (Tmax), (c) minimum temperature (Tmin), (d) relative humidity (RH), (e) solar radiation (SR) and (f) wind speed (WS), at the C57972 climate station for the month of January.
Comparison of the CDFs of observed and RCM-simulated climate variables, including (a) precipitation (Pr), (b) maximum temperature (Tmax), (c) minimum temperature (Tmin), (d) relative humidity (RH), (e) solar radiation (SR) and (f) wind speed (WS), at the C57972 climate station for the month of January.
Availability of blue and green water during the historical period
The total annual average water resources over the Beijiang River basin from 1975 to 2004 is 1,741.46 mm, among which 33.3% (579.52 mm) is green water flow, 2.1% (36.23 mm) is green water storage, and 64.6% (1,125.71 mm) is blue water (Supplementary Table S3). Obviously, the basin was dominated by blue water in the historical period, which is about twice as much as green water. Moreover, the water balance coefficient of this basin was close to 1 for each year from 1975 to 2004 (Supplementary Table S3), implying that the components of the water cycle (i.e. precipitation, blue water, green water flow and storage) were basically in a dynamic balance. We conducted a linear regression analysis using the water cycle components during 1975–2004 (see Supplementary Figure S6), and found that (i) the precipitation and blue water are notably correlated (at a slope of 0.8705), indicating that blue water would mainly come from precipitation and (ii) the correlations of precipitation with green water flow and storage are not significant, implying that green water might primarily be affected by other factors except for precipitation (e.g. temperature, land use and cover, and soil characteristics).
Spatial distribution of annual means of (a) precipitation, (b) green water flow, (c) green water storage and (d) blue water during 1975–2004 (mm/year).
Spatial distribution of annual means of (a) precipitation, (b) green water flow, (c) green water storage and (d) blue water during 1975–2004 (mm/year).
Climate change impact on precipitation, blue and green water
Basin-scale averaged annual (a) precipitation, (b) green water flow, (c) green water storage and (d) blue water during the control period (1975–2004), near future period (2021–2050) and far future period (2061–2090) for two emission scenarios (RCP 2.6 and 8.5) from three bias-corrected RCMs.
Basin-scale averaged annual (a) precipitation, (b) green water flow, (c) green water storage and (d) blue water during the control period (1975–2004), near future period (2021–2050) and far future period (2061–2090) for two emission scenarios (RCP 2.6 and 8.5) from three bias-corrected RCMs.
Relative changes (%) of precipitation under the RCP 2.6 and 8.5 scenarios for the near future period (2021–2050) and far future period (2061–2090) in comparison to the 1970–2005 control period.
Relative changes (%) of precipitation under the RCP 2.6 and 8.5 scenarios for the near future period (2021–2050) and far future period (2061–2090) in comparison to the 1970–2005 control period.
The spatial distribution of future green water storage changes predicted by RMOH, RMPI and RNCC appears to be relatively close, as shown in Figure 8. Under RCP 2.6 in the near future, all three RCMs suggest an overall decreasing tendency in green water storage over the basin, with RMOH forecasting a decrease up to ∼19.3% in the southwestern portion, RMPI predicting a decrease up to ∼12.2% in the southeastern part of the basin, and RNCC predicting a decrease up to ∼18.6% in the central part of the basin. Under RCP 2.6 in the far future, RMOH and RNCC show a 7.8–31.4% reduction in green water storage in most sub-basins, while RMPI suggests a general increase over most areas of the basin with the central part of the basin having a notable increment up to 13.5%. Under RCP 8.5, the three RCMs suggest that the decreasing trend in green water storage is likely to exacerbate significantly. For example, RMOH RCP 8.5 suggests that the green water storage in the southwestern part of the basin would decrease by 16.4% in the near future and continue to decrease by up to 36.5% in the far future, whereas RMPI RCP 8.5 predicts somewhat mild decreases of green water storage (∼10.1%) over the basin in the near future, and significant decreases (∼38.7%) in the basin central in the far future. RNCC RCP 8.5, however, predicts a considerable decrease of green water storage in both near and far futures. In general, the green water storage is projected to decrease under most scenarios (Figure 8(m)–8(p)). Under RCP 2.6, the green water storage in the south-central part of the basin is likely to drop by 10.8–14.9% in the near future, and such a drop would become less significant (i.e. 5.8–12.4%) in the far future. Under RCP 8.5, the green water storage in the basin central area is expected to experience decreases around 11.0–16.5% in the near future, while the decrease would become more significant in the far future (i.e. 29.4–40.2%). It is worth mentioning that the basin-scale averaged green water storage, as shown in Figure 5(c), indicates a minor climate change influence on green water storage (∼9.6% decrease) in the near future under both RCPs and far future under RCP 2.6, with a drastic drop (∼28.1%) in the far future under RCP 8.5.
Figure 9 presents the spatiotemporal information of the simulated annual mean blue water. The basin average information can be referred to in Figure 5(d). The spatial distribution of blue water change predicted by the three RCMs under RCP 2.6 appears to be relatively consistent in the far future, but considerably different in the near term for both RCPs and far one under RCP 8.5. In the near future, RMOH RCP 2.6 predicts a considerable increase of blue water (∼8.1%) near the north and southeast basin and a notable decrease (∼10.6%) in the northwestern part, while RNCC predicts a general increase over the entire basin, with the eastern part of the basin having a notable rise up to 13.9%. Under RCP 2.6 in the far future, all RCMs predict an increasing tendency over the basin, whereas the RNCC predicts an increase up to 39.1% in the northern and southeastern portions of the basin. Moreover, RMPI RCP 8.5 suggests a general decreasing blue water trend over the basin. Particularly, the decreasing levels in the northeast of the basin change from 15.3 to 26.3% when the time window moves from near to far future. RMOH RCP 8.5 suggests a moderate increase (∼12.4%) of blue water in the southeastern part of the basin in the near future, and a general drop (∼6.5%) over the basin in the far future. Under RNCC RCP 8.5, most of the sub-basins are suggested to increase considerably in the far future, except for the northeastern areas which is likely to decrease by 9.3%. The ensemble mean of blue water suggests a basin-wide increase under RCP 2.6, with the southeastern portion of the basin expected to have the largest increase of 7.7% in the near future and a continuous increase to 22.5% in the far future. Under RCP 8.5, the blue water in the southern part of the basin is expected to increase by 6.6–13.3% in the near future, while only a slight increase (∼5.6%) is suggested in the far future. The blue water in the basin north seems to have only slight increases (∼5.3%) in the near future, but they turn to decrease around 2.3–14.2% in the far future. As presented in Figure 5(d), all RCMs suggest that the basin average blue water would continuously increase from near to far future under RCP 2.6, but the changing pattern would be notably different under RCP 8.5. The annual blue water from the RCM mean is expected to increase by 2.8% (29.9 mm) under RCP 2.6 and by 4.8% (51.4 mm) under RCP 8.5 in the near future, benchmarked with the control period. In the long run, the RCM mean suggests that the blue water would experience an increase of 12.4% (134.1 mm) under RCP 2.6 and a slight decrease of 1.7% (18.6 mm) under RCP 8.5. Obviously, the changes of basin-scale averaged blue water and precipitation are in a similar pattern, as blue water is sensitive to the precipitation variability in the Beijiang River basin.
DISCUSSION
From this study, it is revealed that the water resources of the Beijiang River basin are dominated by blue water. This somewhat differs from study results for other basins where green water is normally the major player. For instance, in the Hai River basin of northern China, Zhu et al. (2018) indicated that the annual mean green water (about 74% of the precipitation) is about 2.8 times higher than the blue water. Faramarzi et al. (2009) used SWAT to estimate the water footprint components of 30 provinces in Iran, and indicated that the green water was significantly higher than blue water in more than 20 provinces with annual rainfall being less than 1,100 mm. Schuol et al. (2008b) estimated the blue–green water distribution over the hot, low rainfall, dry continent of Africa, and found that the annual average green water amount was significantly more than the blue one in almost all African countries. These mentioned basins are characterized by low rainfall, dryness and intense evapotranspiration, which leads to low levels of surface runoff and high conversion rates of precipitation to green water. The Beijiang River basin is categorized to have the subtropical monsoon climate and its high precipitation (∼1,800 mm) tends to meet plant retention, infiltration, puddle filling losses and saturated soil layers; this has resulted in an extensive surface runoff generation and thus brought a larger amount of blue water in comparison to green. The previous studies on the Dongjiang River basin, which is adjacent to the Beijiang River basin, have given similar findings (Wu & Chen 2013; Lyu et al. 2017).
By comparing a number of study cases on the annual changes of water components in the literature (see Supplementary Table S4), it is found that the climate change impact on the water components in the studied basin seems to be less significant. This might be due to variations of different climate prediction models, emission settings, downscaling techniques, basin geographical and climatic characteristics, and future projection timeframes. More importantly, accordingly to the projection of ensemble mean of three RCMs, climate change may pose a small increasing effect on precipitation (<10%) in the Beijiang River basin, but there are still areas in the basin where blue and green water may decrease. It is thus essential to explore responsive strategies to reduce adverse influences. For example, creating a contour farming environment by building horizontal terraces or field water storage ditches could enhance the subsurface infiltration rate and soil permeability and increase soil moisture content and green water storage (Srivastava et al. 2010). Some farmland management practices (e.g. plastic mulching and intensive cultivation) could mitigate the loss of soil evapotranspiration and reduce inefficient green water flow (Yang et al. 2015). Improving irrigation efficiency and seeking optimal allocation of water resources for agricultural, industrial, domestic and ecological uses are also effective strategies for coping with shrinking blue water resources.
To explore whether the RCM with 25 km×25 km resolution is sufficient for evaluating the climatic impact on the studied basin, we used the raw RCM data (i.e. uncorrected RCM) to drive the SWAT to calculate the spatial changes of water components (Supplementary Figures S7–S9). A comparison of Supplementary Figure S7 and Figure 7 shows that the areal green water flow changes remain largely consistent for all scenarios, and the magnitudes of changes are very close between using the uncorrected and corrected RCMs. By comparing Supplementary Figure S8 and Figure 8, it is revealed that the spatial distribution of green water storage changes remains largely consistent, but the magnitude of changes somewhat differs. For example, the western central portion of the basin is expected to experience the largest reduction (∼31.4%) in green water storage in the near future based on corrected RNCC RCP 2.6 but the magnitude of such a reduction would be 15.4% based on the uncorrected scenario. A comparison of Supplementary Figure S9 and Figure 9 shows that the areal blue water changes and the relevant ranges demonstrate considerable differences. For example, the increasing trend is projected in the southeast of the basin in the far future based on the uncorrected RMOH RCP 8.5, but such a trend would be reversed under the corrected scenario. Moreover, the southeast portion of the basin would experience the largest increase in blue water (∼31.6%) in the near future based on the corrected RNCC RCP 8.5, but such an increase would rise up to about 54.0% based on the uncorrected scenario. Overall, applying the corrected and uncorrected RCMs to evaluate the climate influences may have notable differences in both the spatial distribution and magnitude of water component changes.
In this work, the blue–green water distribution in the Beijiang River basin was estimated based on the SWAT model. In terms of data, the absence of multi-year land use data and observed evapotranspiration data may somewhat affect calibration, although the performance of the model was still considered satisfactory. In a long run, both land use and climate change are major considerations for water resources management (Pan et al. 2017). Land use is mainly associated with evapotranspiration, runoff, infiltration and soil water redistribution, which are indirectly linked to the blue and green water patterns. This study did not address the impact of land use due to insufficient land use data to predict future land use conditions. Moreover, rivers in the basin are likely to be subject to human interventions such as the construction of reservoirs. However, it is rather challenging to obtain the detailed information on the planned reservoirs in the basin in the long term. Thus, the impact of possible planned reservoirs on water resources is not considered in this study. In terms of computation of water components, SWAT only simulates the hydrological cycle without considering their complex interconversions (Zhang et al. 2020).
CONCLUSION
This study quantitatively evaluated the potential climate change impact on spatiotemporal distributions of blue and green water resources in the Beijiang River basin, southern China. The climate data was based on three CORDEX-EAS RCMs under RCP 2.6 and 8.5 scenarios for three windows. Then, the hydrometric data was predicted by using the SWAT model which was validated using historical data. The QDM method was adopted to reduce the systematic bias from the original CORDEX-EAS RCMs. From the study, the annual average total water resources over the Beijiang River basin from 1975 to 2004 are 1,741.46 mm, of which 33.3% is green water flow, 2.1 % is green water storage and 64.6% is blue water. The basin's water budget is dominated by blue water, which is about twice as much as green. Spatially, the blue water has an annual mean from 796 to 1,640 mm over the entire basin, and is richly distributed in the central and southeast areas. Green water is relatively higher in the north and lower in both central and southwest areas. The average annual green water storage shows a generally decreasing pattern from north to south.
Compared with the control period, the RCM mean of the average annual precipitation would experience somewhat increases (<10%) in most sub-basins in the near term, but a general decrease under RCP 8.5 in the long run. The largest increase and decrease of green water flow from various RCMs are found to be about 2.7% in the northwestern part of the basin under RMOH RCP 2.6 for 2061–2090 and about 17.9% in the central part of the basin under RMPI RCP 8.5 for 2061–2090, respectively. The basin-wide average of green water flow from three RCM averages is projected to decrease by 43.3 mm under RCP 8.5 in the far future. In addition, the relative reduction of future green water storage projected by RNCC is more than 30% in most of the basin under RCP 8.5 in the far future, whereas the absolute change ranges from 3.8 to 17.4 mm under most scenarios. In terms of blue water, RNCC suggests a significant increase in the southeast portion of the basin under most scenarios in the far future; however, RMPI RCP 8.5 indicates a significant decrease in the northeast portion. In general, the ensemble mean of the three RCMs shows a slight decreasing trend in the north portion of the basin and an increasing one in the south under RCP 8.5 in the far future, and an overall increasing trend in most of the basin under other climate scenarios.
Overall, this study helped gain an insight into the future green and blue water resources in the Beijiang River basin. In light of the global climate change, such information would be useful to policy makers in local water resources management. For example, the shrinkage of blue water implies a decrease of easily available water supply, thus, more efficient irrigation practices and rational water allocation schemes are necessary. In addition, some further improvements are still needed. For example, more observed data (such as evapotranspiration and groundwater flow data) can be utilized to validate SWAT and improve its accuracy in computing blue–green water distributions. Also, the contribution of land use alterations to blue and green water components ought to be investigated in detail. Moreover, only three RCMs were used in this study. As the CORDEX-EAS project develops, more reliable studies of climate change impacts using more RCMs projections can be achieved in the future.
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China (52009022) and AcRF Tier 1 project (2019-T1-001-160) from the Ministry of Education (MOE), Singapore. We gratefully acknowledge the CORDEX-East Asia Databank (https://esgf-data.dkrz.de/search/cordex-dkrz/), which is responsible for the CORDEX dataset. The authors are also thankful for the China Meteorological Data Sharing Service System for providing the observed climate dataset.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICTS OF INTEREST
The authors declare there is no conflict.