Response of blue and green water to climate and land use changes: A study in the Wujiang River Basin, China

Blue and green water resources are essential for human livelihood and the sustainable development of ecosystems. It is crucial to thoroughly examine the effects of climate change and land use change on these resources. This study takes the Wujiang River Basin as an example, which, as a typical mountainous basin, has more prominent water resource problems. Firstly, two sets of land use data from 1990, 2000 and 2010 were selected, considering various climate and land use change scenarios. The impact of climate and land use changes on blue and green water resources in the Wujiang River Basin from 1985 to 2014 was then quantitatively analyzed using the soil and water assessment tool (SWAT). Subsequently, the future land-use simulation (FLUS) model was employed to simulate land use in 2040, based on two periods of land use data (2000 and 2020). Finally, the SWAT model, driven by future land use data and two scenarios from the CMIP6 model (SSP2-4.5 and SSP5-8.5), projected the blue and green water resources in the Wujiang River Basin for the period 2015-2100. The results indicate the following: 1) The SWAT model demonstrated good performance in simulating the water cycle process in the Wujiang River Basin, with Nash-Sutcliffe coefficient (NSE) and coefficient of determination (R²) values of 0.94 and 0.94 for the calibration period, and 0.77 and 0.87 for the validation period, respectively. 2) The FLUS model effectively simulated land use in 2020, achieving a Kappa coefficient of 0.764 and demonstrating its applicability for future land use simulations. 3) From 1985 to 2014, land use changes revealed reversibility patterns, with agricultural land being converted mainly into forest land and grassland, forest land being converted mainly into agricultural land and grassland, and grassland being converted mainly into agricultural land and forest land. 4) The period from 1985 to 2014 witnessed an initial increase followed by a decrease in blue and green water resources in the Wujiang River Basin, with blue water increasing by 137.3 mm/a and then decreasing by 127.5 mm/a, and green water increasing by 7.9 mm/a and then decreasing by 12.7 mm/a. Climate change accounts for 99% of blue water change and 85% to 90% of green water change, highlighting its predominant role in these shifts. 5) Urban land is projected to expand almost 1.5 times in 2040 compared to 2010. Both scenarios indicate an upward trend in blue and green water resources when contrasted with the historical base period (1985-2014), with the SSP5-8.5 forward period displaying the most substantial increase. This trend amplifies the risk of flooding and drought occurrences. These findings offer valuable insights for informed land use planning, integrated water resources management, and decision-making concerning future water resource management in the Wujiang River Basin.