Assessing the Impact of Interbasin Water Transfer on Terrestrial Water Budget: A GRACE-Based Case Study in China’s Haihe River Basin

The Haihe River basin (HRB) is the homeland for 120 million people with the 10% of total production of grains in China. HRB has experienced serious water storage depletion due to ongoing socio-economic development, population growth, and additional water stress from climate change, thus resulting in a series of eco-environmental problems, such as land subsidence, seawater invasion, and river blanking. To tackle these issues, multiple water transfer projects have been built to supply water to the HRB from other river basins. One of the most famous is the Middle Route of the South-to-North Water Transfers (SNWT) Project, which began its operation in late 2014. Up till now, the SNWT-diverted water has been used for compensating environmental flow, replenishing reservoirs recharging aquifers, and replacing urban groundwater pumping. This raises the widely concerned issue of whether the terrestrial water storage (TWS) and water budget in HRB have changed unexpectedly after the implementation of SNWT.

This study assesses monthly changes of TWS and water budget in the HRB from 2003 to 2023 by using Gravity Recovery and Climate Experiment (GRACE) satellite and ancillary datasets. We quantify the extent and contribution ratio of each water budget factor to TWS change before and after the SNWT operation using hierarchical analysis. Results show that the annual rate of TWS in HRB shifted from -17.0 mm/a (during 2003-2014) to +4.8 mm/a (during 2015-2023), which can be mainly attributed to the combined impacts of the intense precipitation infiltration in 2021 and the additional water recharge from the SNWT. Precipitation is identified as the main factor dominating regional TWS change, followed by evapotranspiration, runoff, and the water transfer volume. The averaged contribution ratios of these four factors are calculated as 57.0%, 34.0%, 7.49%, and 1.6%, respectively. Most importantly, we found that the contribution ratios of runoff and water transfer volume increased while those of precipitation and evapotranspiration decreased after the SNW'T operation, indicating a new change of HRB's water budget after water transfer.

Although the annual trend of TWS in HRB is increasing over the timespan of SNWT operation (2015-2023), the annual trend of TWS is still decreasing when focusing only on the timespan of 2022-2023. Such decrease in TWS can either be interpreted as a normal recessionary of TWS following the over-recharge from intense precipitation in 2021, or it can be recognized as representing the ongoing TWS depletion in spite of the implementation of SNWT. If viewed from the perspective of regional water budget, the inflow of water from the SNWT operation will certainly moderate the depleting trend of TWS in HRB. Yet whether the SNWT-diverted water can completely halt the TWS depletion or even cause the TWS recovery, still require further analysis and assessment based on the longer-term observational data of TWS change in HRB. The findings of this study highlight the notable impacts of large-scale water transfer and intense precipitation on the water cycle of HRB, and such impacts may become more obvious with the continued operation of SNWT in the future.