Have GRACE Satellites Detected Interbasin Groundwater Flow in Karst Areas of Southwest China?

Interbasin groundwater flow (IGF) is an important water budget component in particular for karst regions. However, IGF is difficult to be quantified and generally neglected. This study attempts to quantify IGF in a typical karst river basin (Wujiang River Basin, 87,920 km2, with ~70% of karst areas) in southwest China by jointly using terrestrial water balance (TWB) and combined land-atmosphere water balance (CWB) approaches. The data sets used (2003-2015) include: (1) in situ- and satellite-based precipitation, (2) three satellite-based evapotranspiration products, (3) in situ streamflow data at the outlet of the studied basin, (4) reanalysis-based atmospheric water flux data from ERA-interim and ERA5, and (5) terrestrial water storage change (TWSC) data based on spherical harmonic and mascon solutions of Gravity Recovery And Climate Experiment (GRACE) satellite measurements. Good agreement is found between in situ- and satellite-based precipitation. Satellite-based evapotranspiration data are validated against in situ-based estimation using eddy covariance method. Monthly (annual) total discharge estimated from TWB agrees well with that from CWB with the highest correlation of 0.82 (0.79) and lowest root mean square difference of 30.7 mm/mo. (97.7 mm/yr). Large imbalance (i.e., total discharge minus streamflow) is found for GRACE-based TWB and CWB with 128.0 and 202.0 mm/yr, respectively, which is mainly attributed to uncertainty with 88.6 and 107.0 mm/yr, respectively, and less attributed to groundwater pumping (3.2 mm/yr). The residual imbalance excluding uncertainty and groundwater pumping ranges from 39.4 mm/yr based on TWB to 95.0 mm/yr based on CWB which implicates the existence of IGF in the karst areas of southwest China. The average estimate of IGF (67.2 mm/yr or 5.9 km3/yr) is larger than the storage (~4.4 km3) of Lake Taihu, the third-largest (in size) freshwater lake in China. This study confirms the existence of IGF in karst areas of southwest China and provides a framework for quantifying IGF applicable for both karst and non-karst river basins. The estimated IGF can help to optimize water resources or recharge estimation, improve groundwater modeling, facilitate transboundary water resource management, and promote the understanding of hydrogeology conditions.