Spatiotemporal variation characteristics and the possible causes of groundwater storage based on GRACE in the Huai River Basin, China

Groundwater management is a relatively weak aspect of water resources management in the Huai River Basin. It is necessary to understand the characteristics and patterns of groundwater changes. Based on GRACE and GLDAS data from 2003 to 2023, the groundwater storage anomalies (GWSA) were calculated in the Huai River Basin. Combined with the Normalized Difference Vegetation Index (NDVI), water resources bulletin, and other datasets, this study used STL time series decomposition and random forest to focus on the spatiotemporal changes and the possible causes of groundwater storage. The results indicated that GWSA in 80.71% of the watershed showed a significant downward trend from 2003 to 2023. The average change rate of GWSA was -0.79 cm/yr from 2003 to 2019, with a lower decline rate in the southeast and a higher decline rate in the northwest. After 2019, the average change rate of GWSA reversed to 1.03 cm/yr. Moreover, the spatial rate of change decreased from north to south, the rate of change in the northwest turned positive, and negative values began to appear in the southwest basin. The dominant factor of the GWSA changes in the basin was the inter-basin water transfer, and it showed a highly significant negative correlation. The secondary factors were the precipitation and actual evapotranspiration. The research studies the spatial evolution trend of groundwater reserves in the Huai River Basin over the past 21 years and conducts the causal analysis, and it could provide a theoretical basis and data support for the groundwater resources management and prediction.