Quantifying long-term drought in China’s exorheic basins using a novel daily GRACE reconstructed TWSA index

• Daily TWSA is reconstructed in China’s nine exorheic basins from 1963 to 2022. • A novel daily standardized GRACE reconstructed TWSA index (D-SGRTI) is proposed. • D-SGRTI shows strong performance in hydrological drought monitoring. • China’s semi-arid basins tend to experience longer-duration hydrological droughts. As global warming intensifies, the frequency of drought events increases, drawing attention to enhanced drought monitoring. While drought indices play a crucial role, most are available only on a monthly scale, and daily indicators remain limited. This study addresses this gap by reconstructing daily Terrestrial Water Storage Anomalies (TWSA) from 1963 to 2022 and proposing a Daily Standardized GRACE Reconstructed TWSA Index (D-SGRTI). The index is applied to nine exorheic basins in China. Results indicate that D-SGRTI shows strong agreement with other drought indices, including the Standardized Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI), Standardized Soil Moisture Index (SSI), and Standardized Runoff Index (SRI). The correlation coefficient (CC) analysis shows that between D-SGRTI and SSI in the Songhuajiang River Basin (SRB) is 0.84, while CCs between D-SGRTI and SRI are highest in the other eight basins, reaching 0.91 in the Middle Yangtze River Basin (MYRB). The semi-arid and semi-humid basins show higher drought frequency, longer duration, and slower recovery over the study period. In contrast, in the humid basins, D-SGRTI reveals lower drought frequency, shorter duration, and quicker recovery. Generally, the drought frequency in autumn and winter is higher than that in spring and summer. This study presents an alternative method for monitoring drought at a daily scale, offering valuable insights for drought assessment across various regions in China. The approach holds potential for broader application in global drought monitoring efforts.