Construction and application of an integrated drought index for the middle and lower reaches of the Yangtze River Basin based on the random forests

Considering the nonlinear relationships among soil moisture, precipitation, air temperature, radiation, normalized difference vegetation index (NDVI), land surface temperature (LST), and vapor pressure deficit (VPD), this study constructed an Integrated Drought Index (IDI) for the middle and lower reaches of the Yangtze River Basin from 2000 to 2023 using a random forest model. The accuracy of the model was evaluated, and the IDI was further applied to identify the temporal and spatial characteristics of the 2022 drought event and the importance of its influencing factors. The results show that: 1) The coefficients of determination (R2) between the predicted and observed soil moisture were 0.91 for the training dataset and 0.50 for the testing dataset, with corresponding root mean square error (RMSE) values of 0.016 and 0.032, indicating good predictive performance of the model. 2) The R2 values between soil moisture and IDI across 14 stations ranged from 0.16 to 0.52 (P R2 values ranged from 0.17 to 0.48 (P < 0.01), demonstrating the high accuracy of the constructed IDI. 3) The anomalies of the integrated drought index were negative during October 2010 to April 2011, July to December 2013, July to October 2019, and June to October 2022, indicating that drought events occurred in these periods. 4) The spatial distribution characteristics show that drought began in the northwestern part of the basin in June 2022, intensified from July to September when over 60% of the region experienced severe drought, and was alleviated in the northern part in October, while drought in the southern part persisted until November, consistent with actual conditions. 5) The feature importance analysis indicates that VPD, LST, and precipitation are the main driving factors influencing drought in the middle and lower reaches of the Yangtze River Basin. This study provides new technical support and a scientific basis for drought monitoring and early warning in the Yangtze River Basin.