RESUMO
Precipitation is the most important factor in determining the drought condition. In addition, the drought condition is also affected by the hydrological process such as evaporation, leakage, runoff, and groundwater. Based on a simple physical model of water income and recession, the effective precipitation index (WAP) can reflect the drought condition in a region on a short time scale. Using a water decline equation to describe the process of preliminary soil moisture, evaporation, runoff, and groundwater, the WAP can be used for the quantitative study of drought. This paper improves the calculation program of WAP with reference to the calculation model of the standardized index. The improved WAP (called SWAP) effectively solves the problem of seasonality in drought. The results showed that SWAP has the ability to capture the drought processes of occurrence, development, peak, and regression in a short time scale, and the SWAP can be used as an effective tool for drought identification, assessment, and prevention.