Abstract:Precipitation is the most important influencing factor of drought and flood. The WAP (Weighted Average of Precipitation) uses precipitation to monitor drought at a daily scale. When there is no precipitation for a period of time and the effect of the precipitation has decayed, WAP is close to 0, but at this time, the soil may still be wet. If in the following days, there is still no precipitation, WAP will continue to remain close to 0, but the soil will become dry. In order to distinguish such difference in the soil moisture (reflecting the evaporation) under the same condition that the WAP is close to 0, the WAP method is further developed, with the evaporation being expressed explicitly in the established model. That is, the flood (drought) state of the change is no longer just forced by precipitation P, but forced by P-E. The basic principle and model of the new method is similar to that of the previous method of WAP; the new index WAPE explicitly considered the evaporation, and its value is positive and negative, close to normal distribution, so it is not necessary to be treated as normal. Results show that the WAPE index can provide more details in the daily monitoring of drought and flood.