The Monitoring Drought Method Including Evaporation At Daily Scale And Its Application

Precipitation is the most important influencing factors of drought and flood. In order to compare with SPI, we used the precipitation alone in our previous research,put forward the WAP method of daily drought and flood monitoring (Lu 2009 GRL).In this method, we consider the extent of flood as an instantaneous variable, and its evolution is forced by precipitation .At the same time, evaporation and other factors are used as the dissipative terms, which play roles in reducing the extent of flood.Finally the WAP is expressed as a linear combination of the non-equal weights of the same day and the antecedent precipitation. It is also pointed out that SPI is a special case of WAP expression, which is equal to the weight, so it always needs a longer scale (more than one month) to be used as the average. The WAP curve showed a skewed distribution, the total value of WAP is positive. When there is no precipitation, and the effects of pre precipitation disappeared, WAP is close to 0, but this time the soil may still wetter; if in the following days there is no precipitation,WAP will continue to remain close to 0 of the value, but the soil will become increasingly dry. In order to distinguish the condition of soil moisture (reflecting the evaporation) differences under the condition of the WAP as close to 0, WAP index will be made further development in our recent work, and the evaporation quantity E is 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, its value is positive and negative, close to normal distribution, so it is not necessary to be treated as normal.In this paper, we used the WAPE index to make preliminary analysis of the monitoring effect in monitoring the daily drought and waterlogging and the phenomenon of drought-flood abrupt alternation, which reflects its practicality.WAPE considers the overall effect of precipitation and evaporation and runoff infiltration, this results in an exponential decay of the comprehensive factors affecting the drought and flood. This attenuation effect eliminates the need for the time scale of the SPEI index. Therefore, the WAPE index is very effective in monitoring drought and flood, especially for daily monitoring, It can reflect the development of drought and flood in a region, the duration, intensity and the change of drought and flood very well.Considering the calculation of evaporation is still a big problem in research filed at home and abroad, in the future research and business application, we can consider Combining the convergence and divergence of the whole layer of water vapor in the atmosphere with P-E, can make full use of observations and model data of theatmosphere; connect the water vapor flux divergence(C) to different value of precipitation and evaporation (P-E); make further efforts of the WAP index and provide a new way to monitor the drought and flood forecasting...