Daily standardized antecedent precipitation evapotranspiration index (SAPEI) and its adaptability in Anhui Province

Agricultural drought and waterlogging are two of the most compelling problems affecting agricultural productivity and efficiency. At present, most of the widely used drought and waterlogging indices are based on ten-day, monthly, and seasonal scales. Their timeliness and accuracy are still far from the desired levels that would enable a sustainable means of preventing and controlling agricultural drought and waterlogging disasters. Based on the data of 78 meteorological observing stations in Anhui Province, selected as the transitional zone between the North Semi-humid and South Humid climates, a daily standardized antecedent precipitation evapotranspiration index (SAPEI) was constructed by fitting the daily antecedent precipitation evapotranspiration index (APEI) series with a 3-parameter log-logistic distribution. In addition, its feasibility and adaptability were analyzed in representative stations that were selected in different dry and wet zones, namely, North of the Huaihe River, Between the Huaihe River and Yangtze River, and South of the Yangtze River. The results showed that the empirical probability distribution curve of representative stations' APEI series highly corresponded with the theoretical log-logistic distribution curve. Representative stations' daily SAPEI curve could describe the changes in farmland water budget in the presence/absence of rain. In the case of the rapid change in drought and flood on June 8 and 28, 2011, SAPEI's drought and waterlogging grade and change trends were in good agreement with soil moisture in the same period. Spatial distribution of drought, waterlogging frequency, and annual mean values of accumulated index based on winter wheat whole development stage's SAPEI was primarily distributed latitudinally. These distribution characteristics conformed to the actual distribution of drought and waterlogging in Anhui Province. The 0-value line of annual mean SAPEI (I_SAPEI) in the winter wheat whole development stage was located north of the "Between the Huaihe River and Yangtze River" zone and along the eastern and southern foot of the Dabie Mountains. The area covered by negative values was drier and was the main area planted with winter wheat in Anhui Province, and the other area covered by positive values was more humid. In conclusion, the daily-scale SAPEI and its accumulated index could reflect the temporal distribution of precipitation and the impact of daily rainfall amount, and accurately depict the changes in drought and waterlogging and the cumulative effects of drought and waterlogging processes in different regions of Anhui Province. Therefore, SAPEI not only retained the advantages of standardized precipitation evapotranspiration index (SPEI) such as strong regional applicability, dimensionless character, and the ability to reflect the influence of temperature, but also had higher timeliness and sensitivity by modifying the precipitation evapotranspiration index from a monthly or seasonal scale to a weighted cumulative precipitation evapotranspiration index at a daily scale. SAPEI can be used in daily dynamic monitoring and the assessment process of drought and waterlogging in Anhui Province.