Assessment of Seasonal Rainfall Drought Indices, Nyala City Sudan

Abstract

Drought is an unpredictable hydrological phenomenon, and climate change has made it difficult to predict and analyze droughts. Nyala city airport metrological station rainfall records from 1943 to 2017 (75 years) were investigated. Four statistical drought indices were used; the standardized precipitation index (SPI), the rainfall anomaly index (RAI), the rainfall decile percent index (RDI), and the percent normal precipitation index (PNI). The study analyzes, assesses, compares, and determines the proper drought index. Results show that annual normal drought class (DC4) percentages for PNI, RDI, and RAI are not significantly different at an average of 42% and 65.3% for SPI at a frequency of 49 years. In comparing the average monthly and yearly drought frequency values and considering the historical dry and wet droughts, results showed the indices performance rank as: SPI, RAI, RDI, and PNI. Result reveals that the SPI was superior in all analyses, but it had some defects in detecting monthly dry drought when precipitation is dominated by rare or zero values (start and end of the rainy season). This was concluded and revealed by conducting a zone chart showing the deviations of standard deviation about the mean. Thus, the SPI index outperforms the other three indices.