Precipitation Deficit Threshold Zoning Based on Shannon Entropy in Karkheh Basin

The entropy of an event is a characteristic of the probability distribution that shows the average uniformity of the occurrence of that event. This theory is used in meteorological studies. In this study, the uniformity of precipitation deficiency is investigated by referring to the concept of Shannon entropy and as a dynamic 12-month in Karkheh basin in southwestern Iran. Precipitation entropy number is introduced as a drought index and the existence of drought is determined according to the average precipitation entropy in the years 1950-2018. This method is evaluated in comparison with standard precipitation-evapotranspiration index (SPEI) for Karkheh basin in spatio-temporal patterns. The results of spatiotemporal evaluation indicated the similarity of the Entropy indicator at the basin on a long-term time scale SPEI. In areas with higher average precipitation, the entropy was sensitive to precipitation uniformity, and with decreasing precipitation uniformity, which is one of the causes of drought, the entropy decreased. In areas with lower average precipitation, the entropy sensitivity was to the volume of precipitation and with decreasing precipitation, the entropy decreased. The long-term mean entropy of the lack of precipitation perfectly indicated the threshold of the drought.