Rainfall Stability and Drought Valuation (Using Spi) Over Southern Zone of Tamil Nadu

Drought is the most extensive hydro-meteorological pattern of prolonged period of water scarcity affecting natural resources and environment. However, it has significantly different characteristics from one region to another. Tamil Nadu agonizes from various disasters including droughts. Generally rainfall deviation from the long-term mean continues to be a widely adopted indicator for drought intensity assessment. The application of this indicator is strongly limited by its inherent nature of its dependence on mean. Thus a standardized procedure that overcomes these limitations becomes a basic need. Moreover stability of rainfall in recent years has been erratic and hence to understand the stability of rainfall over years Precipitation Ratio (PR) was used. Further to understand the drought climatology of southern zone of Tamil Nadu, Standardized Precipitation Index (SPI) was used. PR was studied for three decades (1981-2010) covering seven locations. In most of the locations the rainfall was stable during 1981-90 and thereafter a gradual decline in stability was noticed confirming the weather extremes in the recent decade. SPI had an annual range from +2.69 to -4.94 while Monsoon period had +2.41 to -3.43 and +2.69 to -3.14 respectively for Southwest and Northeast monsoon seasons. As per SPI index, Southern zone is prone to moderate drought followed by severe and extreme drought category. Among the periods studied (Annual, SWM and NEM) number of moderate drought occurrences had no much variation while in severe and extreme drought category, variations could be observed. In particular, during NEM period there was almost no occurrence of extreme drought.