Assessment of Wet and Dry Spells Over North Western Zone of Tamil Nadu Using WASP

Rainfall is the most dependent weather factor that decides the success or failure of a crop in a location. The behaviour of rainfall plays a major role in the selection of crop, cropping system and cropping pattern. It is also essential for planning of water resource management structures. Hence it is essential to study the anomaly in rainfall over a long period of time to identify the deviation in rainfall pattern over the region. Weighted Anomaly of Standardized Precipitation Index (WASP) is an index that can help in identifying the occurrence of dry or wet spells over different time scales (tri, hexa, nona and dodeca-monthly). The analysis was carried out for Salem district of Tamil Nadu over a period of 30 years (1991-2020) which has an average rainfall of 990 mm per year with bimodal distribution. The analysis shows an increase in near normal events, decrease in wet as well as dry events with 3 m, 6 m, 9 m and 12 m WASP. The 3 m-WASP indicates the north east monsoon have more variability in rainfall since more number of wet as well as dry events have occurred during this season. With 6 m WASP During, in all the 3 decades, the consecutive wet as well as dry events have been occurred during the winter and NE monsoon seasons. This shows the risk in crop cultivation during NEM season because of higher instability in rainfall. Considering the longer time scale of 9 m and 12 m WASP indicates the occurrence of prolonged extreme dry or extreme wet events to be very minimal. The negative effects of these events can be handled easily by adopting proper drainage, water harvesting and storage structure within the farms. These structures can also help in recharging the ground water table in addition to supplementing water needs for the crops.