Rainfall variation and groundwater fluctuation in Salem Chalk Hills area, Tamil Nadu, India

Rainfall is a distinctive variable. It reflects influence of multiple meteorological factors locally and globally. Long term rainfall data in respect of Chalk Hills (Magnesite mines) area, Salem have been analysed. From this analysis, an increasing trend in the rainfall from January to May and declining trend in the rainfall during May and June are noticed. The rainfall increases from July onwards, attains a peak value in October and decreases to a low in January. Seasonal rainfall variation shows that the basin receives higher amount of rainfall during the Southwest (SW) monsoon season and equally good amount of rainfall during Northeast (NE) monsoon season. Pre-monsoon season also contributes considerable amount of rainfall but the post-monsoon season rainfall contribution is negligible. The spatial distribution of rainfall in the study area depicts that southwestern part gets more rainfall than the northeastern part during pre-monsoon and northeast monsoon periods. The western part of the study area gets more rainfall than the eastern part during southwest and post-monsoon periods. Frequency distribution analysis over a scale of 10 indicates that the annual variation has the maximum frequency of 9.0 in the range of >1000 mm at Yercaud rain gauge station and the other two rain gauge stations (Salem and Omalur) in the study area shows frequency values 3.0 to 4.0 for the same rainfall range. Groundwater level fluctuation analysis shows that some of the places have deeper water levels during southwest monsoon season. Groundwater starts to replenish the shallow aquifers during northeast monsoon season and reaches high during post-monsoon period when plants are dormant and evaporation rates are less. The water levels remain stable only up to November but during January to May, the water level declines gradually due to water extraction for irrigation.