Statistical analysis of rainfall and groundwater interaction in Bhadra catchment

Interaction between rainfall and groundwater is a complex phenomenon as it varies spatially with respect to local hydrogeological characteristics and temporally with hydro meteorological variations. To understand the resilience of the groundwater system to the changing climate, rainfall-groundwater interaction needs to be analysed in detail. The Rainfall and Time Trends (HARTT) have been widely used in recent studies to model the impact of the rainfall and the groundwater level fluctuation. However, not many such studies have been reported in tropical hilly areas. Bhadra reservoir catchment is a tropical hilly catchment in India which is characterised by heavy rainfall, steep topography and shallow water table conditions and significant subsurface flow conditions. The area is largely dependent on groundwater for irrigation and drinking water supply. Nonparametric statistical methods such as Mann–Kendall (MK) test, Sen's slope estimator and Sequential Mann–Kendall Test (SQMK) are used to study the temporal trends in the rainfall and static groundwater levels. Further, contribution of rainfall to groundwater level fluctuation is analysed using HARTT model. Study shows a decreasing trend in rainfall and groundwater level in the upper catchment. Further, 1 mm increase or decrease in the rainfall anomaly is estimated to cause 0.0012 m decrease or increase in the depth to water table. Only 36% of the groundwater level fluctuation is explained by rainfall induced recharge; nevertheless, the relation is non-uniform throughout the catchment. Spatial variation in topography and hydrogeological conditions is suspected to have significant impact on the rainfall and groundwater interaction leading to poor coefficient of determination for the regression model between rainfall anomaly and the groundwater level fluctuations.