Isotopic characterization and mass balance reveals groundwater recharge pattern in Chaliyar river basin, Kerala, India

• NE winter rains do not contribute much to the groundwater recharge in Chaliyar basin.
• Inverse altitude gradient in δ18O observed in groundwater of Chaliyar basin.
• River water contribution to groundwater recharge estimated in Kerala, India.

Study regionThe Chaliyar river basin, Kerala State, India.Study focusDetailed understanding about spatio-temporal variation in the interaction and exchange of water between surface and sub-surface reservoirs is important for effective watershed management. Spatio-temporal variations in the oxygen isotopic composition (δ18O) were used to understand the interaction between groundwater and river water, and to estimate the groundwater recharge from river water in the Chaliyar river basin.New hydrological insights for the regionBased on the spatio-temporal variation in δ18O values of river and groundwater and fluctuation in ground water levels, following important inferences are made: (1) estimated river water contribution to post-monsoon groundwater recharge is ∼16% in the lowland coastal area of the Chaliyar river basin and 29% in midland region; (2) northeast winter monsoon rains contribute to the groundwater of Chaliyar river basin only in an insignificant manner, and with a delayed response; (3) unlike river water samples which exhibit both seasonal and spatial variation of more than 3‰, the groundwater samples vary only marginally (∼1‰) between the seasons and across the physiographic zones; (4) groundwater samples exhibit inverse altitude gradient in δ18O values in the highland zone, in all the three seasons. This may be due to flow of the isotopically depleted groundwater down the gradient and evaporation of residual water in the upper reaches of the basin.