The researchers studied groundwater recharge in a river basin in Kerala, India by analyzing isotopic compositions of water samples. They found that river water in different elevations showed different isotopic patterns, with some water near the mouth being influenced by sea water. Evaporation also played a role in modifying surface water isotopes. By using oxygen isotopes, they estimated that surface water contributes up to 17% in lowland areas, 14% in midland areas, and only 7% in highland areas to groundwater. This study shows how isotopic ratios can help understand water resources in a river basin.