Replumbing the Bengal Basin: A Shift in Recharge Driven by Groundwater Irrigation Revealed by Stable Water Isotopes

Abstract

Groundwater supports agriculture and provides domestic water for over 250 million people in the Bengal Basin. Our analysis of stable water isotope ratios in rain, surface, and groundwater shows that the proportion of groundwater recharge originating from stagnant surface water bodies has increased by about 50% over the last seventy years while the relative contribution from direct infiltration of rain has decreased. This regional shift in the source of groundwater shows how the simultaneous expansion of irrigated rice, excavated ponds and groundwater pumping has changed the hydrologic system by cycling evaporated standing water through the subsurface. Analysis of water isotope data also reveals that most recharge from standing water enters during the latter part of the dry season (February-April), while most rainwater recharge occurs in the early months of the monsoon (June-August) before aquifers fill to capacity and reject additional recharge of rainwater.