Comparative socio-hydrology to identify dominant controls on baseflow reductions from human irrigation demands

Baseflow plays a crucial role in sustaining river aquatic ecosystems, reduced drought impacts necessitating the need to understand how human activities might influence it. Here, we implement a comparative socio-hydrology based approach to identify dominant controls on baseflow reductions across three regions in India irrigated by large reservoirs. We evaluate the effect of conjunctive use of surface water and groundwater on downstream baseflow in areas irrigated by the Nagarjuna Sagar (NSR) in Krishna basin, Hirakud (HRD) in Mahanadi basin and Indira Sagar (ISR) in Narmada basin, three large reservoirs in India. We apply a socio-hydrologic model to simulate surface water and groundwater withdrawals as a function of reservoir inflows, reservoir characteristics, water demands, and aquifer characteristics of the regions. The model constitutes a reservoir module that simulates water releases from the reservoir, a water use module that simulates how farmers use surface water and groundwater to meet irrigation demands, and a conceptual groundwater module to simulate groundwater levels. Farmers extract groundwater when water supplied from reservoirs does not meet irrigation demands. A classification and regression tree (CART) based algorithm was used to quantify the relative influence of different socio-hydrological factors on baseflow reductions due to upstream irrigation. We found an average annual reduction of 323 MCM (1968-2022), 24 MCM (1958-2021) and 13.72 MCM (2005-2022) in baseflow due to the groundwater pumping for NSR, HRD, and ISR, respectively. These translate to 11 %, 5%, and 3% reduction in baseflow compared to a baseline no pumping scenario. Though the relative reduction in baseflow was primarily governed by the volume of groundwater pumped in all cases, accurate characterization of the reduction required information on climate and reservoir characteristics at annual time scales.