Changes in groundwater-surface water interactions following two centuries of irrigation practices and groundwater use in the Upper Ganges-Yamuna interfluve, North India.

Abstract. The Indo-Gangetic Basin (IGB) is a global hotspot for groundwater overexploitation. Previous studies have shown that groundwater levels initially rose due to enhanced recharge following the construction of irrigation canals, but subsequently declined as agricultural, municipal, and industrial abstractions intensified. However, the relative impacts of separate recharge and abstraction components (precipitation, canal leakage infiltration, irrigation return flow, and irrigation, municipal and industrial abstraction), remain unclear, as do the effects on groundwater-surface water interactions and environmental flows. This study therefore aims to quantify spatio-temporal changes in groundwater recharge and abstraction components over the past two centuries and assess how these changes have impacted groundwater–surface water interactions in the Upper Ganges–Yamuna interfluve in northern India. Groundwater model simulations indicate that canal water infiltration following canal construction after 1830 boosted recharge, but since the 1970s increased abstractions have lowered groundwater tables and reduced river exfiltration. Currently irrigation accounts for roughly 85 % of abstractions, with municipal (15 %) and industrial (< 1 %) uses accounting for much smaller shares. From around 2000, abstraction lowered groundwater tables to such an extent that local rivers likely shifted from draining to infiltrating conditions. As a result, groundwater–surface water interactions in local rivers may have fundamentally changed. This shift threatens environmental river flows, degrades surface water quality by limiting wastewater dilution, and harms groundwater quality where polluted river water infiltrates the aquifer, posing risks to both ecosystems and human health. Although both the Yamuna and the Ganges show reduced groundwater exfiltration, they are not (yet) infiltrating.