Impacts of basin-wide irrigation pumping on dry-period stream baseflow in an alluvial aquifer in the Kosi Fan region of India and Nepal

The Kosi Fan region of India and Nepal hosts a productive aquifer system. Regional hydrology is highly seasonal, and both groundwater and surface water are used for irrigation. Groundwater depletion is not currently occurring, but there is concern that plans to increase groundwater irrigation will reduce river baseflow, potentially affecting downstream water users. This study presents a model-based analysis of the impacts of groundwater withdrawal on dry-period streamflow and evaluation of management alternatives. A sensitivity analysis was performed in which a range of model parameters were tested around a best-estimate, base-case scenario. A high-reduction scenario was then developed which combined the factors that produced the greatest pumping-induced reduction in dry-season baseflow. Management strategies for 2.5, 5, and 10-km no-pumping buffers around the rivers were tested for the base-case and high-reduction scenarios. Simulations show that groundwater withdrawal equivalent to 30% and 60% of dry-season streamflow for the Kosi and Mahananda rivers, respectively, reduces the current dry-season flow by less than 4%. In the base-case scenario, simulated dry-season baseflow reduction is 1.8% and 2.6% for the rivers, respectively; these reduce to ~1% with a 2.5-km buffer zone. For the high-reduction scenario, dry-season baseflow reductions are 4.7% and 7.0% with no buffer; these reduce to 1.3% and 0.9% with a 5-km buffer for the Kosi and Mahananda rivers, respectively. The small reductions in baseflow relative to the total amount of pumping are due to a pumping-induced increase in rainfall recharge, thus the effects of additional pumping are mitigated.