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.

印度和尼泊尔的 Kosi Fan 地区拥有丰富的含水层系统。区域水文具有很强的季节性，地下水和地表水都用于灌溉。目前没有发生地下水枯竭，但人们担心增加地下水灌溉的计划将减少河流基流量，从而可能影响下游用水户。本研究基于模型分析地下水抽取对干旱期径流的影响以及管理备选方案的评估。进行了敏感性分析，其中围绕最佳估计、基本情况情景测试了一系列模型参数。然后开发了一个高减少情景，该情景结合了产生最大抽水引起的旱季基流减少的因素。2.5、5、河流周围的 10 公里无抽水缓冲区针对基本情景和高减排情景进行了测试。模拟表明，分别相当于 Kosi 和 Mahananda 河旱季流量的 30% 和 60% 的地下水抽取量将当前旱季流量减少了不到 4%。在基准情景中，河流的模拟旱季基流量减少分别为 1.8% 和 2.6%；在 2.5 公里的缓冲区中，这些减少到约 1%。对于高减排情景，旱季基流量减少 4.7% 和 7.0%，没有缓冲；在 Kosi 和 Mahananda 河的 5 公里缓冲区中，这些分别减少到 1.3% 和 0.9%。相对于抽水总量，基流的小幅减少是由于抽水引起的降雨补给增加，因此额外抽水的影响得到了缓解。