Estimation of pumpage and recharge in alluvial fan topography under multiple irrigation practices

Summary In this study, we estimate small-scale temporal variations in groundwater pumpage and recharge in alluvial fan topography. We study these variations on a monthly scale using groundwater storage hydrograph and isotope analysis along with area rainfall hydrographs and data of historical irrigation pumpage variations. We consider multiple irrigation practices whose effect was measured only on an annual basis in mostly previous studies. In previous studies, pumpage for non-irrigation or irrigation purposes was regarded as known but illegal pumping was not accounted for. Also, recharge sources include rainfall, rivers, boundary inflow, and groundwater-irrigation, which have not been individually accounted for in previous studies. Pumpage can be estimated from the pumping rate and the number of pumping days. In alluvial fan topography, the actual pumping rate, including that because of illegal wells, can be deduced from the recession slope of the groundwater hydrograph. The annual pumping rate of non-irrigation pumping is assumed to be constant, and can be estimated as the slope of the groundwater hydrograph over consecutive non-rainy days during the non-irrigation periods of the dry season. For cases where there are multiple irrigation practices, the temporal distribution of the irrigation pumping rate can be estimated from the slope of the groundwater hydrograph and also from records provided by the Irrigation Association. In this study, we regard recharge as the difference between inflow and loss. Different types of inflow can be inferred from the storage hydrograph in rainy or non-rainy days, and the loss can then be obtained by using the groundwater balance equation. The isotope mass balance equation is then used to quantify various sources of recharge. In this study, we apply the proposed methodology to the Cho-Shui River alluvial fan, Taiwan. The results show that the proposed methodology can overcome previous restrictions of groundwater hydrograph and isotope analyses to effectively and rationally estimate monthly variations in pumpage and recharge, thus making it possible to research fine flow mechanisms within a multi-layer groundwater system.