QUANTIFYING THE TOPOGRAPHY-DRIVEN GROUNDWATER FLOW IN THE DISCHARGE AREA OF AN ENDORHEIC WATERSHED

Discharge area, which is usually associated with topographic lows, is an area where the flow of groundwater is directed upward with respect to the water table. Water table has been found to be shallow in discharge areas under all climates, which inevitably leads to high groundwater evaporation in (semi-)arid climates. In fact, the shallow water table in low areas is controlled not only by precipitation recharge (R) and groundwater evaporation (E_g), but also by topography-driven groundwater flow from high to low areas (Q_up). However, due to the simultaneous existence of groundwater evaporation and topography-driven groundwater flow, each of them is difficult to quantify. Hydrologists in the field of groundwater evaporation usually assume that the flux of groundwater flow should be predetermined, while hydrologists in the field of groundwater modeling use the flux of groundwater evaporation as a precondition in a model.

In this study, we analyzed the mechanisms of water table fluctuations in a shallow well in the discharge area of a semi-arid catchment and divided the unfrozen period into four stages with different trends of water table change. We first use water table rises associated with rainfalls to estimate precipitation recharge, then we use the water balance equations of the four stages to obtain two unknowns, i.e., the upward component of topography-driven groundwater flow and groundwater evaporation. In the unfrozen period, R is found to be 161 mm, Q_up is found to be 180 mm, and E_g is found to be 439 mm, which lead to a decreased water table. In a hydrologic year, i.e., including the seasonal frozen period, Q_up could be as high as 280 mm. This indicates that topography-driven groundwater flow plays a more important role than precipitation recharge on the shallow water table. We also compared the ratio of R to E_g in different stages to identify the boundary conditions in the discharge area, and found that that a specified-flux boundary with net evaporation should be used to characterize groundwater circulation in low areas in most times.

The method proposed in the current study could be applied to similar study areas to quantify groundwater budgets, as well as aid developing groundwater models with suitable boundary conditions in the discharge area of a basin.