A Study On The Regime And Budget Of Groundwater In The Discharge Area Of Basins

The flux of topography-driven groundwater flow, which plays a major role on maintaining a shallow water table in topographic lows, especially in arid or semi-arid regions(Fan, 2015), is difficult to quantify. In the arid or semi-arid regions where potential evaporation exceeds precipitation, groundwater flow modelling has become a routine to quantify the flux of groundwater inflow into and outflow from a specific study area. However, precipitation recharge and groundwater evaporation, are difficult to quantify due to the complicated processes in the unsaturated zone. This brings great uncertainty on the boundary condition assigned at the upper boundary of a groundwater flow model.Precipitation recharge, evaporation, seasonally frozen soil and regional groundwater joint control the water table. Precipitation recharge could lead to rises in water table and evaporation could result in falls in water table. Seasonal frozen soil absorb groundwater during freezing, in turn return the original ground water when melted. In topographic lows with shallow water table, the upward component of the topography-driven groundwater flow could also lead to rise in water table. Due to the simultaneous existence of topography-driven groundwater flow and groundwater evaporation, it is difficult to quantify groundwater evaporation without determining the upward component of the topography-driven groundwater flow even if recharge from other sources is absent.In this study, The Wudu Lake is one of the numerous endorheic lakes in the Ordos Endorheic Region. Take the latest research results of regional groundwater flow system of as a guide, the water table fluctuation in the topographic lows of Wudu Lake catchment is analyzed. In the case of the water table fluctuation of well DK2-10,The mechanisms of water table fluctuations are examined and the amount of precipitation recharge is estimated based on the water table rises corresponding to precipitation events. We mainly estimate the components of groundwater circulation and groundwater budget in the unfrozen period, four stages with different trends ofwater table change can be identified. Based on the groundwater balances in different stages of water table fluctuation, groundwater evaporation and the upward component of topography-driven groundwater flow are quantified.Based on the quantitative description of each stage with different trends of water table. The upward component of topography-driven groundwater flow is found to exceed total precipitation recharge in the unfrozen period, which demonstrates the great contribution of groundwater flow to maintaining the shallow water table. We also find groundwater evaporation exceeds precipitation recharge in most time of the unfrozen period, which indicates a specified-flux boundary with net evaporation, and is less than precipitation recharge in a limited time, which indicates a specified-flux boundary with net recharge.This study enhances our understanding on the budget of groundwater in discharge area. The method could be applied to similar study areas. Moreover, this study could help could aid developing transient groundwater models with correct boundary conditions in the low areas of a drainage basin.