Estimation Of Groundwater Recharge Using Multiple Approaches

Groundwater recharge represents the renewable rate of aquifers and is one the key parameters for the management and rational development of groundwater resources. In arid and semi-arid regions, quantitiative estimates of groundwater recharge are remely difficute as the rate is very low. The Ordos Plateau is located in the east part of NW China and abundant natural resources are stored in the study area that makes it the largest energe base in China. Due to the sparce precipitation, strong evaporation, and undeveloped surface water sources, groundwater is the most important source of water supply. With development of local economy and the construction of the energy base, groundwater mining will increase gradually that requires effective management of groundwater resources. Therefore, groundwater recharge estimation is ndded to proper management of groundwater and has a pratical and theoretical siganificance.Due to uncertainties associated with each method for estimating recharge, the use of many different approaches is recommended to constrain the recharge estimates. Based on previous studies, eight methods were used to quantify groundwater recharge in the Ordos Plateau. Firstly, isotopic data of precipitation and groundwater were used to identify source and mechanism of groundwater recharge. Secondly, seven methods were used to estimate groundwater recharge based on surface water, soil water and groundwater, including the chloride mass balance method, the water table fluctuation method, darcian method for saturated and unsaturated zones, the water balance method based on remote sensing, the empirical method and the baseflow separation method. Finally, based on cases in the world, the spatial and temperal features of 16 commonly used methods were summarized.The isotopic data of precipitation and groundwater show that groundwater scaters around the local meteoric water line, indicating groundwater is of meteoric orgion. There are two mechanisms of groundwater recharge, the piston type and the preferential type. The dominant mechanism is the piston type. There is great difference among results from different methods. Generally speaking, The largest recharge was usually estimated from the Darcian method for saturated zones and the smallest estimates were usually from the chloride mass balance (CMB) method. The difference is caused by two reasons. The first reason is that each method has its space/time scale. The second reason is the distinctions among the various terms used to refer to groundwater recharge. Generally, groundwater recharge rates are higher in the eastern part where the land surface is covered by permeable sand that is favorable to infiltration. The total recharge rate in the Ordos Plateau varies between 3.35 billon m~3/yr and 7.43 m~3/yr. The average values of recharge in this study as a whole ranges from 14 to 30 % of precipitation, which is acceptable for arid and semi-arid regions.The guidelines for selecting groundwater recharge methods were proposed with respect to study goals, groundwater recharge type/mechanism, available data, and time/fund. A comprehensive summarization was made for 16 methods in terms of recharge type, spatial scale, temperal scale, accuracy, computation complexity, data aqucisition and fund. Generally, methods based on surface water and groundwater have large spatial and temperal scale, suitable for groundwater resources management, while methods based on unsaturated zones have smaller spatial and temperal scale, suitable for contamination studies.