Introducing the Apparent Recharge Rate to Present Spatio-temporal Variability of Groundwater Recharge and Groundwater Sustainability

Groundwater sustainability of a certain watershed assumes the balance of recharge and discharge from natural and anthropogenic processes. Estimation of groundwater recharge has been developed long time in Hydrological Science; however, most techniques are limited by their own constraints in temporal and spatial variation of recharge. In this study, we suggest a new method to estimate groundwater recharge, containing intrinsically the variation of specific yield (Sy), using relations between the amount of precipitation for events and the corresponding rises of groundwater level from almost 10-year monitoring data. This approach is basically similar to the Water Table Fluctuation (WTF) method, however, different from the WTF in using the amount of precipitation of an event as a cumulative amount of daily precipitation during the event. The corresponding water-level rise accounts for the maximum change starting from the precipitation. Thus, there is no fixed time unit such as one-day or one-month etc., instead temporal steps are defined as an independent event no matter how long it continues. This relation provides the relation (R/Sy) as recharge rate (R) divided by specific yield (Sy) of the monitoring points. Sy presents the site specific characteristics of recharge processes, and R presenting water-level rises due to precipitation events are various depending upon rainy-and-dry seasons. Thus, the apparent recharge rate, R/Sy, could indicate the variation of groundwater recharge of the specific time and location unit. The rate was validated with 10-year data, and applied to the small watershed in Korea to estimate groundwater sustainability. This approach of apparent recharge rate is direct and clear because it is obtained from the field monitoring data, and more applicable to the field conditions because it contains the natural variability of specific yield of surface materials.