Estimating spatio-temporal variation of groundwater recharge from event-based precipitation and cumulative water-table fluctuation data

The water-table fluctuation (WTF) method is a simple and direct approach to estimate groundwater recharge, represented as follows: [R={Sy•(Δh/Δt)}/P], and here R, Sy, (Δh/Δt), and P are the recharge rate, the specific yield, the rate of water-table rises as time, and the amount of precipitation, respectively. However, the method is limited by two factors, the time-scale of data measurement and the specific yield of the unconfined aquifer. Thus, in this study, time-scale of precipitation and the water-table rises were changed into the event base and the summation of total rises corresponding to the event, respectively. Then, the relation between event precipitation and cumulative water-table rise per event becomes proportional as follows: ΣΔh/ΣP = (R/Sy). Hereby, we called (R/Sy) as the apparent recharge rate (Ra), which hold the meaning of spatial variation of aquifer materials intrinsically, and consequently being site specific values. The apparent recharge rate (Ra) was calculated at four sites with the National Groundwater Monitoring Wells and the automatic weather stations in Korea. 10 years of daily groundwater level and precipitation data were collected from 2005 to 2014. Considering the sites are under Monsoon climate conditions, all data were grouped separately for seasonal variations, and the number of precipitation events ranged from 68 to 130 depending on different seasons. The estimated Ra values shown significant seasonal and site-specific variations, as expected. The apparent recharge rate (Ra) method could provide simple-and-direct quantitative assessment of groundwater recharge without uncertain measurement of Sy in local scale for sustainable water-resource management.