Modeling of water flow and heat transport in the vadose zone: Numerical demonstration of variability of local groundwater recharge in response to monsoon rainfall in Korea

Abstract

The rainfall of Korea in the summer monsoon period occupies more than 50% of the annual precipitation in most areas, and thus groundwater recharge to shallow aquifers is dominantly controlled by the amount and the pattern of monsoon precipitation. This paper presents two numerical models that demonstrate linear relationships between precipitation and recharge. First, a simple heat transport model employing a lumped parameter approach is presented for estimating two lumped parameters related to water flux and thermal diffusivity in the vadose zone. The model determines the parameters by a simple optimization process that minimizes the root-mean-square error between simulated and measured temperatures. The model is applied to 22-year time series data of soil temperatures measured at a synoptic station of Korea. The impact of monsoon precipitation on the thermal regime is clearly reflected in the simulated results by illustrating a linear relationship between precipitation and the water flux in the vadose zone. Secondly, an infiltration model is presented for analyzing variability of precipitation recharge in relation to the monsoon rainfall. The model simulates the unsaturated flow from time series data of precipitation and pan evaporation, assuming immediate removal of surface ponding, a linear relationship between the evaporation rate and the soil water content, and a static water table. Numerical simulations were performed for three soil textural groups by using 20-year meteorological data. The results demonstrate that the annual recharge is linearly proportional to the annual precipitation with varying degrees of the correlation coefficient depending on soil types. Sensitivity analyses show that the uncertainties in evaporation-related model parameters significantly affect the model results with controlling tradeoff between recharge and evaporation estimates.