Integrating finite-element-model and remote-sensing data into SWAT to estimate transit times of nitrate in groundwater

Abstract

This study integrates a water-table estimator into the hydrologic model SWAT (Soil and Water Assessment Tool) to simulate transit times of nitrate in groundwater for Fish River basin in Alabama, USA. Three water-table estimators were developed using 3D finite-element-modeling data and spatiotemporally dynamic remotely sensed data. Initially, the study estimates spatiodynamic groundwater levels by coupling the spatial percolation from SWAT with a finite element model. The study then improved the spatial resolution of Gravity Recovery and Climate Experiment (GRACE) remote sensing data by the inverse distance weighted technique and generated the spatial groundwater levels. Next, empirical equations describing the relationship between the soil moisture and groundwater levels were obtained based on the SWAT-modeled groundwater levels and remotely sensed soil moisture contents. The results showed that the correlations of the groundwater levels between real stationed data and those obtained from the finite element method, GRACE approximations and the soil-moisture function were 0.691, 0.335 and 0.652 respectively. The spatiodynamic-groundwater-level estimates obtained from the best performing integrated model (SWAT and finite element model) were used to predict transit times of nitrate in groundwater with the aid of remotely sensed data. The result indicates that the transit time increases progressively from 1 year at 0.2-m depth of soil layer to 23 years at 0.80-m depth of soil layer in the Fish River basin. The transit times were positively related to the amount of spatially contributed nitrate in the groundwater that flows into river reaches in the basin.