Constraining spatial variability in recharge and discharge in an arid environment through modeling carbon‐14 with improved boundary conditions

Abstract

AbstractCarbon‐14 (14C) has been widely used to estimate groundwater recharge rates in arid regions, and is increasingly being used as a tool to assist numerical model calibration. However, lack of knowledge on 14C inputs to groundwater potentially limits its reliability for constraining spatial variability in recharge. In this study, we use direct measurements of 14C in the unsaturated zone to develop a 14C input map for a regional scale unconfined aquifer in the Ti Tree Basin in central Australia. The map is used as a boundary condition for a 3‐D groundwater flow and solute transport model for the basin. The model is calibrated to both groundwater 14C activity and groundwater level, and calibration is achieved by varying recharge rates in 18 hydrogeological zones. We test the sensitivity of the calibration to both the 14C boundary condition, and the number or recharge zones used. The calibrated recharge rates help resolve the conceptual model for the basin, and demonstrate that spatially distributed discharge (through evapotranspiration) is an important part of the water balance. This approach demonstrates the importance of boundary conditions for 14C transport modeling (14C input activity), for improving estimates of spatial variability in recharge and discharge.