Improving punctual groundwater recharge estimate modeling by including stable water isotopes

Groundwater recharge (GWR) is a key parameter for sustainable groundwater management and for predicting future groundwater availability. Its importance is increasing as global environmental and anthropogenic pressures intensify stress on groundwater resources. This study aims to improve point-scale estimates of GWR by constraining unsaturated-zone models at three sites that differ in vegetation and soil cover. Model optimization was performed using volumetric water content (θ) alone and θ in combination with soil water stable isotopes (δ¹⁸O and δ²H).

The newly implemented isotope module in HYDRUS-1D, which accounts for isotope fractionation, was successfully applied under semi-arid conditions. Strong monsoon dynamics caused large temporal variations in observed θ, which were challenging for the model to reproduce accurately, whereas vertical profiles of isotope concentrations were simulated more precisely. Estimated recharge rates over a 236-day period ranged from <1 cm to 31 cm for models optimized with θ and isotopes, and from <1 cm to 15 cm for models optimized with θ alone. Despite similar soil classes and short spatial distances, GWR exhibited pronounced heterogeneity across the study area in Tamil Nadu, South India. These findings highlight the use of multiple data types for model calibration, with isotope data providing additional constraints on the simulated recharge rates.