Improving the Reliability of Groundwater Recharge Estimates by Modelling Stable Water Isotope Transport in Soils

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ABSTRACT Groundwater levels are declining in various regions worldwide. A thorough understanding of groundwater recharge is crucial for ensuring the sustainable use of groundwater resources. In this study, three 1D models of the unsaturated and saturated zones were developed to estimate groundwater recharge. Model parameters were optimised with a partial optimisation framework using volumetric water content ( θ ) alone as well as θ combined with soil water isotope (δ 18 O and δ 2 H) measurements collected at regular depth intervals. The results highlight the advantage of including soil water isotopes, in addition to θ , during the optimisation process. Although extreme wet conditions during the dry and rainy seasons were difficult to simulate with respect to θ , isotope signatures over depth were successfully reproduced, demonstrating their usefulness in constraining the space of possible solutions and thus increasing model reliability. The study also demonstrates the applicability of the new HYDRUS‐1D isotope module, which accounts for isotope fractionation, under semi‐arid climatic conditions. Groundwater recharge was found to be highly heterogeneous even across small areas, depending on soil properties and vegetation cover at the study site in Tamil Nadu, South India. Recharge rates ranged from < 1 to 31 cm and from < 1 cm to 15 cm over a period of 236 days for models optimised with θ in combination with isotopes and with θ alone, respectively.

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