Using insights from water isotopes to improve simulation of surface water-groundwater interactions.

Understanding interactions between surface water (SW) and groundwater (GW) is challenging because of the lack of information available and the complexity of the processes involved. In this paper, SW-GW interactions are simulated using numerical modelling integrated with insights from water isotopes. Isotope analysis can be used to (i) determine the relative contribution from different sources of water into aquifer recharge and (ii) identify areas with high SW-GW interactions. The results, for the Shiraz catchment in Iran, show that (i) SWAT-MODFLOW has better performance than stand-alone SWAT (R improvement from 0.50 to 0.54) and (ii) SWAT-MODFLOW calibrated with insights from isotope data (SWAT-MODFLOW-ISO) has significantly better performance than SWAT-MODFLOW (R improvement from 0.54 to 0.68 and RMSE reduction from 1.67 to 1.33). This demonstrates that insights into SW-GW interactions that are revealed by isotopes can be used to improve hydrological modelling performance.