The identification of precipitation amount effect with a water isotope-enabled threshold model in vadose zone: a case study in Ordos Plateau

Accurate estimation of precipitation amount recharged to the groundwater is essential to the water balance calculation of a groundwater system in regional scale. In the torrid zones, the proportion of heavier water isotopes in precipitation presents a negative correlation with the precipitation amount. The physical processes explicating this so-called amount effect have not been well understood. This paper focuses on the δD relationships of local precipitation and groundwater to explain the isotopic variability in Ordos Plateau, China. In order to better understand the physical process interpreting the amount effect, a numerical threshold model in the vadose zone to relate the ineffective precipitation amount and isotopic abundance of precipitation and groundwater was supposed. By comparing the modelled δD with that in groundwater, ineffective precipitation, a value of 6.12 mm was obtained in Ordos Plateau. Thus, the potential groundwater recharge from the precipitation in the region can be deduced. The methodology proposed in this study will provide useful insights into the estimation of precipitation contribution to groundwater in other similar areas in the world.