An Analysis of Stable Water Isotope distributions across Namibia: Rainfall and Groundwater Isoscapes

Abstract:

Global precipitation isoscapes are an important toolset that aids our understanding of global hydrologic cycles. The Global Network for Isotopes in precipitation (GNIP) database is a starting point for modelling global isoscapes; however, the network has deficiencies that affect the accuracy of the isoscapes in several regions. These deficiencies include low or non-existent data coverage in regions of strong hydrological interest (e.g., drylands), discontinuous data and poor correlation between temperature and isotope distributions in the tropics. To evaluate the efficacy of global isoscape interpretation in the regional context and investigate precipitation to groundwater recharge to groundwater, in this study, we downscaled a global isoscape model (Regional Cluster Water Isotope Prediction (RCWIP)) to the geographic extent of Namibia and compared the predicted versus observed ones. Furthermore, we generated precipitation and groundwater isoscapes based on local isotopic studies and compared the data-driven precipitation isoscape with RCWIP model. Results indicated that there was no correlation between the RCWIP model values and the observed values. The local meteoric water line for Namibia determined from the RCWIP model reflects the global meteoric water line suggesting that the model reflects global trends missing out on local variations. This raises the question of how relevant are these global isoscapes to inadequately covered or represented regions such as drylands. Extending the isoscape concept to groundwater over the same area and juxtaposition to the rainfall isoscapes we can determine areas of potential recharge as groundwater is the most important water resource in these dryland systems. Although this does not quantify the available groundwater resources, it nonetheless enables us to make interpretations on whether aquifers contain modern or paleo waters and the areas of potential recharge. These interpretations are important for sustainable urban planning.