Integrating in-situ data with satellite-derived products to assess surface-groundwater interactions and sustainability of groundwater resources in semi-arid environment

In arid and semi-arid regions, water scarcity is nowadays a primary challenge, because of continuously increasing spatio-temporal rainfall variability and high evapotranspiration, both implying a decline of freshwater resources. Moreover, it is expected that this problem will worsen with the ongoing climate change. It is therefore important an in-depth investigations of the spatio-temporal surface-groundwater (SW-GW) interactions and sustainability of groundwater resources, which can be optimally realized through application of an integrated hydrological models (IHMs). This study proposes an approach to integrate satellite-derived products with in-situ measurements to assess the spatio-temporal SW-GW interactions and sustainability of groundwater resources in the data scarce Zamra catchment (ZC), northern Ethiopia. The research approach consists of four study chapters (chapter2-5). Chapter 2 focusses on the integration of daily satellite rainfall with in-situ rainfall. The study demonstrated that the Geographically Weighted Regression approach, could substantially reduce the daily biases between satellite and in-situ rainfall products in topographically complex areas, indicating further validation and improvement can be achieved by increasing in-situ gauge network and eventually considering more accuracy-effective explanatory variables. Chapter 3 focusses on the spatio-temporal estimate of interception loss (EI), characterized by various land use types and the overall estimated EI demonstrated high spatial and temporal variability, ranging on annual basis from zero at bare lands to 30% in forested areas. Chapter 4 focusses on derivation of PET as the product of RS-based, FAO-Penman-Monteith ETo and NDVI-based land use land cover factor (Kc). The NDVI-based Kc demonstrated high spatio-temporal variability, ranging from ~0.15 (bare land) to ~1.4 (forest), which resulted in higher PET values than the bias-corrected RS-based ETo in locations where Kc >1 and vice versa (i.e. in locations where Kc <1), which underlined substantial difference between ETo and PET. Chapter 5 focusses on IHM assessment of the spatio-temporal variability of SW-GW interactions and on sustainability of groundwater resources in the hydrologically complex ZC. RS approaches to estimate driving forces (Chapter 2-4) were applied as inputs of the MODFLOW 6 IHM (MOD6-IHM). The calibrated MOD6-IHM, showed high spatio-temporal water fluxes variability in the ZC, largely influenced by high spatio-temporal rainfall variability, in which the only source of water input. Throughout the MOD6-IHM solution, that rainfall (P) was partitioned into the two dominant sinks, evapotranspiration (ET=53.2% of P) and stream outflow (q= 46% of P). As the net recharge (1.6% of P) constrains groundwater resources sustainability, the issue of sustainability of ZC groundwater resources is crucial considering their future utilization for agricultural purposes.