Evaluation of watershed scale changes in groundwater and soil moisture storage with the application of GRACE satellite imagery data

- The long-term change in water storage of the Tana Basin is studied by GRACE data.
- The basin shows a decreasing soil moisture and groundwater storage.
- This change in storage is not uniform all over the basin, however.
- Change in storage sites are delineated by combing GRACE and local aquifer potential.

As a hidden resource, our understanding and knowledge on the distribution of groundwater and its quantity is limited. In areas where such data is scarce, geo-observatory products such as the Gravity Recovery and Climate Experiment (GRACE) can provide an accurate estimate of changes in total terrestrial water storage at a coarser, regional resolution. However, in the Lake Tana basin, Ethiopia, groundwater is stored in localized aquifers, and its flow is controlled by physical variables (local geology, soil cover, slope, elevation difference, topographic index and drainage density) and climate (rainfall). The objective of this study is to utilize GRACE (CSR RL05) for the evaluation of spatial and temporal changes of soil moisture and groundwater storage at watershed scale. A geographic information system (GIS) based multi-layer analysis of the physical variables is implemented to delineate aquifer potential zones. The aquifer potential zone of the basin is combined with the GRACE (CSR RL05) based change in total terrestrial water storage data. Accordingly, the very high and high potential aquifer zone classes demarcate hot spots of high groundwater storage fluctuations recorded by the satellite data. The change in storage fluctuation of the 11-year monthly data, from 2003 to 2013, reveals that the total terrestrial water storage of the basin has declined by 18.37 cm. This is equivalent to the loss of 2.21 BCM of water within 11 years. Groundwater and soil moisture gains are a response of recharge, hence it is highly recommended to promote groundwater recharge, soil moisture harvesting, and sustainable use. On the other hand, the study highlights the possibility of application of the regional GRACE (CSR RL05) product for watershed-scale evaluations.