SPATIOTEMPORAL ANALYSIS OF GROUNDWATER STORAGE CHANGE, RECHARGE, AND QUALITY IN TANA SUB-BASIN, ETHIOPIA PH.D. DISSERTATION IN WATER RESOURCES ENGINEERING (GROUNDWATER ENGINEERIN

Abstract

Water resources' quantity and quality deterioration have posed a bigger panic for the sustainability of the resources in the global context attributed to anthropogenic and natural reasons. The Tana sub-basin, Ethiopia, is one of the regions where high competitions of water use for different demands has been taken place. Hence, evaluation of terrestrial water storage anomaly (TWSA) and drought events, groundwater storage anomaly (GWSA), recharge, and groundwater quality studies are indispensable for sustainable water resources management and the socioeconomic development of the region. To this end, the present dissertation is an endeavor to fill the lack of such investigations in the Tana sub-basin. The Gravity Recovery and Climate Experiment (GRACE) and Global Land Data Assimilation System (GLDAS) datasets were used to assess the long-term TWSA, drought incidences, and GWSA. The spatiotemporal recharge analysis was carried out usinga a recently coupled model (SWAT+gwflow) of surface water and groundwater in main gauged watersheds (Megech, Ribb, Gumara, and Gilgel Abbay) of the Tana sub-basin. A calibration process of the coupled model was carried out using a parameter estimation (PEST) tool was with monthly observed streamflow data until performance of the model was in a good acceptable range. Modified Mann-Kendal test and Sen's slope estimator were applied for trend analysis. The spatial and seasonal groundwater quality assessments were conducted for drinking and irrigation suitability using major physicochemical parameters of fourty samples, indices, and support vector machine (SVM). The trend results of TWSA and GWSA revealed a significant (p<0.05) increasing trend and gained 50.68 cm and 40.43 cm terrestrial water and groundwater thickness, respectively from 2003-2022. Nevertheless, both terrestrial and groundwater losses were recorded in the first decade of the study period (2003-2012) attributed to drought events identified by using weighted water storage deficit index (WWSDI) mainly in 2005, 2006, and 2009. The most sensitive parts of the study area to large fluctuations of GWSA were predominantly the nearby southern and eastern directions of Lake Tana. The average volumetric recharge estimates from 1997-2015 of Megech, Ribb, Gumara, and Gilgel Abbay were 16.575, 68.786, 105.641, and 469.903 million cubic meters (MCM), implied the sub-basin gained 661.904 MCM mean annual renewable water from these watersheds only. The spatial variation of the recharge rate showed the least to highest in Megech (northern) to Gilgel Abbay (southern) part of the sub basin, highlighting its consistencey with GWSA spatial variation. The groundwater quality anlysis according to drinking water quality index (DWQI) and irrigation water quality index (IWQI) revealed, in general, that the groundwater samples were suitable for drinking and irrigation purposes. However, higher nitrate and salinity level in some samples especially in the northern Lake Tana shore need a special attention for public and crop health, respectively. The findings of this dissertation may have far-reaching applications for policymaking plans and sustainable surface- groundwater resources managemen