Sustainable Uses of Surface Water and Groundwater Resources in the Bilate Sub-basin, Ethiopia

Abstract

Ethiopia's Bilate sub-basin is highly vulnerable due to a number of factors, including shifting land use and land cover, erratic rainfall patterns, frequent climate changes, and rising temperatures. These variables may have a significant impact on evapotranspiration, runoff, river flow conditions, groundwater recharge, storage, and water level fluctuations. Greenhouse gas emissions, agricultural expansion, poor land use management, and extensive deforestation contribute to significant effects on the water balance systems of the sub-basin. Previous studies and recent field observations indicate that groundwater levels in deep boreholes are declining. However, the local community lacks technical knowledge about perennial water resources and river flow conditions in ungauged catchments. The study also observes that predictions of the effects of climate change on actual evapotranspiration and groundwater recharge rate, as well as evaluations of groundwater storage anomalies, have not been investigated in this sub-basin. The main objective of this research is to raise awareness of the rainfall-runoff and flow conditions of the Bilate River in an ungauged catchment, actual evapotranspiration, groundwater recharge, and trace the groundwater storage anomalies under the effects of climate change and anthropogenic stresses by coupling Station-based hydro-climatic data, including precipitation, temperature, wind, humidity, and radiation, were collected from the Ethiopian National Meteorology Agency from 1989 to 2020, along with stream flow data and well completion report data from the Ethiopian Ministry of Water Resources and Energy. Spatial data, including soil texture, topography, and slope data, were also used for this study. Projected precipitation and temperature were acquired from the CORDEX Africa platform using RCP4.5 and RCP8.5 scenarios. Static groundwater water levels data from 16 wells were measured using a dip meter. GRACE/GRACE-FO terrestrial water storage data from JPL, CSR, and GFZ, GLDAS data including SMS P, Q, ET, and CWS from CLM, NOAH, and VIC portals were obtained. The study employed R-programs, Arc-GIS-10.3, ENVI-5.3, and ERDAS-2015 for land use land Page | xii Abera Shigutie Nannawo; August 2023 cover classification. Multiple linear regression-based regionalization was used to generate stream flow at ungauged catchments of the sub-basin, while the MIKE-11 NAM model was used for model parameter calibration and verification. The WetSpass-M model was employed for modeling water balance components. The artificial neural network-backpropagation algorithm was used for filling the gap between GRACE/GRACE-FO, as well as downscaling groundwater storage anomaly from 1ºx1º to 0.25ºx0.25º. The simulated result indicated a maximum mean annual inflow of 56.7 m3/sec from streams originating from the northern, northwestern, and southwestern highlands of the Bilate sub-basin. The central area of the sub basin had the lowest mean annual streamflow of 33.8 m3/sec, while the mean annual streamflow of ungauged catchments was estimated to be 39.1 m3/sec. The land use land cover analysis for a period between 1989 and 2019 indicated that the agricultural land increased by 27.06%, leading to a reduction in grasslands, shrubs, and forests by 14.87%, 9.41%, and 4.0%, respectively. The calibrated WetSpass-M model estimated the annual interception, groundwater recharge, surface runoff, and actual evapotranspiration to be 82.85 mm, 127.34 mm, 614.95 mm, and 517.59 mm, respectively. This study reveals that the sub-basin's actual evapotranspiration and groundwater recharge rate are expected to change significantly in the future. During the baseline period (1986-2015), the maximum annual actual evapotranspiration is predicted to range from 2573.2 to 2815.8 mm. However, under the RCP4.5 scenarios, the maximum annual actual evapotranspiration is expected to increase to a range of 2806.4-3019.2 mm and 3016.8-3212.1 mm for the mid-term (2041-2070) and long term (2071-2100) periods, respectively. Meanwhile, under RCP8.5 scenarios, actual evapotranspiration is estimated to range from 2859.3-3116 mm and 3168.7-3352.2 mm for the mid and long-term periods, respectively. During the baseline period, RCP4.5 and RCP8.5 for both mid-term and long-term periods, the study predicted that the maximum annual groundwater recharge would be 442.5 mm, 371.6 mm, 347.6 mm, 319.6 mm, and 327.41 mm, respectively. However, the maximum annual groundwater recharge may decrease by 83.3 mm to 138.7 mm in the RCP4.5 scenario during the years 2041-2070, while in the RCP8.5 scenario during the years 2071-2100, the maximum annual groundwater recharge may decrease by 26.1 mm to 72.3 mm. The study found that the maximum monthly positive value of groundwater recharge ranged from 20.80 mm in the southernmost basin to 39.49 mm in the central-southeastern basin throughout June to mid-October, while the maximum monthly negative value was recorded as -158 mm in March 2011. Despite facing challenges such as difficult terrain, remote location, and limited data availability from gauging stations, Page | xiii Abera Shigutie Nannawo; August 2023 the MIKE 11-NAM model and multiple linear regression-based regionalization revealed to be beneficial in the study. The study also found that WetSpass-M performed well in replicating the hydrological water balance in the Bilate sub-basin. The study predicted that climate change would cause a decrease in actual evapotranspiration between the 2041-2070 and 2071-2100 periods, except for maximum actual evapotranspiration, resulting in sporadic actual evapotranspiration due to a decline in precipitation and an increase in temperature. The study's results create awareness of climate change's impacts on actual evapotranspiration and groundwater recharge systems in the watershed. Furthermore, the study emphasized the importance of assessing groundwater sustainability challenges by evaluating groundwater storage anomalies with land surface models and satellite data. The study's findings provide valuable scientific knowledge for developing regulatory frameworks for sustainable groundwater and surface water management by considering both groundwater storage anomalies and surface water storage changes to address these challenges. The study recommends taking actions to reduce deforestation rates and restore vegetation cover, as well as implementing appropriate soil and water conservation measures in the sub-basin. Monitoring of stream flow and groundwater level heads throughout the year is important to understand the deficit or depletion of both surface water storage and groundwater storage during the dry season and the surplus amount of them during the wet season. Water planning and management strategies should consider the anticipated decline in annual surface runoff and groundwater recharge, as well as the rise in maximum actual evapotranspiration. Finally, the findings of this study help to plan and manage sustainable uses of surface water and groundwater resources in the Bilate sub-basin