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UNESCO-IHE Institute for Water Education, Delft (2010)

Groundwater recharge estimation and base flow analysis for the mescoscale Chemoga catchment, Upper Blue Nile (Abbay) river basin, Ethiopia

Gurmu, M.G.

Titre : Groundwater recharge estimation and base flow analysis for the mescoscale Chemoga catchment, Upper Blue Nile (Abbay) river basin, Ethiopia

Auteur : Gurmu, M.G.

Etablissement de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2010

Résumé
Chemoga catchment (the study area) is one of the catchments found on the hillside of Choke mountain chain, in the Upper Blue Nile River (Abbay) basin (UBNRb), Ethiopia. The catchment has three rainy months (June, July and August). It is gauged at its outlet on Chemoga River and the river stage is recorded twice daily, by the Ministry of Water Resources. In dry season, springs and streams of the area either run dry or extremely reduce in discharges. This seeks well understanding of the hydrogeology of the area in relation to the annual groundwater recharge and base flow analysis, in large. Unfortunately, there is no data for both the groundwater recharge and base flow, except some data on long-term rainfall and stream discharge from the gauging station. This study has estimated annual groundwater recharge from precipitation in the study area using three different methods : Chloride Mass Balance (CMB), Base Flow Analysis (BFA) and Water Table Fluctuation (WTF).The base flow analysis with river hydrograph separation has been given a further focus. A combination of tracers (18O and Cl-) based analyses and a base flow analysis tool (Hysep2k6) was implemented as a method. Despite the fact that there was very limited time and money, extensive field works were conducted for ten weeks (from 04-July-2010 to 17-September-2010) in the study area, to collect the appropriate data. Water levels, using three mini-divers as well as manual methods, were monitored in different hand dug wells. Water samples were collected from rainfall, groundwater and river for isotope, Chloride and further hydro-chemical analyses. Moreover, field observations were carried out to understand the general hydrogeological setting of the study area. According to this study ; the groundwater recharge estimates from the CMB, WTF and BFA methods are 165 mm/a, 171 mm/a and 142 mm/a, respectively. So, the average annual groundwater recharge in the mesoscale Chemoga catchment is 159 (around 160) mm/a. This represents only 11% of the mean annual precipitation (1441 mm/a). Subtracting the average base flow from total runoff, which is 514 mm/a, the direct runoff from the direct precipitation is 355 (25%). Therefore, the total runoff is 36% of the mean annual precipitation. The remaining 64% is for ET. On the other hand, Base Flow Index (BFI) for the area has been estimated from a short term data to be 0.14 from BFA using Cl-as a tracer and 0.50 from BFA using 18O as a tracer. Using these BFI values and implementing in the Hysep2k6 tool, the BFA has given annual base flow estimations of 73 mm/a and 232 mm/a, respectively. Using the base flow analysis tool alone the estimation is 121 mm/a. Generally, it has been concluded that the study area has low annual groundwater recharge relative to the annual direct runoff.

Sujets  : groundwater recharge ; catchment areas ; water table ; aquifers ; Upper Blue Nile ; Ethiopia

Présentation

Page publiée le 7 janvier 2017, mise à jour le 14 octobre 2018