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UNESCO-IHE Institute for Water Education, Delft (2011)

Analysis of lake-groundwater interaction, Awassa Lake, Ethiopia

Eromo, M.C.

Titre : Analysis of lake-groundwater interaction, Awassa Lake, Ethiopia

Auteur : Eromo, M.C.

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

Grade : Master of Science (MS) 2011

Résumé
Understanding the interactions between the lake and groundwater is vital for proper planning, management and protection of total water resources. Lake Awassa is used for fishing, tourism, and has also significant social and ecological functions. Since the last three decades, the water level in the lake has increased and over-flooded adjacent residential areas. Thus, in this research, lake water balance computation and steady-state numerical modelling for the whole lake Awassa catchment were used to analyze the causes of the lake water level rising and the interaction between the lake and the groundwater system. The lake water balance calculation and analysis of annual water balance components show that the annual lake water level is positively proportional to the river inflow and the annual rainfall in the previous year, and negatively proportional to evaporation and net groundwater discharge. The lake water levels fluctuate with alternating wet and dry years, higher lake water levels in wet years, and lower lake water levels in dry years. However, the continuous increasing trend of the lake water levels is caused mainly by the increase of river inflow. The increase of river inflow might be attributed to the disappearance of a swap in the basin and the decrease of evaporation since mid 1990s. The inflow components to the lake are, in the order of the importance, river inflow and direct precipitation. The outflow components are mainly evaporation and discharge to groundwater. The net groundwater flow calculated as the residual term of the lake water balance indicates that on average there is more lake water discharge to groundwater. Since there is no surface outlet of the lake, and the lake is a fresh water lake, discharge to groundwater controls the salinity in the lake. The advantage of numerical simulation of the interaction between the lake and groundwater can reveal where groundwater discharges to the lake and where the lake discharges to groundwater. A steady state groundwater model consisting of two aquifer layers was constructed and used to quantify water exchange between the lake and groundwater. The model estimates that the main recharge is from precipitation infiltration, followed by the lake discharge and river leakage. The main groundwater discharges include the subsurface outflow to the downstream basin, discharge to the lake, evapotranspiration, abstraction, and discharge to the river and springs. Groundwater discharges to the lake in the south, east and west sides, whereas the lake discharges to groundwater in the north. Therefore, the lake Awassa is a flow-through lake in relation to groundwater, which is the mechanism to maintain a freshwater lake. However, the net groundwater flow is to the lake. The lake water balance and the groundwater model gave conflicting results of the net groundwater flow to the lake. The lake water balance indicates the net groundwater flow is to the groundwater while the groundwater model shows the net flow to the lake.

Sujets  : lakes water balance groundwater flow groundwater modelling Ethiopia

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Page publiée le 5 avril 2021