Characterization of Groundwater- Lakewater Interaction in Lake Hawassa Basin.
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Date
2014-11
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Addis Ababa University
Abstract
Being at the center of booming economic and social development, Lake Hawassa Basin is one of the most prone to pollution lake waters in Ethiopia. Shallow groundwater table near by the lake suggests there is a direct link through unconfined aquifer between the lake water and groundwater. Thus the lake water could affect or affected by any anthropogenic and/or natural activities done on the nearby groundwater. Lake Hawassa level rising has also pronounce as a problem of part of Hawassa city and surrounding residence is the main issue which need, investigation to alleviate the problem. This research has made its objective in analysis of the lake water and groundwater interaction. Since groundwater recharge area might extend well over the location where the lake water and groundwater interfaces, the study extends its area of analysis to the natural hydrologic (surface water) catching boundary of the lake.
The Hawassa lake basin is part of the central main rift of Ethiopia which is topographically closed catchment cover an area of about 1440 km2. For understanding of Lake Basin ground water behavior a numerical groundwater flow model was used for steady state and three dimensional flow conditions. The basin have been discretized in to 13500 triangular prism elements of average 500 m edge lengths. The whole study area was divided in six hydrogeological settings. The Lake water and the perennial rivers within the basin are considered as constant head boundaries while the entire basin boundary is treated as a groundwater divide (Hydraulic no flow boundary). To make sure that the model parameters (Hydraulic conductivities in three dimensions) are appropriate to the area, a number of trail values of these parameters are given to the model to predict the 68 inventoried groundwater tables within the basin. The parameters appropriateness is evaluated based on Mean error, Root Mean Squared Error and r2 statistics, the result obtained were -2.73m, 12.33m and 0.755, respectively. From the data quality used in the model, the model extent and hydrogeological regions used in the model the result obtained was taken as acceptable. The hydraulic conductivity obtained for the whole region is in the range of 2.5 x 10-8 and 1 x 10-6 m/s.
The calibrated finite element numerical flow model (TAGSAC model) of this research was simulated for different lake water levels observed during 1969 and 2013. The model
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simulated groundwater flow direction is towards the lowest elevation of the basin (Lake Hawassa). The lake water level variation has significant effect on the groundwater on its western part than locations in the other directions. Larger groundwater abstraction on the western part than the other parts in the basin will bring about the same variation in the lake water level. Thus it can be suggested that lake water level variation affects or affected significantly by any water resources management made on the its western boundaries. From pollution point of view the reverse is true, i.e. any pollution that takes place in the eastern part of the lake will bring about larger pollution at a faster rate to the lake environment. This is because of the higher gradient, there by higher groundwater velocity in this location. Unfortunately this part of the lake (eastern part) is susceptible to pollution due to the vast, economic and social development. The presence of the Hawassa city is a clear threat to the lake water pollution not only from surface water pollution but also from groundwater pollution point of view.
Within the analysis period (1969 – 2013), the region which was not affected by the lake water variation is again located in the eastern part of the lake. This region has shown no change in the groundwater level. No change in groundwater level at a location imply that this location either receives water from some other location or is itself a source. The location being at higher elevation in the lake basin makes it the major recharge source of the basin. The site which includes the highest development (social and industrial) makes the biggest threat to groundwater thereby lake water pollution.
Finally, for any groundwater and/or lake water planning work, the model created in this research should be applied with caution, considering all drawback and limitations related to model input parameters.
Key Words
Groundwater-Lake water interaction; Groundwater model; Hawassa city; Hawassa Lake; Numerical modelling; TAGSAC;
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Keywords
Groundwater-Lake water interaction, Groundwater model, Hawassa city, Hawassa Lake, Numerical modelling, TAGSAC