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Groundwater Flow Modelling assisted by GIS and RS Techniques (Raya Valley - Ethiopia)

Hagos, Mohammedsultan Abdella (2010) Groundwater Flow Modelling assisted by GIS and RS Techniques (Raya Valley - Ethiopia).

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Abstract:The study focused on modelling of groundwater flow system in the Tigray Region of northern Ethiopia, assisted by GIS and RS techniques. The study area - Raya Valley- is a large fertile valley enclosed by high mountain ranges on the edge of the Afar Rift system. This part of the Tigray Region has abundant resources with respect to groundwater, fertile land, livestock potential and agro-climatic conditions. Ambitious project plans have been made to implement a pressurized irrigation project of about 18,000 hectare using the area’s groundwater potential. The aquifer is characterized by unconsolidated material, which includes gravel and coarse sand with some pebbles and cobbles as colluvial deposits at the foot of the escarpment. The valley aquifer is replenished by direct recharge from rainfall and by runoff from the ephemeral river systems flowing from the western mountains. Several perennial springs are found in the western part of the valley. The groundwater flow system in the unconsolidated deposits of the Raya Valley was modelled using PMWIN (Chiang et al., 1998) as pre- and post-processor for MODFLOW (McDonald and Harbaugh, 1988). The model was developed for steady-state conditions in an unconfined aquifer, represented by a single layer with a constant thickness of 100m. The grid cell size of the model was taken 250x250m and 17600 of active cells were used to represent the entire study area which is 1085 km2. Model area and the elevations of top layer were delineated by the ASTER DEM optimisation and use of the topographic maps. Aquifer properties were assigned based on the analysis of pumping test data. The Chloride Mass Balance Method (CMB) was employed to estimate the recharge. Optimised parameters (hydraulic conductivity and recharge) were spatially distributed over the model area. The model was calibrated using hydraulic head observations from more than 80 wells. A combination of trial and error and automatic methods were performed to calibrate the model using the observed hydraulic head until the root mean square error (RMSE) reached about 11m. The sensitivity of the calibrated model was tested by systematically changing the input parameters individually. The summary of the entire domain water budget showed that the model calculated inflow and outflow terms are in balance. The outflow was modelled through MODFLOW’s drain and groundwater evapotranspiraton packages. The outflow components of the water budget of the Raya Valley were estimated as 92mmy-1 and 22mmy-1 for drainage and groundwater evapotranspiration respectively. While the model calibrated total recharge rate of the valley was 114mmy-1. Beside this, annual groundwater recharge of the two sub-basins in the valley which are Mohoni and Alamata sub-basins using the inverse modeling were estimated as 66mmy-1 and 170mmy-1 respectively. All the available measured hydraulic heads were used for calibration. Therefore, the model was calibrated but not verified. Hence, the results obtained here should not be interpreted as a perfect simulation rather as system response within fairly realistic model input parameters. Finally, some recommendations were made for management and monitoring of the valley’s aquifer. Key Words: Raya Valley, Alamata, Mohoni, groundwater modelling, recharge, irrigation
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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