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International Institute for Geo-Information Science and Earth Observation (ITC) 2009

Groundwater resource assessment of the Aynalem wellfield through transient flow modelling, Mekele, Ethiopia

Teferi, G.G. (Gebrehaweria Gebrekirstos)

Titre : Groundwater resource assessment of the Aynalem wellfield through transient flow modelling, Mekele, Ethiopia

Auteur : Teferi, G.G. (Gebrehaweria Gebrekirstos)

Etablissement de soutenance : University of Twente International Institute for Geo-Information Science and Earth Observation (ITC)

Grade : Master of Science in Geo-Information Science and Earth Observation 2009

Résumé
The study focused on assessment of groundwater resources and its spatial and temporal variability of the Aynalem wellfield using transient state flow modelling. Most of the water for Mekele town is from the groundwater of the Aynalem wellfield that is facing rapid lowering due to increased abstraction for domestic and industrial water demand. Despite of this, the effect of abstraction with respect to recharge is not well assessed and the aquifer storativity and groundwater flow pattern was not known. The groundwater recharge of the system was mainly from direct rainfall. WATBAL was employed to estimate the recharge. The WATBAL model was calibrated with river flow and groundwater level by minimizing the difference between the observed and simulated values. The groundwater recharge was estimated 32 mm/year (5% of the annual rainfall). The finite difference schematization of the modelled area was discretized into a uniform squared grid size 250 by 250m, comprising 32 rows and 82 columns while 1705 numbers of active cells are used to represent the entire study area which is approximately 106 square kilometres in extent. Later the grid was refined into 125 by 125 cell size. The transient model was simulated with MODFLW 5.3 for a period of five years (2003 to 2007) for one layer aquifer of 50 meter thickness. The initial aquifer parameters (transmissivity and hydraulic conductivity) and initial hydraulic heads were used from the steady state model. The storage coefficient was obtained from the pumping test analysis. The model was calibrated with trial and error in two steps. First the storage coefficient was adjusted then the groundwater recharge was adjusted by comparing the observed and simulated well hydrograph. In MODFLOW average groundwater recharge was simulated 30 mm/year (5% the annual rainfall). The water budget of the transient model shows that the aquifer storage declines some 17 mm/year and that the groundwater flow to the Aynalem River and outflow in the western outlet of the catchment decrease respectively some 68% and 14% for the simulation period. After four years of groundwater recharge without abstraction the wellfield recovers to the natural situation. On the other hand after abstraction of groundwater with 18000m 3 /day for ten years, the groundwater declines with an average 53m.

Version intégrale (ITC)

Page publiée le 30 janvier 2018