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International Institute for Geo-Information Science and Earth Observation (ITC) 2005

A three dimensional ground water model of the aquifer around lake Naivasha area, Kenya

Yihdego, Y. (Yohannes)

Titre : A three dimensional ground water model of the aquifer around lake Naivasha area, Kenya

Auteur : Yihdego, Y. (Yohannes)

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 2005

Résumé
Lake Navaisha and aquifer surrounding the lake are important water resources in the area and are used extensively for irrigation and domestic water supplies. Continued or increased withdrawals from these sources have the potential to affect water levels in these aquifers. This thesis presents the design of a 3-D conceptual model of ground-water flow, the development and calibration of a numerical model for steady state groundwater simulation.Part of this study also includes updating the three-dimensional hydro-geologic framework model to serve as the foundation for the development of a steady-state regional ground-water flow model on the basis of integration of geology, hydro-geochemistry, geophysics, isotopic analysis and selection of mathematical boundary conditions. Groundwater flow in the Navaisha basin was modelled numerically with the ground water modelling system(GMS 5.0)and is used to simulate ground-water flow in the aquifers and lake–aquifer interaction. A four layer system was designed from which, the upper two layers represent the sediment aquifer, and the lower layers represent the volcanic aquifer. The regional model area was divided into grid blocks 300 meters areal space while the local and site model have 150m and 80m areal grid spacing respectively. The Navaisha lake is considered as an integral part of the ground water flow system since heads and flow patterns in surficial aquifers can be strongly influenced by the surface Navaisha lake that are are direct contact ,vertically and laterally with the aquifer. The lake was simulated by specifying a high hydraulic conductivity for lake-volume grid cells, the “high K” technique. The model was calibrated to static water level measurements in wells. Pilot points were used as a device for characterisation of parameter spatial variation in conjunction with the regularization in the ground water model calibration. Overall, the finite difference groundwater model result was comparable with measured well data .The simulated head and flow distributions mimic the important aspects of the flow system, such as magnitude and direction of the head contours. Also, simulated lake level varies in a manner determined by the water budget computed for the lake in the model grid. This process is crucial in making the model serve as simulator of the response of lake stage to hydraulic stresses applied to the aquifer and variation in climatic condition, a capability desired by resources manager. The sensitivity of lake level computed using high–K method was tested to the choice of K2/K1, where K1 is the hydraulic conductivity of the lake nodes and K1 is the hydraulic conductivity of the aquifer. The results indicate that values of K2/K1 less than 1000 produced a significant head differential across the lake (computing four wells at the lake surface), which could result in erroneous calculations of seepage to and from the lake. A value of K2/K1 greater than 1000 but less than 1,000,000 gave acceptable solution, produced no gradient across the lake

Version intégrale (ITC)

Page publiée le 28 janvier 2018