A GIS-based flow model for groundwater resources management in ...

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Adelana ch004.tex 21/7/2008 14: 9 Page 4848 Applied groundwater studies in AfricaThe geological database was extensively used in the modeling process to calculate themodel layer bottom, top, and thickness of the aquifer structure and the formulation ofparameters and boundaries of the numerical groundwater model. GIS tools were usedfor: the interpolation of layer surfaces; the interpolation of hydraulic conductivities; theformulation of outer and inner boundaries; and the calculation and spatial description ofgroundwater recharge areas.The interpolation tools in GIS are needed to get a spatial distribution of model parameters.For geological purposes statistical and geostatistical analyses of the data have to be carriedout before the interpolation. Hydrogeological data with a normal distribution (e.g. elevationdata or measured groundwater levels) can be interpolated with kriging and geostatistics.The map of the basement top elevation (Figure 1) represents one example of an outputfrom the created GIS. Data sets without normal distributions (e.g. hydraulic conductivities)first have to be log-transformed to obtain a normal distribution and then analyzed andinterpolated with geostatistical tools. In GIS the data can be pre- and post-processed,proved and corrected.GIS was also conveniently used to control the surfaces of the layers of the numericalgroundwater model so that there are no intersections. Although ArcGIS (ESRI, 2005) isnot fully 3D capable (only 2.5D) it is sufficient to prepare the 3D structural model forthe numerical groundwater models. The GIS database was also used for the calibration ofthe model. It is a useful tool to visualize the deviation between modeled and interpolatedmeasured water levels as well as statistical calculations.4 NUMERICAL GROUNDWATER FLOW MODEL4.1 Model setupTwo 3D numerical modeling systems were chosen as basic tools for the simulation ofthe Nubian Sandstone System: A finite difference modeling system (Modflow with thePre- and Postprocessor Visual Modflow 3.0 (Waterloo Hydrogeologic Inc., 2002)) and afinite element modeling system Feflow 5.0 (Diersch, 2003). The grid covered an area of2.2 million km 2 and the modeled area about 1.65 million km 2 . For the finite differencesolution a grid of 10 × 10 km was used. In the finite element model the area of the trianglesranged from 10 km 2 to 100 km 2 . The main reasons for the choice of a 3D modeling systemis to model:• the large distance flow from Chad to the Qattara Depression;• the climatic change from wet to semi-arid to the present arid conditions during the last25,000 years, which is the flow time of this large distance flow;• the possibilities to build a transport model with implementation of slices with verticallydifferentiated flow and transport parameters.The model was designed as a closed system. In this way, reliable no flow boundary couldbe identified at the outcrops of the basement. As the saltwater-freshwater interface seemsto be highly stable, it is also implemented as a no flow boundary (Neumann condition).All groundwater flow, recharge and discharge occurred within the model. Groundwaterrecharge was implemented on the top layer. Grid cells of the Nile River were considered asconstant head cells with spatially varying time constant heads in this long-term simulation(Dirichlet condition).
Adelana ch004.tex 21/7/2008 14: 9 Page 49A GIS-based flow model for groundwater resources management 49Figure 3. Cross sections of calibrated hydraulic conductivities (m/s) for the Nubian Aquifer Systemas they are implemented in the numerical groundwater model. Original data based on drillinginformation from 1960–2005 (after Sefelnasr et al., 2006).The hydraulic conductivity was evaluated for the entire area using the available drillinginformation up to year 2001 in the newly developed areas, e.g. Tushca (southwest ofAswan)and East Oweinat (Dahab et al., 2003), as well as the results published inThorweihe & Heinl(1999) and shown in the cross sections in Figure 3. Based on the variability of hydraulic conductivityvalues and the general stratigraphic setting, the Nubian Sandstone Sequence wasdivided into 10 layers. The layers 2 to 9 were further divided into three layers each to ensurea representative value of hydraulic conductivity and other flow parameters particularly in alocal-scale solute transport model (not reported here). For the same reason, the bottom andtop layer were also divided into two layers each. The confined part of the system in the northwas considered as ‘leaky aquifer’, allowing vertical water exchange between the Nubian
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