Geology and geomorphology maps were identified to be categorical maps

Geology and geomorphology maps were identified to be categorical maps. Categorical maps have known and definable boundaries. To assign scores to different classes of each factor map in categorical maps, each factor map was first converted to raster and a score column was added to its attribute table. Then, a suitable score corresponding to the influence of each class to groundwater recharge occurrence was assigned. The vector and raster maps are joined to link the attribute table, and the resulting vector map is then converted to raster with scores. Sandstones are typically permeable and porous; therefore, they allow percolation of water and can store those large quantities of water, thereby making them good aquifers, However, those of the Wilgerivier Formation form poor aquifers due to limited faulting; (Meulenbeld and Hattingh, 1999) hence, it was assigned a score of 1. Shales have very small interstitial spaces due to very small particle sizes, but can store large quantities of water; however, their transmission is limited due to very low permeability, therefore, making it an aquiclude. An aquiclude is an impervious unit through which virtually no water moves. The shales of the Ecca Group are very dense and should not be ignored as possible sources of groundwater. The borehole yields are between 0.5 to 2 l/s with a fractured or intergranular aquifer system; (GCS, 2006), thus, shale was assigned a score of 2. The diamictite of the Dwyka Group is massive, with little jointing and shows stratification in some places. It has very low hydraulic conductivity ranging from 10-11 to 10-12 m/s and shows no primary voids. The Dwyka diamictite forms an aquitard with very small yield quantities of water ranging from 0.5 to 2 l/s confined within narrow fractures and joints; (GCS, 2006), hence, it was assigned a score of 4. An aquitard is a saturated low permeability unit that can store groundwater but restrict the movement of groundwater. Diabase intrusion is highly fractured and weathered, yields appreciable quantities of water and therefore forms good aquifer. It was assigned a score of 10 (Figure 6.2.1). Lineament density, slope and drainage density maps were identified to be continuous maps. Continuous maps define a surface where each location is measured from a fixed registration point. For the continuous maps, each criterion map is reclassified into 10 classes using the reclassify tool under spatial analyst tool, the method of classification used is quantile and a score column was added as well as a suitable score. The classification method used to reclassify the continuous map is quantile method which assigns the same number of data values to each class, hence, there are no empty classes or classes with too few or too many values. The higher the drainage density, the higher the score given. Lineament densities range from 0 to 140.6 and the 10-percentile interval classes were assigned scores from 1 to 10 respectively in accordance to its relative contribution to groundwater recharge occurrence and storage The slope of the study area ranges from 0 to 70ยบ with 10-percentile interval classes assigned a score from 1 and 10 for the steepest to the flattest slope classes, respectively. This is due to gentle slopes being good groundwater recharge prospecting zones than steep slopes which favor surface runoff (Figure 6.3.2). Drainage density is the inverse of lineament density; hence, the drainage density class scores are opposite than those in lineament density. The drainage densities range from 0 to 252.4 and were assigned scores from 10 to 1 (Figure 6.3.3).


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