The Weighted Index Overlay Analysis

The Weighted Index Overlay Analysis (WIOA) is one of the multi-criteria decision-making tools used to assigned weights and scores to each factor and classes of each factor respectively, to determine groundwater recharge potential zones (Danee and Santhi, 2014; Jasrotia et al., 2007 and Nag and Ghosh, 2013). Individual factors maps with weight and class scores were integrated using the raster calculator in the spatial analyst tool in ArcGIS software. The integrated map was then classified into; excellent, good, moderate, poor and very poor groundwater recharge potential zones and lastly correlated and validated with the field groundwater data obtained from the article written by Meulenbeld & Hattingh, 1999 and from Department of Water Affairs (Figure 6.4). The output groundwater recharge potential zones map was validated with the field data related to the yield potentiality of different state-drilled wells or boreholes found within the study area The classification method used for the output groundwater recharge potential zones map is quantile. This classifier was found suitable to represent the generated data since there are no empty classes or classes with too few or too many values, features are grouped in equal numbers in each class, thereby producing all-inclusive, reliable and conspicuous results. It was observed that most of the boreholes are sited on excellent to good groundwater recharge potential zones where the geology is mainly sandstone, diamictite and close to contact zones with diabase intrusions. The rest of the boreholes are sited on poor to very poor groundwater recharge potential zones with a sandstone rock mass. According to Hattingh, 1996, the sedimentary rocks of the Wilgerivier Formation makes poor aquifers whereas, the cracks and fissures in intrusive rocks form the main aquifers, hence, groundwater occurs in a fractured aquifer system. The boreholes close to diabase intrusions make good aquifers. Borehole yields are limited, especially in sedimentary rocks, they are below 0.5 l/s, however, those sited on faults and fractures in intrusive rocks, can yield higher than 3 l/s. Typical borehole depth ranges between 40 and 120 m while the average range of depth of water level is between 10 to more than 40 m below ground level (mbgl) (Meulenbeld and Hattingh, 1999; DWA, 2011). As seen in figure 6.5.2, the groundwater occurrence ranges from 0.1 to 0.5 l/s indicating the presence of groundwater resources in the study area, even though they occur in small quantities. This shows a good correlation with the produced groundwater recharge potential map of the study area; hence, further exploration may be done to delineate possible groundwater resources on excellent groundwater recharge potential zones.Delineation of groundwater recharge potential zones on the eastern part of the Middelburg Basin, Mpumalanga using Weighted Index Overlay Analysis with the aid of Remote Sensing and GIS techniques is found to be effective. Satellite images and conventional data were used to prepare the factor maps of geology, lineament density, drainage density, geomorphology and slope. The factor maps were assigned weights and scores to classes and then integrated to produce the output groundwater recharge potential zones map. From the results of the groundwater recharge potential zones map, Middelburg Basin has been classified into five different groundwater recharge potential zones; “excellent”, “good”, “moderate” “poor” and “very poor”. For validation of delineated groundwater recharge potential zones, field groundwater data from existing boreholes from an article written by Hatting and Meulenbeld, 1999 was used as reference. The results revealed good correlation with respect to derived groundwater recharge potential zones. To ensure sustainable groundwater utilization in the study area, the groundwater recharge potential map can be used as a guide for (a) conducting detailed geological and geophysical exploration of groundwater potential and (b) future planning of location for artificial recharge project in the study area.


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