Approximately 94% of the United Arab Emirates (UAE) is desert and the water demand continuously increases with the progressive evolution of human activity and land development. It is, therefore, necessary to develop a sustainable groundwater management plan to accurately utilise this important resource, which, in turn, requires delineation of groundwater potential zones
There are several methods such as geological, hydrogeological, geophysical and remote sensing techniques, which are employed to delineate groundwater potential zones. Integration of remote sensing and GIS offers the advantage of having access to large coverage, even in inaccessible areas. These methods are rapid and cost-effective tools in producing valuable data on geological and geomorphological parameters that helps in deciphering groundwater potential zones.
The case looks at how groundwater potential zones were mapped in Al Dhaid, UAE. As a first step of groundwater potential mapping, five essential geological, terrain and hydrology parameters derived from satellite images were prepared and verified in the field and against previous studies. These parameters were geological fractures, drainage network, slope, relief, and convergence index.
Subsequently, weights and scores were assigned based on level of contribution towards groundwater potentiality and spatial distribution of groundwater wells. The geological fractures, which control terrain and hydrological parameters, were given the highest level of contribution value (22%) and the topographic convergence was assigned the least level of contribution value (18%). In the same context, each parameter class was assigned a score value. The score value was calculated by multiplying the parameter weight value by the number of groundwater wells. For example, if the weight value of geological fractures is 0.22 and the total number of groundwater wells within <75m from geological fractures, the score value of this class (<75m) will be 14.3.
Frequency ratio analysis was performed on the resultant groundwater potential zones map and locations of groundwater well and farms. Relative frequency of groundwater wells and farm locations was calculated from the ratio of the percentage of total number of the pixel and area of each groundwater potential zone. Finally, the groundwater potential zones map was validated by comparing with the previous hydrological maps of the study area and locations of groundwater wells and farms and hydrological data collected from groundwater wells.
The map of groundwater potential zones calculated by integrating remote sensing data and GIS is shown in Figure 1. The map comprises five classes from very high groundwater potential zone at the foot of Oman Mountain and alluvial plain to very low groundwater potential zone in hard rocks of Oman Mountain. The map showed that the areas having high and very high probability of groundwater potential are located where geological fractures are highly intersected and stream order is higher.
Fig 1: Groundwater potential zones map of the study area. Blue and black points highlight groundwater locations
Frequency ratio analysis was performed on the groundwater potential zones map and spatial distribution of groundwater wells. Frequency ratio was calculated by dividing the percentages of each zone and the total number of its pixels in the produced groundwater potential map. The frequency ratio increases from very low to very high since the very high potential class generally tends to more groundwater accumulation than other classes of the study area. The ratio values are gradually increased from very low potential class to very high potential class.
In order to estimate the accuracy of the obtained results, the produced groundwater potential zones map was checked by comparing with previous hydrological maps of the study area and groundwater well and farm locations. The result of validation showed that the high and very high zones in the computed map using the proposed method coincide with areas of confirmed and expected groundwater potential zones. In the same context, the low and very low potential zones agree well with previous hydrological maps of the study area. Additionally, the result of spatial analysis showed that the highest number of groundwater wells is located within high and very high potential zones suggesting a positive correlation. Out of 85 groundwater wells, 71 were found to be located within high, very high and moderate classes.
The estimated transmissivity and storability of groundwater wells were found to be a higher in the alluvial plain, along wadi courses and at the foot of the Oman Mountain than elsewhere in the study. When considering these validation results, the produced groundwater potential zones map was considered as a satisfactory agreement between the computed result and the drilled groundwater wells and agricultural areas.
In comparison to the previous geological and hydrological maps, the integrated approach provides confidence in the spatial association between terrain, geological and hydrological parameters and groundwater accumulation. The produced groundwater potential zones map permits a better understanding of the hydrological setting of the Al Dhaid area and can be employed by hydrologists to detect new zones may be carry groundwater potential in shallow and deep aquifer with time and cost effectively.
Integration of remote sensing and GIS has been applied to integrate five essential geological, hydrological and geomorphometric parameters, which control groundwater movement and accumulation. The spatial distribution of geomorphological, geological and hydrological features have been precisely mapped and analyzed for the first time. These results have allowed a better understanding of the hydrological setting. They have also emphasized the usefulness of employing integrated data sets extracted from various remote sensing data for groundwater exploration. The integrated approach shows that the locations of groundwater potential can be mapped over multiple scales with timely and cost effective. The proposed approach can be applied in arid and semi-arid region where the potentiality of groundwater is needed.