THERMAL ANOMALIES IN EASTERN ARABIA: IMPLICATION TO GROUNDWATER RECHARGE

The desert plain of the northern United Arab Emirates (UAE) appears in satellite images as a sandy surface that reflects the arid nature of the region. Thermal infrared (TIR) data reveal several cool patches in a particular segment of the plain. The recurring anomalies are the result of subsurface moisture, which is not depicted in optical images. The anomalies persist for up to one week in the TIR data after a rainfall in the adjacent highlands. To understand the cause of recurrence of such wet patches in only certain locations, potential water transport mechanisms were considered. The drainage basins and their associated channel networks, east of these locations, were computed using the GIS watershed delineation routines. Furthermore, regional fault zones were traced from ASTER satellite images and SRTM elevation data.

Analysis of both daily MODIS thermal data and Tropical Rainfall Measuring Mission (TRMM), of the period between 2000 and 2004, emphasized the potential of subsurface water transmission to the area following Monsoon rainfall events over the mountains to the east. Summer daytime is the best period for detecting subsurface moisture with MODIS TIR data where evaporation processes are efficient and enhance the contrast of surface temperatures, making cooler subsurface areas more pronounced. Sites of the most frequent subsurface moisture occurrences in the TIR data were identified and mapped using the GIS technique. The main drainage basin of Wadi Er-Rafiah coupled with the trace of the Hatta fault zone play a significant role as rainwater conduits between the mountain range and the adjacent sandy plain. Frequent occurrence of water transmission suggests recharge of groundwater aquifers in this region. Drilling of several wells at two of these sites confirmed the existence of highly productive groundwater aquifers.

Furthermore, geophysical surveys are being conducted in an attempt to define the extent of these aquifers. It is concluded that thermal data, which are able to detect near surface accumulation of rainwater, are useful in delineating sites of groundwater recharge in arid regions.