Geomorphological and geophysical information system analysis of major rainwater-harvesting basins in Al-Baha region, Saudi Arabia

摘要

In the present study, a morphometric and geophysical analysis of three major rainwater-harvesting basins in Al-Baha province, Saudi Arabia, has been carried out using geographical information system (GIS) techniques and geophysical analysis. GIS techniques aimed to create a spatial database for three basins, and it was used to determine most of the morphometric parameters and stream attributes. The morphometric analysis revealed that the maximum elevation in the basins upper stream is about 2350, 2230, and 2450 m above sea level for Al-aqiq, Medhas, and Mathalmat basin, respectively. The maximum elevation decreases with steep slopes in the three basins toward the dams' site. The general flow direction of the streams in Al-aqiq basin goes from southwest to northeast following the main direction of the Arabian Shield structure; in Medhas basin, flow goes from south to north with wide flat areas in the downstream portion of the basin and near dam site and from southeast to west for Mathalamt basin. The morphometric parameters of Al-aqiq basin imply that the basin has the largest area with about 309 km(2); the perimeter is about 119 km and the total stream length is about 533 km. Medhas basin comes as a second largest area with 278 km(2); the perimeter is about 110 km and the total stream length is about 477 km. While Mathalamt basin has an area of about 220 km(2), the perimeter is about 104 km and the total stream length is about 374 km. The three basins have medium drainage density with a value of 1.72 km(-1) for Al-aqiq and 1.7 km(-1) for Medhas and Mathalamat basins. A relief ratio of 0.1 in Al-aqiq basin, 0.08 in Medhas basin, and 0.12 in Mathalamt basin was noticed. These ratios indicate a high permeable landscape in Al-aqiq and Mathalamt basins while there is a medium permeable landscape in Medhas basin. Al-aqiq and Mathalamt basins have a similar elongation and circularity of 0.73 and 1.85, and 0.73 and 1.8, respectively, which indicates that the two basins have fairly elongated shape, while elongation and circularity is estimated at 0.62 and 2.56 for Medhas basin, respectively. Drainage pattern of Al-aqiq and Medhas basins has a high dendritic shape, while Mathalamt basin has a simple dendritic shape. The geophysical survey was performed using vertical electrical sounding (VES) resistivity. The geophysical analysis reflected the presence of several different layers of resistance. High resistance was more than 2000 a"broken vertical bar at a 2-m depth due to dry river sediments in the dams' location while in other areas because of the presence of the basement rocks and climatic change. These results confirm the output of the geological study and give an indication of the state of recharge and rainwater harvesting in the three basins.