Abstract
Runoff coefficients are widely used as a diagnostic variable of runoff generation in process studies and as an important input parameter in hydrologic design. Many regions of Saudi Arabia do not have sufficient historical records and the detailed runoff information needed for physically based distribution models. The US Department of Agriculture, Natural Resources Conservation Service Curve Number (USDA-NRCS-CN) method was used in this study for determining the curve number and runoff depth for Asir region, Saudi Arabia. Runoff curve number was determined based on the factors of hydrologic soil group, land use and land cover, and slope gradient in the Asir region of Saudi Arabia using the geographic information system (GIS). The region was not subjected before for any previous studies concerning runoff generation or rainwater harvesting by using GIS and remote sensing. ArcGIS 9.3 and the global positioning system were used to develop the soil map on the basis of samples collected during a field survey. Soil textures were identified and used to build a map of the hydrological soil groups. Unsupervised and supervised classification of Landsat images was used to generate land use and land cover maps. The land cover distribution in the study area revealed that the largest class in the area, barren and sparsely vegetated land, occupied 62,832.38 km (super 2) of the study area (more than 80% of the total area), followed by shrubland (15,212.66 km (super 2) ), cropland and pasture (52.34 km (super 2) ), mixed shrubland/grassland (40.89 km (super 2) ), and grassland (12.27 km (super 2) ). Built-up land occupied only 11.45 km (super 2) . The slope map for the Asir region was generated from a 15-m digital elevation model. The GIS technique was used to develop the CN for the region based on the Soil Conservation Service method. The annual runoff depth was derived from the annual rainfall and CN per pixel using the raster calculator tool in ArcGIS. The rainfall distribution in the study area showed the wise identification of suitable sites for rainwater harvesting, as most of the constructed dams were located in the areas with higher rainfall. The analysis also revealed that the annual runoff for the study area ranged from 27 to 69% of the total rainfall, with variation as low as 80 to 300 mm/year. This is the highest amount of runoff that can be generated in Saudi Arabia. The runoff volume was calculated for the entire region using the previously developed runoff depth map and basin areas. The results showed that the runoff volume (M3) in the Asir region varied from as low as 237,000 m (super 3) to a maximum of 2,140,000 m (super 3) ; this result revealed a significant yearly amount of runoff that can be harvested for any use. Moreover, the calculated runoff depth in the area agreed with the actual dam capacities for the existing dams in the Asir region. This approach can be applied in other regions in Saudi Arabia for rainwater harvesting and groundwater recharge. Copyright 2015 Saudi Society for Geosciences