Abstract
This paper presents the effect of the threshold (S) used in the extraction of the stream network on the morphological characteristics, fractal dimensions, and the hydrologic response of arid basins. Four basins with areas vary between 252 and 2726 km (super 2) are considered as a sample in Makkah Al-Mukarramah region, Saudi Arabia (SA). Three different values of the threshold are used for each basin: 0.5 km (super 2) , 1.0 km (super 2) , and 2.0 km (super 2) . The methodology estimated the fractal dimensions of the mainstream and the stream network of the basins based on Horton-Strahler ratios and compared them with measurements from GIS software using the box-counting technique. Two theoretical procedures were used to compute the fractal dimensions. The fractal dimension of the mainstream (d2) is closer to the measured (d) when compared to (d1). Both d1 and d2 overestimated the d values. In contrast, the fractal dimension of the stream network (D1) is closer to the measured (D) values when compared to D2. Both D1 and D2 overestimated D. The values of d range between 1.05 and 1.13 while the D values range between 1.36 and 1.71. Based on the theory of geomorphological instantaneous unit hydrograph (GIUH), it has been shown that the GIUH varies depending on the threshold. The computed GIUH parameters for different thresholds showed that the values of the peak discharge (q (sub p) ) varies between 0.46 and 1.96 (1/hour) and the values of time to peak (t (sub p) ) varies between 0.28 and 1.11 hour. The comparison between observed and estimated storm hydrographs of the gauged Dara basin showed high variability with different thresholds. The relative error (RE) in peak discharge (Q (sub p) ) varies between 0 and 59%. While the RE in runoff volume varies between -7.9% and -58.4%. The RE in lag time (T (sub lag) ) varies between 6.3% and -275%. The GIUH based on gamma distribution seems not to be a representative model to the hydrographs in arid regions and other models should be investigated. A new approach has to be followed to develop a GIUH that does not depend on the threshold or finding an appropriate threshold that can be used for accurate flood predictions.