Comparative analysis of estimation of slope-length gradient (LS) factor for entire Afghanistan

Abstract

Slope length gradient (LS) is one of the crucial factors in the Universal Soil Loss Equations (USLE, RUSLE). This study aimed at estimating the slope-length and slope-steepness (LS) factor for the entire watersheds of Afghanistan by using three different methods, namely; (1) LS-TOOLMFD (Method 1); (2) The Method of Equations (Method 2); and (3) The approach of Moore and Burch (Method 3). The first method uses the digital elevation model (DEM) in the ASCII format, and the other two methods use the DEM in the spatial domain. The results show that the LS-factor of the study area ranges from 0.01 to 44.31, with a mean of 5.24 and standard deviation of 6.95, according to Method 1; 0.03 to 163.49, with a mean of 9.6 and standard deviation of 13.58, according to Method 2; and 0 to 3985, with a mean of 7.16 and standard deviation of 29.7, according to Method 3. The study reveals that Methods 1 and 2 are more appropriate than Method 3 because Method 3 yields high LS-factor values close to or at streamlines located near mountainous regions. The highest LS values are found to be in the northeast, north, and central regions of Afghanistan, which is consistent with the high mountains and deep valley geomorphology, indicating that these regions are particularly vulnerable to soil erosion by rainfall-runoff processes. The sediment delivery ratio (SDR) for the Upper-Helmand River Basin (Upper-HRB) is also estimated by the RUSLE, employing the LS factors produced by the three methods. The results revealed that the average annual soil loss is found to be, respectively, 9.3, 18.2, and 11.1 (ton/ha/year) by using the three methods, corresponding to SDR of 23.5%, 12.1%, and 19.9%. Abbreviations: ABD: Asian Development Bank; ASCII: American Standard Code for Information Interchange; Bsh: cold semiarid steppes; Bsk: cold semiarid steppes; Bwh: warm and cold deserts; Bwk: warm and cold deserts; CMS: Convention on Migratory Species; Csa: humid subtropical; Csb: Mediterranean; D: humid continental; DEM: Digital Elevation Model; ET: extreme tundra; GCS: Geographical Coordinate System; GIS: Geographic Information System; GUI: Graphically User Interface; Ha: Hectare; HMRB: Harirod-Murghab River Basin; HRB: Helmand River Basin; KRB: Kabul River Basin; LS: Slope–Length and Slope–Steepness; Masl: meters above sea level; MFD: Multiple-Flow Direction; NRB: Northern River Basin; PARB: Panj-Amu Darya River Basin; RUSLE: Revised Universal Soil Loss Equations; SDR: Sediment Delivery Ratio; SFD: Single–Flow Direction; SRTM: Shuttle Radar Topography Mission; SY: Sediment Yield; UCA: Unit Contributing Area; USGS: United States Geological-Survey; USLE: Universal Soil Loss Equations