عنوان مقاله [English]
Assessing the impacts of climatic and land use changes on rates of soil erosion by water is the objective of many national and international research projects (e.g., Van Oost et al., 2000 Nearing., 2001 Vanacker et al., 2003 Chaplot et al., 2005). However, over the last decades, most of the related studies have mainly focused on sheet and rill erosion processes operating at the plot scale. Field-based evidence suggests that sheet and rill erosion as measured on runoff plots are not realistic indicators of total catchment erosion nor do they indicate satisfactorily the redistribution of eroded soil within a field. It is through gully erosion that a large fraction of soil eroded within a field or catchment is redistributed and delivered to water courses. Indeed, most sediment produced by interrill and rill erosion in uplands is often deposited at the foot of hill slopes or in depressions within the landscape and therefore does not reach the river channel. Hence, other sediment-generating processes in catchments such as gully or channel erosion must play an important role in the production of sediments which are transported by rivers and which cause reservoir infilling. Gully erosion is defined as the erosion process whereby runoff water accumulates and often recurs in narrow channels and, over short periods, removes the soil from this narrow area to considerable depths ( Poesen, 2003). Maragheh catchment is a sub-catchments of the Mazlaghan river in Markazy Province due to loose alluvial deposits, sparse vegetation and topographic conditions are severely affected by gully erosion. This penomena not only caused the loss of soil and degradation of agricultural and pasture land but also aggravated offsite effects of water erosion (e.g. flooding, pollution) with transferring runoff and sediment from uplands to valley bottoms and permanent channels. This research has been performed to investigate effective factors on gully erosion and designing gully erosion susceptibility zonation map in Maragheh catchment by using Fuzzy Multi- attribute Decision Making.
Maragheh catchment, covering about 8892 ha, is located 70 Km away from west Saveh between 35Ë 19' and 35Ë 00' latitude North and between 49Ë 41' and 35Ë 00' longitude East. The study area is situated at an altitude of 1608 to 2987 meters from sea level and has a cold semi-arid climate.
Materials and Methods
In this study, the factors of litology, slope, slope aspect, elevation, distance from road, distance from river, land use, soil, precipitation and land suitability were studied for zoning of gully erosion susceptibility. These factors were obtained by using topographic maps (scale 1:25000), geologic map (scale 1:100000), aerial photographs (scale 1: 40000), IRS image (2003), data rain gauge stations, results of laboratory analysis of soil samples, and GPS tool was also used to record field observation. For production and analysis of maps, we have used software Ilwis 3.3 and ArcGIS 9.3. In the current research, zoning of gully erosion susceptibility has been performed by using Fuzzy Multi Attribute Decision Making via Maximum-Minimum operator. In this method, decision making is done in two steps. The first step, the weights of classes of effective factors was computed by way of Frequency Ratio and its normalization. After that membership functions were defined for each class of various factors. The second step, weight of each effective factor was obtained through Analytical Hierarchy Process (AHP). Then, weights obtained through AHP were used in determination of that membership functions on gully erosion. Determination of membership functions and fuzzy analysis were performed by using the software Matlab7.1. Then, output of software computed susceptibility for each pixcel in case study map transformed to ILWIS software environment and gully erosion susceptibility zonation map was made by using fuzzy Triangular and Gaussian membership function.
Results and Discussion
The results of this study showed that very high susceptibility areas were affected by litology of loose alluvial sediments of Quaternary (Qt1), soil of sandy loam or sandy clay loam, land type Plateaus and Upper Terraces, land use of poor range as the main factors. Also, this areas were affected by other factors such as slope 15-30% , maximum daily rainfall with return of two years 23/7- 26/4mm, distance from rivers 0-200m, slope aspect of the west and south and altitude of 2000-2400m.
In this research, gully erosion susceptibility zoning of maps were obtained in four classes by using Fuzzy Multi Attribute Decision Making (MADM) and via the Maximum-Minimum operator composition. Index Quality Sum shows that type function has no much effect on results of zoning. Solving problems via Fuzzy Multi Attribute Decision Making were done in two stages. In the first step, membership function was defined for each of the classes of factors by normalized Frequency Ratio. In the second step, priority of the factors assigned by using AHP model and then membership function was determined for the factors based on AHP model. Therefore, this method has great capability of preparing gully erosion susceptibility map zonation. As in Maximum operator numbers are trending toward zero, therefore a greater numbers of pixels are locating in very sensitivity class to gully erosion. For this reason, this operator has very little sensitivity in zoning. In Minimum operator, numbers are trending toward one. Therefore, a greater number of pixels are locating in low sensitivity class to gully erosion and has low sensitivity in zoning. In this study, for adjusting of very high sensitivity of Minimum operator and low accuracy of Maximum operator, we were used the Maximum- Minimum operator composition.