M.R. Ansari; F. Soleimani; A. Ahmadi
Abstract
Introduction: Soil erosion is the most widespread form of soil degradation jeopardizing food security worldwide. In Iran, gully erosion is important because about 90% of the country has arid and semi-arid climates and rainfall is not adequately distributed. In such conditions, the absence or lack of ...
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Introduction: Soil erosion is the most widespread form of soil degradation jeopardizing food security worldwide. In Iran, gully erosion is important because about 90% of the country has arid and semi-arid climates and rainfall is not adequately distributed. In such conditions, the absence or lack of vegetation cover with increasing runoff causes more than 2 billion tons of soil losses annually. Therefore, conducting basic and applied researches on soil erosion via different methods for comprehensive management of natural resources is emphasized. The prerequisite for all kinds of erosion, such as gully erosion, is the prediction of the risk of gully formation in different areas susceptible to erosion. Materials and Methods: The study area is located in Ghaleh Gorg watershed which sub-basin of Shahid Modarres that large part of these farmlands has been destroyed by gully erosion. The purpose of this study was to apply fuzzy logic operators for gully erosion zoning. In this research, six effective parameters on soil erosion including mean weight diameter of aggregate, sodium adsorption ratio, salinity, percent of fine sand, silt and clay were determined according to standard laboratory methods. After re-classification, standardization of prepared layers was carried out by the Fuzzy method. Hence, Fuzzy-based layers were integrated using operators of Fuzzy algebraic sum, Fuzzy algebraic production and Fuzzy gamma with 0.2, 0.5, 0.8, 0.9 values and the obtained results were evaluated. The validation of prepared maps was done based on two methods of map matching percentage of areas with very high and high sensitivity with gullies map of the study area and quality sum index (Qs). Results and Discussion: The Fuzzy results of raster layers showed the relative accumulation of silt, clay and fine sand grains in the middle to western parts of the region due to leaching and soil aggregation of Aghajari formation with a high slope of >20%. The deposition of this sediments was with 0-5% slope in the middle and western part of the basin. Results of layer integration indicated the fact that the fuzzy summation and multiplication method are not suitable approaches for final mapping because of their high increasing and decreasing effects, respectively. About Fuzzy Gamma operator 0.2, the results revealed that about 17.07% of the area was in the high and very high-risk zone and 67.07% of the area was in the low risk zone. In Fuzzy Gamma 0.5, about 31.16% of the area was in high risk and 55.38% in low risk zone. And only 60.38 percent of the gullies was in the high-risk area. Thus, these both operators 0.2 and 0.5 cannot be an acceptable method for preparing the final fuzzy map. The results of gamma operator 0.8 showed that about 43.21% of the area was in high and very high risk classes and 42.45% of the area was in low and very low risk classes. In the gamma operator 0.9, about 60.92% of the area was in high and very high risk zone and 17.1% of the area was in low and very low-risk zone. Also, regarding the distribution range of gullies, 94.93% of gullies was in high and very high risk classes, which is more acceptable and better than gamma 0.8. According to the obtained results, Fuzzy gamma 0.9 with 94.93 matching percentage of areas containing very high and high sensitivity and maximum quality sum index (0.73) among different operators was selected as the best method for preparing Fuzzy map in the study area. Conclusion: According to the results of this study and its comparison with field observations, effective factors contributing to the initiation and development of gully erosion were sensitivity of the geological formations, soil texture type, salinity and alkalinity and non-implementation of biological and biomechanical operations to the soil and vegetation cover restoration. Around 3855 hectares (60.38%) out of 6327.5 hectares of the total studied area were at high and very high erosion risk. Furthermore, 2056 hectares (94.93%) out of the 2166 hectares of the gullies area were at high and very high risk of erosion, denoting the high accuracy of the final map. Among the available methods, Fuzzy Gamma 0.9 with the highest overlap between the area of high and very high-risk classes of the gully map (94.93%), and the highest qualitative sum index (0.73), was selected as the best fuzzy method for zoning in the study area.
Freidoon Soleimani; Majid Soufi; Aziz Arsham
Abstract
Introduction: Khuzestan province with 4.6 million hectares area has suitable agricultural lands that gully erosion destroyed some parts of them. One of the major basins in which much of its agricultural land threatened by gully erosion is located in Modarres watershed of Shushtar. Gully erosion progression ...
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Introduction: Khuzestan province with 4.6 million hectares area has suitable agricultural lands that gully erosion destroyed some parts of them. One of the major basins in which much of its agricultural land threatened by gully erosion is located in Modarres watershed of Shushtar. Gully erosion progression in this basin causing the loss of farmland and thus the farmers are leaving farming and consequently would lead to poverty and migration to urban areas and social consequences. Therefore, presenting a model to determine the factors that impact on gully erosion and erosion hazard zonation for conserving and stability of the land need to research and investigate on gullies of Khuzestan province.
Materials and Methods: Study area is located in Shahid Modares basin that large parts of these farm lands destroyed by gully erosion. The aim of this project is to identify the main factors affecting on linear gully development and modeling them. For this purpose, distribution maps of climate and the gully was determined. Then a Watershed from each climate class with the highest losses was selected. At last Haddam watershed with warm-arid climate and Sharif watershed with warm-semi arid climate were selected. And 15 gullies were selected from each watershed then some parameters studied and measured in these gullies. Finally relationship between the length and volume development of gully with watershed characteristics, Physio-chemical soil properties, the percentage of vegetation cover, pebbles, bare soil, litter, slope and upstream area of head cuts, rainfall and different land use area a 20-year period (1993-2012) were analyzed. Modeling of the gully development was done using multivariate regression.
Results and Discussion: The results showed that in Haddam watershed the gully number A17 had most elongation (78.8 meters) while the gully number B2 had lowest elongation (3.8 meters). In Sharif watershed the gully number B3 had the most elongation (108.1 meters) while the gully number B4 had the lowest elongation (1993-2012). Gully clustering results showed that gullies of Haddam’s watershed were in two clusters and gullies of Sharif’s watershed in the three clusters. The final equation for the development of Haddams gullies in the cluster one shows the amount of development in these gullies depends on three variables including upstream area of initial point (X4), the percentage of bare soil (X6) and slope of initial point (X2). At cluster two the amount of progress in these gullies depends on silt percent (X18) and PH(X23). The final equation for the development of Sharif’s gullies in the cluster one shows the amount of progress in these gullies depends on upstream area of head cut(X3). At cluster two the amount of development depends on two variables upstream area of initial point (X4) and Sodium content (X22). In Haddam’s watershed more than 80 percent of land use is covered by rain fed and irrigated land. Unlike Sharif’s watershed that more than 80 percent of pasture land form. In addition, measurement of upstream watershed area reflects very poor range condition. In this region, due to the slight slope in gully head cut the surface runoff has not velocity but also due to lack of canopy cover in gullies’ watershed and low soil permeability because of silt frequency and high salinity in the soil profile increases the risk of gully development. In this research, slope of head cut upstream and slope of initiates point is less than 5 percent at total gullies that are correlated with development of gully elongation. However, due to poverty and lack of vegetation cover conditions provided for gully linear growth.
Conclusions: Gully erosion is a form of progressive erosion that allocated the largest portion of soil degradation in different climates to itself. Gullies behavior against its spreading and growth is affected by different environmental factors. In this research, available data analysis in Haddam’s watershed showed that the most important factors on the gully development were watershed characteristics on gully headcut (area and slope), the fine geological formation (silt), acidity and high soil salinity. These factors along with poor vegetation cover and agricultural land had created an excellent platform for gully elongation so that over a 20 year period have at least 3.8meter per year and a maximum elongation 78.8 m. In Sharif ‘s watershed showed that the most important factors on gully development were upstream area of head cut, upstream area of initiate pointed amount of sodium which caused to 10.1 and 108.1 meters minimum and maximum linear development respectively.
