Document Type : Research Paper
Authors
Associate professor, Department of Environment, Faculty of Natural Resources, Semnan Universiy, Semnan, Iran
Abstract
Introduction
Human activities are at the core of global environmental change and Humans play a key role in global warming, land degradation, air and water pollution, rising sea levels, eroding the ozone layer, extensive deforestation, and acidification of the oceans. Soil erosion and degradation is a natural phenomenon altering the relief of the landscape. Erosion is often capable of causing several on-site and off-site impacts. Erosion and soil loss are common in hilly areas, but their severity will vary depending on the geoenvironmental factors including, Steep sloping, geological characteristics, vegetation and climatic factors making it more vulnerable to erosion. One of he most important kind of erosion is badland erosion. The term of badlands currently refers to areas of unconsolidated sediment with little or no vegetation, which are useless for agriculture because of their intensely dissected landscape. Badland erosion is observed mostly in arid and semi-arid regions, and the interaction of precipitation with geological materials is responsible for the development of badlands in arid and semi-arid regions. Because soil erosion is a complicated process that is influenced by the properties of the land surface and the soil as well as by environmental factors, quantitatively accurate forecasts of soil erosion and susceptibility mapping are challenging. The main goal of this research is to map badland erosion suscepibility in Firozkuh watershed using frequency ratio model.
Materials and methods
Firozkuh watershed was selected as study area because in this watershed, badlands are the most important contributors to soil erosion because of the condition of climatic, hydrologic, topographic, and reduced vegetation conditions, and as well as presence of susceptible soil and geology formations in this region. The first step in this research is to prepare distribution map of the badlands and determine their location on the map. This was done using Google Earth imagery and field surveys. The maps of condiioning factors were prepared from different sources and entered into the GIS environment. Digital Elevation Model (DEM) map with the cellsize of 30 meter was prepared using the elevation points and lines in the topographic maps prepared by the National Cartography Center of Iran. Slope aspect, slope degree, plan curvature, TWI and elevation classes maps was creaed using DEM map in ArcGIS10.3 and SAGA-GIS environment. The geology map of the watershed was extracted from the geologic map of Iran with the scale of 1:100000. River and road maps were extracted from 1:25000 topographic map and the distance from these features was calculated in ArcGIS10.3 environment. The land use of Firozkuh watershed was created from LANDSAT 8 images of year 2020 using a synthetic method. To map soil characteristics, 30 samples were taken from depth of 0-30 centimeter and analyzed in the laboratory. Aaverage annual rainfall map was developed using rainfall data from meteorological stations. After classifying conditioning factors maps, the weight of each map was calculated using the frequency ration model. In the next step, by combining the weights, the final badland erosion susceptibility map was prepared. The ROC curve and the area under the curve were used to assess the accuracy of the frequency ratio models.
Results and discussion
The relationship between badland erosion and conditioning factors was investigated using the frequency ratio model. The results showed that the highest weight of the frequency ratio is related to the elevation class of 1710 to 2286 meters, rainfall 400 to 550, slope more than 35, northwest aspect, distance less than 1150 meters from drainage network, marl, limestone and shale formations, ranglands, Convex and concave slopes, clay 25 to 33%, silt 27 to 35%, hydrological group C, soil depth 57 to 120 cm, pH 7.6 to 1.8, TWI class 6 to 11. Accuracy assessmen of the freency ratio model was done using ROC and area under this curve. The area under the ROC curve was 0.71 that showed frequency ratio model is acceptable for badland erosion susceptibility mapping in the Firozkuh watershed. Despite its simplicity, the freqency ratio model provides acceptable results due to the creation of a logical connection between the badlands and conditioning factors. Other studies, including investigating the potential of underground water, landslide susceptibility maping, and the vulnerability to floods, have also been conducted with this model, and its accuracy has been confirmed.
Conclusions
Because of topographical, climatic and geological conditions, the badland erosion is a dominant phenomenon in the Firozkuh watershed. In this research, badland erosion susceptibility map was prepared using the frequency ratio model. Accuracy assessment showed that frequency ratio is a suitable model for badland erosion susceptibility maping in this watershed. The results showed that about 50% of this region has high and very high susceptibility to the badland erosion, so it is necessary to pay attention to this phenomenon and prepare a susceptibility map.
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