Introduction
The soil erosion is one of the effective factors in reducing agricultural production and threatening food security, in which the soil fertile layer is destroyed. The risk of soil erosion is different depending on the watershed topography, soil characteristics, conditions of local weather, land use and methods of land management, which the land use change can be exacerbated. Land use changes are one of the important factors in changing hydrological flow and soil erosion. As a result of land use change and vegetation destruction, a large part of the rainfall has turned into runoff, which in addition to soil loss and sediment production, causes a lot of financial and human losses caused by floods. Therefore, the knowledge from erosion process in different time periods can help in determination of the distribution pattern of erosion and its effects over time. So that, the decision-makers, while monitoring and evaluating the conducted measures about the soil and water conservation take the appropriate decisions for the current and future conditions of watersheds. Therefore, in the present study, the effects of land use change on soil erosion have been evaluated using the RUSLE model in Chardavel watershed of Ilam province for the years 2005 and 2020.
Materials and methods
In the present study, the effective factors in the Revised Universal Soil Loss Equation were prepared in the form of raster layer. These factors include the rainfall erosivity, length and degree of slope, vegetation management, soil erodibility and conservation operations. The factor of rainfall erosivity was calculated for each of the stations inside and outside the watershed and the erosivity layer was prepared in the GIS environment, by using the Inverse Distance Weighting method. For the length and degree of slope were used from the digital elevation model map and cumulative flow map. The layer of soil erodibility was obtained using the soil texture map. The NDVI index was calculated using Landsat 8 of OLI sensor (2020) and satellite images of five ETM Landsat satellite images (2005) and using bands of 4 and 5 (for Landsat 8) and bands of 3 and 4 (for Landsat 5) in the remote sensing software, and then, the management factor was obtained in years of 2005 and 2020. In order to obtain the factor of conservation operation, the land use map was used for years of 2005 and 2020, and according to each land use was determined the its conservation value. In the next step, the amount of annual soil loss was estimated by multiplying these layers in the GIS environment. Finally, to determine the amount of erosion in each of land uses, the layers of erosion and land use placed on top of each other to the years of 2005 and 2020.
Results and discussion
The results showed that the rainfall erosivity was from 220.5 to 309.8 MJ mm ha-1 h-1, the length and degree of slope were 0 – 403.7 at the pixel level, the vegetation management was 0.16 - 0.57 (in year of 2005) and 0.18 - 0.61 (in year of 2020), the soil erodibility was 0.03 - 0.06 t. ha h ha-1 MJ-1 mm-1 and protection operation was 0-1 for both years of 2005 and 2020. The average erosion was 13.23 t ha-1. year-1 for year of 2005 and this value was 20.13 t ha-1 year-1 for year of 2020. The barren land uses due to the lack of vegetation produces the highest amount of erosion, so that its amount has increased years of 2003 to 2005 from 0.65 to 3.8 t, and residential areas due to the presence insulation, asphalt and proper drainage have the more role of transferring runoff. In agricultural areas, it has been exposed to erosion during this period due to not following the principles of proper cultivation and usage of proper tools.
Conclusion
Considering the extensive effects of soil erosion on various aspects of human life, its continuous monitoring and evaluation can help to make the right decisions to confronting with this crisis. In the present study, the RUSLE model was used to investigate the effects of land use change on soil erosion for the years of 2005 and 2020 in Chardavol watershed. In general, the results showed that the erosion amount has increased in the studied watershed from 2005 to 2020, which it indicates that there is the dangerous situation for the watershed in the future. Therefore, it is suggested to use soil and water conservation operations, especially in the steep areas and also in the upstream areas of the watershed. Also, according to the change of land use in Chardavol watershed, the measurements should be taken for soil and water conservation in areas with high erosion potential. It also seems that the adoption of appropriate laws to prevent land use change can help reduce soil erosion. But, because the most important part of the watershed are its residents, these laws should be in such a way that while attracting people's participation and satisfaction, it guarantees their stable income. Therefore, the development of management scenarios in the region will help natural resource managers in prioritizing and usage of appropriate management methods in controlling soil erosion in high-risk areas of Chardavol watershed. |
