Scenarios Simulation of Biological Operations of Soil Conservation on Changing Erosion Using the RUSLE Model (Margav Watershed - Farim Section of Mazandaran Province)

Soil loss is a phenomenon that it causes the land degradation and due to the low productivity of agriculture, it has a negative impact not only on natural resources but also on people's livelihood. From an economic point of view, soil is one of the natural resources that is effective on the national power of a country and is evaluated as a major axis in economic power. The average annual soil erosion in Iran is about 24 t.ha-1 year-1. This figure is very significant and includes the economic value of more than hundreds of billions of Rials. Economic, social and political tensions, especially the tensions resulting from the migrations of villagers and nomads, are also largely related to this issue. On the other hand, the ever-increasing growth of the population in Iran, followed by the consumption of more food, has forced the farmers to use sloping lands that are sensitive to erosion, and this is one of the main reasons for the destruction of natural resources. Therefore, before any action, it is important to predict the intensity of soil erosion, to plan in determining land use, to examine the risks of erosion and to evaluate the effects of changes in land use. Changing land cover and land use affects many natural processes such as soil erosion and sediment production, floods, and physical and chemical properties of soil. The purpose of the present study is the investigating the different scenarios of biological operations on the amount of soil loss in Margav watershed of Mazandaran province.

The watershed area of Mergav is in the range of east longitude 53° 9′ 6″ to 53° 14′ 41″ and north latitude 36° 6′ 25″ to 36° 12′ 34″, with an area of 4401.7 hectares, upstream of Shahid Rajaei Sari Dam and in the functions of the city of Frame, Dodangeh The average annual rainfall of the basin is equal to 1.580 mm, and December and autumn are the rainiest months and seasons respectively, and August and summer are the least rainy months and seasons. The average temperature is also calculated as 11.7°c. Also, the physiographic parameters the minimum and maximum height above the sea level are 779 and 1787 meters respectively, the weighted average height is 1204.2 m, the weighted average slope is 35.79% and the concentration time is 2.7 hours. In the end, the climate of the area has been wet, cold and semi-humid according to Amberje and Dumartin methods. In order to implement the scenarios of changing the current land use of the basin to the use of pasture and medicinal plants, first, the current land use map of Mergao watershed, was drawn using field visits and Google Earth software in the GIS environment. Then, using the RUSLE model, the erosion of the entire basin was calculated. Since it was not logical to change the use of forest, residential and dam lands to other uses in the basin, a scenario with an area of 561.54 hectares was proposed to be removed from the simulation process and applied to other uses.

The obtained results from present study showed that the average soil loss in agricultural lands, gardens and degraded forest areas of Margav watershed using the RUSLE model was the rate of 25.4 t.ha-1.year-1. Also, the simulation results of the scenarios of biological operations and soil conservation using the RUSLE model showed that scenarios Pasture (first scenario), pasture and horticulture (second scenario) and cultivation of medicinal plants (third scenario), decreased the soil erosion with rates of 18.9, 8.9, 5.4 t.ha-1.year-1, respectively. According to the role of rain erosion, its amount increases from the north of the basin to the west and southwest of the basin, which is related to the decrease in rainfall and uneven spatial distribution of rainfall in the basin. The index of soil erodibility (K) in the use of destroyed forest and agriculture was more than other uses. Areas with higher slopes were more eroded, which shows the role of LS factor in this matter. In other words, the agricultural lands that were placed in the medium to high section for the LS factor make this section more sensitive to soil erosion. According to the map of the vegetation factor on the amount of erosion, the most sensitive areas to the erosion of degraded forest lands and then agriculture are those that increase or decrease the amount of erosion by considering the increase or decrease of the slope. Therefore, the most important factor that can reduce the negative effect of slope on erosion in the basin is vegetation.

The negative consequences of the erosion of the entire watershed of Mergav can be a gradual decrease in the fertility of the surface soil and arable agricultural lands and endanger people's livelihood and food security. In fact, considering the effective role of vegetation on soil erosion, the implementation and development of garden lands or the cultivation of medicinal plants according to the principles of strip cultivation in sloping lands in this basin is suggested. In fact, in line with conservation targeting, economic targeting can be considered in choosing the type of vegetation and choosing a plant species that can both prevent soil erosion and generate income from its cultivation. Therefore, according to the instructions for building a garden and cultivating rainfed medicinal plants in sloping lands in 2016 and considering the economic aspects, three scenarios were proposed for this basin, the best scenario is the conversion of agricultural lands to rainfed medicinal plants, which reduces erosion from 4 25.4 to 5.4 tons per hectare per year (scenario 3) because the cultivation of medicinal plants does not require tillage every year and compared to crops such as wheat, it can be cultivated according to the soil conservation model, which reduces soil erosion and cultivation costs.Therefore, the considering the social and economic issues based on sustainable development in the implementation of watershed operations can be an important factor in the better management of country watersheds.