Investigating Land Use and Slope Effects on Soil Properties, Runoff and Sediment Using Rainfall Simulator Case Study of Kechik Watershed in Golestan Province

Soil erosion and its issues are among the most important environmental challenges. In many areas, soil erosion affected valuable natural resources and soil fertility. The recognition of the factors affecting runoff and soil erosion, and the determination of their issues are essential for soil and water management conducive to sustainable development. The assessment and analysis of the changes associated with the soil characteristics; runoff and sediment lead to proper planning and management.
THEORETICAL FRAMEWORK: Review of former studies shows that the land use change is one of the main effective factors regarding the erosion intensity whose effect is sometimes more than the rainfall intensity and slope. For quantifying the effects of land use change and slope on runoff, sediment and soil nutrients, the present study was carried out with the aim of comparing the chemical and physical properties of soil in different land uses and slope classes, and of investigating their effects on runoff and sediment.
This research was accomplished in Kechik watershed on loess formation located in Maravetape township, Golestan province.Rainfall simulations were done in 4-square-meter plots on five land use including forest (Natural forest and Reforestation), rangeland, farmland (Watermelon and Harvested Wheat) and four-slope classes including 3-12%, 12-18%, 18-25% and 25-40%. Based on the three-nested design, 96 simulation experiments and samplings were done. Output runoff and sediment and soil samples were collected. Nutrient amount (Phosphorus, Nitrogen, and Organic matter), Cation Exchange Capacity, Antecedent moisture content and Soil density were measured at the laboratory.
The result showed a change in the type of land use (natural forest, forestation, rangeland, watermelon farmland and harvested wheat) due to the increase in runoff from 12.8, 40, 25.2, and 40.7 to 54.9 lit., respectively. The average sediment changed from 57.5, 230.9, 119.1, and 1369.6 to 1190.1 regarding the land uses, respectively. The reduction of phosphorus was found from 13.4, 8.8, 10.8, and 6.3 to 4.4 in different land uses. The organic matter reduced from 6.4, 4, 3.2, and 1.6 to 1.1 in different land uses, respectively. The cation exchangeable capacity reduced from 16.8, 10.3, 9.1, and 6.2 to 7.6 c mol kg-1 in different land uses, respectively. The soil moisture changed from 22.2, 6.4, 12.8, and 6.8 to 6.7 in different land uses, respectively. The soil density in different land uses increased from 1.05, 1.16, 1.25, and 1.63 to 1.57 gr cm3, respectively.
According to the results, organic matter is the only variable that has significant difference affected by slope changing in all of the land uses. Also, the investigation of the relation between measured variables of runoff and sediment showed that the organic matter, phosphorus, cation exchange capacities have the highest effects on runoff and sediment, respectively. The increase in runoff in farmland comparing to the forest shows the importance of forests in reducing runoff, flood management, and ground water recharge. On the other hand, sediment production in farmlands 20 times more than in forests shows the important of forest in preventing sediment production, filling the reservoir of dams, and losing valuable soil.