Changes in runoff, Soil and nutrient loss in different vegetation cover type in Loess lands (Case study: Kechik watershed, Golestan province)

Recognition of effective factors on erosion and determines their negative consequences are essential to proper management of soil and water resources and achieve to sustainable development. Previous studies indicate that land use type and slope are important in runoff volume, soil erosion and nutrient loss.
In most researches due to difficulty operation of simulating rainfall in large-scale plots led to use of small plots. In some cases, the slope changes at this level havent proper representative of natural conditions.
The objective of this study is assessing the effect of different vegetation cover type, slope and rainfall intensity on changes in runoff, soil and nutrient losses in relative large-scale plots.
The study area (Kechik watershed) is located in 25 Km east of Maravetapeh city in Golestan province at 55° 57´ 52" to 55° 57´ 10" longitude and 37° 42´ 15" to 37° 46´ 25" latitude. This watershed has a uniform petrology and loess formation almost covered entire watershed. Rainfall simulation sites were selected by three nested design. Five Land use include of forest (Natural forest and Reforestation), rangeland, farmland (Watermelon and Harvested Wheat) and four slope classes include of 3-12, 12-18, 18-25 and 25-40 were considered. In land uses 4, 2, 4, 4 and 2 slope class was selected, relatively. Rainfall simulation using two rainfall intensity of return period 50 and 100 year (90 and 105 mm hr-1 relatively) were accomplished. Under considering of 3 repeat for each plot, 96 plots were selected. Output runoff and sediment of 1*4 meter plots throughout simulation were collected. Sediment concentration, runoff coefficient, nutrient amount (phosphorus, nitrogen, organic matter) and Cation Exchange Capacity were measured at the laboratory. Results using SAS, Minitab and SPSS were analyzed.
Analyzing plots outputs showed, in forest (Natural forest and Reforestation), rangeland and farmlands (Watermelon and Harvested wheat) mean of organic matter (OM) changes 9.1, 6.7, 3.6, 2.2 and 1.8%, mean of N 0.60, 0.30, 0.28, 0.17 and 0.15 %,mean of CEC 15.8, 11.2, 9.6, 6.4 and 7.7 Cmol kg-1, mean of P 16.4, 11.31, 15.4, 8.6 and 5.9 mg kg-1, and runoff coefficient 11.1, 26.6, 16.7, 27,1 and 36.5% and sediment 20, 57.7, 29.7, 342.3 and 297.5 gr m2, respectively. According to results, slope factor had different effects on measured variables. The most and fewest slope effect was observed on sediment of farmland (watermelon) with 9.1 times difference and nitrogen of rangeland with 1.04 times difference. Measured variable didnt show any trend with rainfall intensities. The highest significant positive correlation was found between runoff and sediment (r=0.93**).
Measuring of physical and chemical variables in different land uses and slope classes showed when land use type gradually changes from forest to rangeland and farmlands and also slope increase, significant decrease in chemical variables and significant increase in physical observed. According to results, proper management of steep farmlands can have an important role to decrease the runoff coefficient, soil and nutrient loss.