Investigating the Effects of Land Use, Land Slope and Soil Properties on Sediment Yield in the Sub-Catchment of Karkheh Watershed Basin in Lorestan Using an Artificial Rainfall Simulator

Extended abstract Introduction The present study was performed to investigate the effects of slope percentage and land use type on the produced sediment by an artificial rainfall simulator in Kakasharaf watershed, a branch of Karkhe watershed (Lorestan province).   Methodology The rainfall simulator used in this research was designed and constructed by the Soil Conservation and Watershed Management Institute. This device is a portable non-pressurized piston gun with the dimensions of 119.5 × 83 cm and a height of 1.60 m. The rainfall simulator was calibrated and adjusted to the location of the test based on the maximum rainfall intensity of 30 minutes. In order to select the location of the test, after designing the units, land use, and mapping of the slopes of the studied basin, six sub-basins with the three types of forest, pasture and agriculture were selected. After collecting and recording the data, SAS and Excel softwares were used for statistical analysis and plotting. One-way analysis of variance and comparison of means were done based on Duncan test at 1% level.   Results  The average of sediment yield obtained from rainfall simulator in three slope classes and three land uses in sub-basins of the study area showed that in three different land uses, by increasing slope percentage, the amount of sediment yield increased. This trend was not observed in the forest and agricultural land uses in sub-basins 2 and 4, and the amount of sediment yield did not correlate with the increase in slope percentage. The reason for this can be attributed to the effect of some soil characteristics such as the percentage of lime, organic matter, clay content, soil nitrogen and pH of these hill slopes. The amount of sediment yield in different sub-basins, i.e. in rangeland uses in basin of six with a mean sediment of 95.9, and in sub basin five with a mean sediment of 95.75 g in the plot, respectively, had the highest and lowest amount of sediment in pasture land uses. In agricultural land use, the highest sediment yield measured in sub-basin 2 indicated an average of 85.86g and the lowest sediment in the sub-basin showed a mean sediment of 33.31 in the plot area. Also, in 5th basin with a sediment of 1.444 g and in the sixth basin with a mean sediment of 32.27 g, the highest and lowest amount of sediment was produced on the plot area and in the forest land uses. In general, the average sediment yields from the rain fall simulator in the entire Kakashpour area at 10, 20 and 30 percent slopes were 7.49, 23.41 and 61.66 grams, respectively. By increasing the amount of slope percentage, the amount of sediment yield increased as well. The average amounts of sediment at the plot area in the forest, pasture and agriculture were 46.07, 30.88 and 15.02, respectively.   Discussion & Conclusions The results showed that the average quantities of sediment resulted from the plot area with one square meter of rainfall simulator in Kakasharaf watershed were obtained as 7.49, 23.41 and 61.06 g in the slope degrees of 10, 20 and 30%, respectively, and 46.07, 30.88 and 15.02 g in the three land use types of forest, pasture and agriculture, respectively. Therefore, the highest sediment quantity was produced in the slope degree of 30% and forest land use due to the mature forest soils exposed to erosion because of the destruction of forest soils by farmers. Analysis of variance showed a significant difference (p<0.05) between the different sub-basins from the viewpoint of the produced sediment. Also, the interaction effect between sub-basin and land use type on the produced sediment indicated a significant difference at 0.01. Also, the results of Duncan`s mean comparison test indicated a significant difference between the land slope degrees and also between three land use types of pasture, forest and agriculture in terms of the produced sediment and the sediment increased significantly with an increase in the slope  rate from 10% to 30%. Based on the results of the Pearson correlation matrix, soil properties did not have a meaningful correlation with soil loss. The results of sediment yield in different uses indicated that the highest amount of sediment was in the forest and the lowest amount referred to the agricultural land. The reason of this result could be the activity of microorganisms and earthworms that increase porosity and organic matter, and lower specific gravity of soil particles that are more easily displaced by runoff. Also, the soil of forest land use in this area has become dense and hardened due to the easy access of the people of the area, and the continuous cutting of trees. In agricultural lands due to the use of organic fertilizers and the presence of straw after harvesting (which can increase organic matter) as well as the continuous tillage operations in soil, soil permeability can increase. Therefore, the amount of sediment yield was higher in rangelands and forests due to the excessive grazing and the frequent transportation of livestock and, consequently, a decrease in the permeability rate of the soil. The results also showed that the increase in the slope amounts would trigger the sediment yield increase as well. The reason is its effect on the speed of runoff and partly due to the intensification of the effects of rain drops (Toy et al., 2002). Their results showed that the effect of slope percentage on runoff and sediment was significant. Slope amount had a positive effect on the amount of separation of soil particles due to the impact of rain drops.The results of the analysis of variance and the comparison of the mean of the sediment yield in the six sub-basins showed that there is a significant difference between the sediment yields of the sub-basins at the 5% level. The reason for this difference can be attributed to various conditions such as topography of the area, geology, soil characteristics, land use in each sub-basin, as well as the impact of human and managerial factors in the sub-basins, which ultimately affect the amount of sediment produced in the six sub-basins.