Soil Loss Comparison under Overgrazed Pasture and Rainfed Agriculture in Gorgak Sub-Watershed, North Karoon Watershed

One of the main factors accelerating soil erosion in the watersheds in Iran is mis-management including overgrazing and improper landuse changes of pastures into agriculture. This problem is true in the whole Karoon watershed. Therefore, the objective was to study the effects of landuse changes on soil loss in Gorgak sub-watershed. The statistical design was split plot design with pasture and rainfed agriculture as main factors. A drop maker rainfall simulator with 40 mm/hr intensity and 30-minute rainfall period was used on five field locations under two soil moisture conditions of air-dry and field capacity and three slope classes of 10-20, 20-30, and 30-40%. A total of 180 sediment and runoff measurements were made using the above rainfall simulator under field conditions for two landuse systems of rainfed cereal and overgrazed pastures. All the measurements were done in triplicates (5 fields x 2 landuses x 2 moisture content x 3 slope classes x 3 replicates = 180 plots). The results showed the soil loss under overgrazed and air-dry soil condition was 45.6 times more than the rainfed agriculture under the similar soil moisture condition. However, the results showed the soil loss under overgrazed and field capacity soil moisture condition was 2.33 times more than the rainfed agriculture under similar soil moisture condition. Once the soil loss was compared for different slope classes of 10-20, 20-30, and 30-40 slope percent, the results indicated the soil loss under overgrazed and air-dry soil condition were 4.77, 5.19, and 2.34 times more than the rainfed agriculture under similar soil moisture condition. Overall, soil loss under overgrazed condition was 3.38 times more than rainfed agriculture. Therefore, although most soil conservationists generally believe that the rainfed agriculture in Iran is very mis-managed and it has high soil erosion rates but the results of this study showed the overgrazed pastures in the north Karoon watershed need much more priority for any soil loss measures and controls. Key words: North Karoon Watershed, Gorgak watershed, Rainfall simulator, Landuse, Watershed Management, Soil loss.