Effects of Rainfall and Slope Components on Runoff and Soil Erosion in Dry Lands (Case Study: Gachsaran Dry Lands)

Low efficient sloping rainfed lands are one of land uses being exposed to severe erosion in Iran which is definitely one of important sediment sources for rivers and reservoirs. These lands are most sensitive to erosion while being plowed and fallow. This period in most regions of Iran coincides with the rainy season. A large part of rainfed agricultural lands of Gachsaran city is much erodible while the income of many residents of this area is from agriculture. Therefore, the aim of this study is to investigate the impact of various rainfall intensities in different slopes on the amount of erosion and runoff in rainfed lands of Gachsaran city in Kohgiluyeh & Boyerahmad Province.

First, the soil surface was saturated in order to prepare the environment for the experiment. Then, rainfall intensities of 33, 64 and 110 mmh-1 on three slopes of 6, 12 and 25% were generated using a rainfall simulator. Therefore, nine treatments with four replications were analyzed. Runoff and sediment were collected using special containers and transferred to the laboratory. After data collection, statistical analyses of runoff and sediment were conducted by SPSS software and analysis of variance method along with Duncan test. The other analyses were also accomplished in terms of the effect of factors on the amount of runoff, sediment, quality elements of water, soil and sediment.

Comparison of mean of the studied parameters showed significant (p<0.05) effect of rainfall intensity and slope percentage on the amount of runoff and sediment; so that, the rainfall intensity of 110 mmh-1 caused runoff and sediment 4 and 1.7 times higher than the 33 mmh-1. Also, the slope 25% caused runoff and sediment 2.3 and 1.7 times higher than the 6% on the three measured slopes. Comparison of the results of rainfall intensity and slope impact on the studied components indicated that rainfall intensity has a more considerable impact than the slope percent, on the amount of runoff and sediment. Also, measuring the amount of nutrients in soil and sediment indicated the impact of soil erosion on nutrient transport and reducing fertility; so that, higher amount of carbon, phosphorus and potassium in the collected sediments than the soil.

According to the results, slope percent and rainfall intensity have a significant effect on runoff and sediment production independently and in interaction with each other, in the experimental plots. Based on the result analysis of sediment concentration, only high amounts of rainfall intensity affect sediment concentration and weight of sediment. Moreover, increase of slope caused amount, volume and runoff coefficient; but this increase of runoff in the slope of 25% was much higher than the slopes of 6 and 12%. So, as the slope increases, the runoff will also increase.