Evaluation of the effect of range exclosure on soil erosion at plots scale (Case study: Sanganeh Soil Conservation Research Site)

Considering the large extent of the country's rangelands, studying their hydrology and soil erosion is important for choosing management scenarios. Rangeland exclosure is one of the watershed management methods that is used to improve vegetation and also control soil loss. Rangeland's exclosure of the Sanganeh Soil Conservation Research Site (SSCRS) started about 25 years ago, which has led to the improvement of vegetation in most of the slopes compared to the area under livestock grazing. However, vegetation has not been established on some slopes of the exclosure region. This research site has provided suitable conditions for soil erosion studies and assessment of rangeland management measures at the scale of the plot and small watersheds by having erosion plots of different lengths located on slopes with and without vegetation. In this regard, the present research is planned with the aim of determining the effect of rangelands exclosure 1) on soil loss in plots with and without vegetation, and 2) with different lengths.

In this research, a comparison of soil erosion was made in th e area under free livestock grazing (E6) and exclosure watershed with similar conditions (E4). In this regard, six erosion plots with lengths of 5, 10, and 15 m (with areas of 10, 20, and 30 m2, respectively) in two vegetation situations (with and without vegetation cover) were selected in each study watershed. After collecting the soil erosion data under 24 natural rainfalls, the effect of plot length and vegetation situation were compared by paired t-test.

The results of data analysis indicated that the maximum intensity of 30 minutes is in the range of 2.4 to 32 mm per hour with an average of 0.9 mm per hour. It can be clearly understood from the distribution of precipitation data based on the seasons of the year that the average rainfall of three seasons, spring, autumn and winter, is almost equal. However, the average rain erosion index (EI30) in spring is 3.02 and 4.40 times higher th an the corresponding values in autumn and winter, respectively. The reason for this is the occurrence of heavy rains in spring. The comparison of two fields with cover and without cover in the areas of exclosure and under livestock grazing in terms of soil erosion showed that in both areas (with cover and with out cover) the amount of soil erosion in the plots under grazing is significantly higher than in exclosure area. So, in similar rainfalls in the region, the soil erosion in the watershed under grazing at different slopes is from a minimum of 282% to a maximum of 550% more than the exclosure watershed. By increasing the length of the plot, soil erosion per unit area decreases. The decreasing trend of specific sediment with the increase in the length of the flow path is mainly due to the decrease in the amount of specific runoff and as a result, it is not possible to move the eroded materials in the plot. In other words, due to the dominance of the surface erosion process and the lack of development of rill erosion in the investigated plots, the re-infiltration of runoff and the deposition of transported materials are the most important reasons for the decreasing relationship between the increase in the length of the plot and erosion per unit area. In both slopes (with and without vegetation), the effect of waterlogging on soil erosion has increased with the increase of the area of the plot. In other words, grazing pasture in 10 m2 plots has reduced soil erosion by 287.7 and 324.6 percent, respectively, in slopes without and with vegetation. Meanwhile, in the plots of 30 m2, the reduction of soil erosion was observed by 472.4 and 613.7 percent. On the other hand, in the slops with vegetation, grazing has a greater effect on reducing soil erosion. In this context, it can be stated that surface erosion and sheet currents are the dominant phenomena of erosion and sediment transport in the studied slopes, therefore, if the roughness and permeability of the soil increases due to the operation of pasture exclosure, it can be expected that the runoff flow in longer routes (from the slope 10 m2 to 20 and 30 m2) to increase the amount of material deposition. In this case, exclosure in the domain with a longer length has a greater effect on reducing soil erosion.

SSCRS with erosion plots in different conditions (length, vegetation conditions and pasture management) is a suitable place for scientific research in the field of pasture hydrology. In this research, the effect of plot length and vegetation status on erosion was evaluated using soil erosion data measured in two areas of exclosure and under livestock grazing. The results of this research showed that in both ranges (with and without vegetation), the amount of soil erosion in the plots under grazing is at a significant level of 1% higher than in exclosure region. So, in similar rains in the region, soil erosion in the watershed under grazing in different lengths is at least 282% more than the flooded watershed. Also, the results indicated that soil erosion per unit area decreases with increasing plot length. The decreasing trend of specific sediment with the increase in the length of the flow path is mainly due to the decrease in the amount of specific runoff and as a result, it is not possible to move the eroded materials in the plot. Finally, the results showed that in both fields with and without vegetation, the effect of waterlogging on soil erosion increased with increasing plot length. On the other hand, in the range with vegetation (compared to the range without cover), grazing has a greater effect on reducing soil erosion.The results of the surveys conducted at the SSCRS confirm the sensitivity and fragility of the dry grassland ecosystem in this region on behalf of the country's dry grasslands. In such a way that after a long period of flooding in the region, some slopes still lack vegetation. Therefore, the destruction of vegetation in some areas, especially steep slopes and sensitive formations, may not be easily returned to the original state, and great care must be taken in their management. Because these domains have high erosion and runoff production. Due to the existence of recorded information related to the ideal situation of the region (exclosure region), it is possible to create different grazing systems and evaluate its effects. Finally, it is possible to compare the information obtained from exclosure region, managed grazing and open grazing.