چكيده لاتين :
Introduction
Manʹs undue interference, livestock overgrazing, soil texture disorganization, and change in the geometry of channels are the main factors which could destroy the hydrologic balance of the watershed in an ecosystem. The systematic imbalance in a watershed could destroy the earth crust and increase flooding. It could also result in the erosion and soil sedimentation behind the reservoir dams and the destruction of the agricultural lands, roads, and villages. In addition, it could reduce the production ability of productive sources. Soil erosion is one of the environmental problems that threatens natural resources, agriculture, and the environment. The objective of the present study was to evaluate the biological, managerial, and constructional watershed projects and their effects on reducing erosion, sedimentation, and parameters of land protective cover. This study was conducted between 2001-2010 in the Khorrambid urban watershed from the Sivand dam in Fars province.
Methodology
A. Primary Collection Method
The preliminary data for Khorambid urban watershed from Sivand dam was collected by the executive detailed reports (Natural Resources and Watershed Management Office of Fars Province, 2001).
B. Estimating the parameters of land cover layer, erosion, and sedimentation before implementing the watershed projects
Before implementing the watershed projects, the vegetation parameters, erosion, and sediment yields in the executive and detailed reports were used to evaluate the parameters.
C. Estimating the parameters of land cover layer, erosion, and sedimentation after implementing the watershed projects
In the first step, the vegetation classification mapping was prepared and the vegetation parameters were sampled in the paired plots. It must be mentioned that one of the most effective items in estimating the erosion and sedimentation is to know the land cover parameters. To determine the rates of erosion and sediment exited from the network, it was required to calculate the rate of erosion and sediment on the domain. It was also required to calculate the controlled sediments on the waterway or at the rear of structures. In the first step, the locations of all structures executed in the intended sub-basins were specified by GPS to determine the sediments trapped at the rear of the structures. Meanwhile, their types, structural materials, and location maps were prepared. In the second step, the waterway slope, overflow height, reservoir length, waterway width, reservoir volume, and sediment volume were respectively determined by preparing some special forms via clinometer and meter. In addition, to estimate the erosion and the sedimentation of the domain, the experimental model of MPSIAC, modified by ArcGIS10, was used. When the nine layers of the MPSIAC model were combined and overlapped, the homogeneous units map was prepared. Next, the scores of nine factors were estimated and calculated to extract the sedimentation degree of each homogeneous unit according to the following equation:
Qs = /253 e0.036R (Equ.1)
When the sedimentation map was prepared, the erosion rate was calculated by the Sedimentation Delivery Rate (SDR) equation in the level of homogeneous unit. Regarding the factor of the basin area ( based on square mile), the following equation was used to determine the percentage of SDR.
Log SDR = 1.8768 – 0.14191 log(10A) (Equ.2)
To evaluate the differences between sedimentation changes in the sub-basins for the pre and post watershed operations, the normality test and Paired T-test in SPSS software were used.
Discussion
The results indicated that the factors’ score of river erosion and surface erosion which were respectively 30.77 % and 30.51% had the most effects and the land cover factor, with 15.05%, had the least effect on reducing the sedimentations to 21.97%. Also, the area of sediment class of 75-100 was 1375 hectares, which was equivalent to 15.78% of the basinʹs area. Indeed, in terms of the spatial distribution, it was located in the basin outlet and changed to class 50-75. The sediment class of 50-75 was 7339 hectares, equivalent to 84.22% of the basinʹs area. It reduced to 28.09 % and changed to the lower class of 25-50. Since there was a significant change in the sedimentation of the sub-basins in 0.05 levels, the results of this study indicated that the watershed projects were efficient in reducing the sediments. To manage the water and soil resources, to raise the effectiveness of watershed operations in reducing erosion and sediments, and to raise the land cover layer in Khorrambid watershed, the combination of both biological and constructional programs and treatments undertaken in the basin should be applied.
Conclusion
With regard to the findings of this study, before the watershed operation, the sedimentation degrees in two classes of 75-100 and 50-75 were respectively 15.78% and 84.22%. According to the sedimentation class map, the sedimentation degree of 75-100 was located in the basin outlet. Since the structural watershed operation has been extended more than biological watershed operation, the sediment class with the area of 1375 hectares experienced lower sediment degree and changed to 50-75 class after the watershed operation. The sediment class of 50-75 was 7339 hectares before the watershed operation; however, by implementing biological programs, 2447 hectares of that class changed to the lower class of 25-50. Therefore, 15.78% of class 75-100 before the watershed operation reduced to 0% after the watershed operation. In addition, 84.22% of class 50-75 was reduced to 71.91%. Regarding the equation of SDR based on the area factor, the rate of SDR for the basin was 33.03%. Thus, the sediment rates for pre and post watershed operations were respectively calculated as 23755 and 14390 tons, which indicated 39.42 % drops in the total sediments. The results of this study showed that the watershed operation, which was a combination of protective, managerial, and biological programs, was implemented well in the measures of domain and waterway (structural) in the basin. Consequently, the sedimentation mean in Khorambid watershed decreased to 39.42%. The findings of this study are in line with the findings of researchers such as Roghani (2012), Salmanpour (2013), Azami Rad (2013), and Agharazi (2016).
