جستجوی مقالات مرتبط با کلیدواژه « رسوب ویژه » در نشریات گروه « کشاورزی »

Extended AbstractIntroductionAccelerated soil erosion and severe sediment production disrupt the natural balance in watersheds and have off-site impacts on river channels and downstream reservoirs. There are different estimates of soil erosion and sediment yield in Iran. However, what the researchers agree on is that it is more than tolerable erosion. Rangelands cover more than half of Iran, and poorly covered lands play a large role in flood and sediment production. Therefore, the study of hydrology and soil erosion is necessary for sustainable use of pastures. The prerequisite for these studies is long-term monitoring of sediment, runoff, vegetation, soil, etc. A careful review of previous studies on rangeland hydrology shows that most studies have focused on the effect of grazing management at the plot scale or have used simulation approaches and experimental models, as well as in a short period of time. Therefore, it is very necessary to conduct scientific research using long-term monitoring data in experimental watersheds. In this regard, the present study was proposed with the aim of evaluating the sediment yield of small watersheds in pastures with grazing exclusion vs. overgrazing. The results of this research can provide useful information to researchers, promoters, planners, and ranchers.Materials and methodsSanganeh Soil Conservation Research Station (SSCRS) with an area of 30 ha was established about 25 years ago in Kalat County (Razavi Khorasan Province, Iran). In addition to measuring erosion in plots and monitoring vegetation, by building several ponds at the outlets of six small watersheds (SWs), their runoff and sediment yields have also been recorded since 2006. The present study was conducted with the aim of evaluating the erosion and sediment production of these SWs (1200 to 17000 m2). For this purpose, the runoff and sediment of 69 events were collected at the outlets of six SWs. Also, the time series of NDVIs were calculated for SSCRS, adjacent rural and nomadic (outside the village) rangelands on a seasonal scale. To determine the time series of NDVIs, all satellite images of the study area were downloaded and after pre-processing and corrections, the images were processed. Then, the soil erosion amounts of SWs were estimated in terms of the sediment delivery ratio of the study area (Noor, 2020) based on the average sediment yields (according to their long period of 15 years). In the next step, the amounts of soil erosion obtained were compared to the amounts of tolerable soil erosion for arid climate rangelands proposed by Skouti Oskouei and Arabkhedri (2018). Finally, the soil erosion of two similar watersheds, one in the overgrazing area (E6) and the other in the grazing exclusion area (E4), were compared.Results and DiscussionThe results showed that NDVI is influenced by livestock grazing intensity. The highest value of this index was observed in the grazing exclusion area (E1 to E5 SWs), then in the Sanganeh village rangeland (E6 SW), and the lowest value was observed in the nomadic rangelands. In terms of time scale, the biggest difference in NDVIs between overgrazing and grazing exclusion areas was observed in spring. The results of the investigation of sediment production indicated an inverse and non-linear relationship between the specific sediment yield and the area of SWs. This study showed that the amount of soil erosions in E2, E3 and E6 SWs are more than tolerable erosions which suggests the need for more conservation measures. Finally, the comparison of two similar SWs (E4 vs. E6) indicate a significant reduction in annual sediment yield (582%) due to grazing exclusion in the area. Also, the results showed that the sediment productions of E6 SW in the spring and autumn seasons are significantly higher than E4 at 1% and 5% levels, respectively. Furthermore, it is remarkably (but non significant) higher than than the E4 SW in the winter season.ConclusionGrazing exclusion in SSCRS rangeland led to a significant reduction in erosion and sediment production compared to overgrazing condition outside the station. However, in some SWs, the erosion amounts were still more than the tolerable values, which indicates the difficulty of restoration of destroyed rangelands on steep slopes with sensitive formations in arid climate. Implementation of management measures including scientific grazing (especially in spring) is necessary to reducing damage caused by floods and sediments.

Watershed erosion and sediment load of rivers are serious water resources management challenges, which have negative consequences on the operation of water installations and dams. This study has done with the aim of investigating the relationship between hydro geomorphologic characteristics and calculation the amount of especial deposition around watersheds, because of depending on the volume of runoff and soil erosion rates or sediment discharge with hydrogeomorphologic characteristics in the watershed. In order to achieving to the object of research and investigating the role of percent of vegetation cover density, slope rates and soil permeability in fancies formation, Zonouz Chai catchment, one of sub-basins of Zilber Chai watershed, which has a higher number of drainage density than other sub-basins was selected and the weights of all effective factors were calculated according to the analytic hierarchy method (AHP). Afterwards, based on the multiple regression model using stepwise method the correlation coefficients (R2) were determined at 95% confidence level and amount of runoff volume was calculated in Zonouz Chai and Zilber Chai based on SCS method. Finally, correlation coefficients of watershed characteristics were estimated and were replaced as exponential coefficients for discharge-sediment equation, which was suggested. Then especial deposition rates were calculated in Zonouz Chai and Zilber Chai respectively, 163 and 672 (ton/km2/y). Comparison of computational results with observational rates, based on 35 statistic data years, was found, in the two watersheds has acceptable accuracy at 85% and 91%, respectively.

Sediment yield estimation for small basins is essential in watershed management planning projects. However، almost all of these basins are ungauged. In this study، in order to determine a range for specific sediment yield and to develop a regression model for estimating annual sediment yield، sedimentation behind small dams within 25 basins in Western Azerbaijan province، were measured. The selected basins with an area ranged 29 to 1200 ha، cover a range of lithology، land uses and climates and all of the sediments had been trapped behind the check dams. The specific sediment yields showed a two digit differences ranged 0. 12 and 12. However، the weighted average of specific sediment yield for all basins was calculated a low value of 1. 3. Study of relationship between annual sediment yield and 18 characteristics of watersheds using correlation matrix showed that the sediment yield is significantly affected by geologic erodibility، basin area and land use factors. From several regression models which were developed، an equation including two independent variables، geologic erodibility and basin area، with a high determination coefficient (R2=0. 899) was chosen.