A. Sharifi; H. Shirani; A.A. Besalatpour; E. Esfandiarpour
Abstract
Introduction: Interrill erosion is one of the major types of erosion playing key role in the transport of fine particles of the soil, particularly in arid and semi-arid regions, which leads to the decrement of soil fertility and surface water pollution. Land-use change is one of the main ways which reflect ...
Read More
Introduction: Interrill erosion is one of the major types of erosion playing key role in the transport of fine particles of the soil, particularly in arid and semi-arid regions, which leads to the decrement of soil fertility and surface water pollution. Land-use change is one of the main ways which reflect the interaction of human activities and the natural environment and can impact soil aggregation, aggregate stability, and erodibility. Hence, this research aimed to evaluate the susceptibility of soils under different land-use types (four types) to interrill erosion using both rainfall simulation test and soil aggregate stability indexes. The location of study area was around Jiroft city.
Materials and Methods: This study was conducted in four types of land use around Jiroft city in southern Iran, including disturbed pasture, undisturbed pasture, protected forests, and artificial forest. For each land use, 25 points were selected (A total of 100 points for all land uses). To measure soil physical and chemical properties, disturbed and undisturbed soil samples were collected from each point at a depth of 0–20 cm. The samples were transported to the laboratory where these samples were then air-dried. Some soil properties such as texture, organic carbon, electrical conductivity, soil acidity, calcium carbonate equivalent, and bulk density were measured, and available nitrogen, phosphorus, and potassium in the soil and sediment samples were also determined. Furthermore, some characteristics of soil particles, including the geometric mean diameter, geometric standard deviation, particulate organic matter, water-dispersible clay, tensile strength of soil aggregate, mean weight diameter and fractal dimension of aggregates were determined. To assess how susceptible are soils to interrill erosion, rainfall simulator was used to generate rainfall with an average intensity of 60 mm/h.
Results and Discussion: According to the results, the undisturbed pasture revealed the highest content of organic matter, particulate organic matter, clay, and tensile strength, while the minimum values of bulk density, sand percentage, and fractal dimension have been observed in this land use. For this reason, it is assumed that the aggregates of undisturbed pasture (intact rangeland) show more stability than other three land uses. The maximum and minimum values of bulk densities were observed in the protected forest (1.58 g cm-3) and undisturbed pasture (1.43 g cm-3), respectively. On the other hand, the highest value of aggregates fractal dimension, as well as minimum values for mean weight diameter and dispersible clay in the protected forest demonstrated that this land use had either no aggregate or its aggregates were very fine. As a matter of fact, lack of organic matter and insufficient clay content can be considered to be the reasons for poor aggregate stability in this land use. The highest and lowest values for tensile strength of soil aggregate were found in the undisturbed rangeland (64.82 kPa) and protected forest (34.38 kPa), respectively. The variations in the tensile strength of soil aggregate can be attributed to the changes in the contents of clay and organic matter in different land uses. Moreover, despite the maximum amount of total organic matter in the undisturbed pasture (or intact rangeland), the amount of sediment organic matter in this land use was lower than the other three land uses. It is because of the fact that most of the OM in this area was of a stable organic matter type, which was under the soil surface and was accordingly protected from surface erosion. The particle size distribution of sediment was smaller in the undisturbed pasture, whereas it was found to be larger in the protected forest. The reason can be attributed to the coarse-textured soil in the forest compared to the finer-textured soil in the undisturbed pasture (or intact rangeland). In addition, the highest sediment concentration and the highest rate of erosion were observed in the disturbed pasture. The artificial forest accounted for the minimum sediment concentration, while the artificial forest, as well as the protected forest, revealed the lowest erosion rate.
Conclusion: The results of the current research demonstrated the high rate of interrill erosion in all land uses so that the disturbed pasture and artificial forest accounted for the highest and the lowest rate of erosion (7 and 2 ton/ha) respectively. According the results, intrinsic soil characteristics such as soil texture played major role in some land uses, while for the others, the slope impact was more crucial. On the other hand, both erosion rate and sediment concentration revealed the same trend under four different land uses of the study area. Therefore, because of the fact that the highest and the lowest rate of erosion, as well as sediment concentration, were found to be in the disturbed pasture, and the artificial forest, respectively, therefore the sediment concentration can be considered to be an important index for soil erosion. Due to high rates of erosion occurring in the study areas, some measures have to be undertaken to prevent and control soil erosion in this area. To achieve this aim, preventing people from entering the vulnerable area, avoiding livestock grazing, protecting existing plants and restoration of native plants can be mentioned as efficient measures to improve conditions.
Malihe Ebrahimi; Hossein Asadi; Arezoo Sharifi; Eisa Ebrahimi
Abstract
Introduction: The study of physical properties of suspended sediments is one of the main topics in river studies. Sediment size distribution is one of the sediment physical properties which indicate the relation between the sediment source and its sedimentation process in watersheds. It is also important ...
Read More
Introduction: The study of physical properties of suspended sediments is one of the main topics in river studies. Sediment size distribution is one of the sediment physical properties which indicate the relation between the sediment source and its sedimentation process in watersheds. It is also important for prediction of the load of non-point source pollution, and for planning sediment trap structures. The Anzali Wetland, located on the southern coast of the Caspian Sea in northern Iran, is a large complex of freshwater lagoons with extensive reed-beds, shallow impoundments and seasonal flooded meadows. Environmental conditions in the Anzali Wetland have been degraded due to the increased inflow of sewerage, wastewater and solid waste from the industry, agriculture and urban area, and sediment from the upper stream mountainous area. The lagoon has decreased in size since the 1930s to less than a quarter of its former extent. The aim of the present study was to assess the changes in size distribution of suspended sediment in Pasikhan River as the most important river interring to Anzali Wetland.
Material and Methods: Pasikhan River originates from the South Mountains, has two branches namely Siahmezgi and Imamzadeh Ebrahim. The sampling carried out during a seven month time period (October 2013 to April 2014) at two hydrometric stations; Mobarakabad (upstream) and Nokhaleh (downstream). The samples were collected in 15 days intervals by depth-integration technique at normal condition. Particle size distribution was measured by Pipette method based on Stocks law. The mean weight diameter (MWD) of sediment particles was calculated, the sediment size distribution curve was drawn and the median grain size (d50) was calculated. According to the European classification, the particles size distribution was divided into four groups of fine sand (0.2 mm), coarse silts (0.06 mm), medium silt (0.02 mm), and fine silt and clay (equal to and less than 0.006 mm). The data were compared for each sampling time for both Stations. Flow discharge and suspended sediment load were also determined at each sampling date.
Results and Discussion: At the Nokhaleh station, the maximum observed flow discharge and sediment concentration were 51.4 m3/s and 4.162 g/L, occurred in February 4, 2014 and November 3, 2013, respectively. The highest flow discharge and sediment concentration of the Mobarakabad were 9.8 m3/s and 2.633 g/L which were observed on February 19, 2014 and April 4, 2014, respectively. These changes and differences were partly due to topography and land use differences between upland and lowland and partly due to rainfall pattern. Results showed that the MWD and d50 were 0.062 and 0.052 mm on average, respectively at Mobarakabad station, and 0.055 and 0.051 mm, respectively at Nowkhaleh station. The maximum values of MWD and d50 were observed to be 0.07 and 0.061 mm, respectively at normal condition at Mobarakabad station. The study of sediment size distribution indicated that the particles smaller than 2 mm comprised 83-94 percent of the suspended sediment at Mobarakabad station, and 87-99 percent at Nokhaleh station. The percentage of particles smaller than 0.02 mm were observed to be 12-33 and 10-64 at Mobarakabad and Noukhaleh stations, respectively. Also the amount of fine silt and clay in suspended sediment were 3-16 and 5-24 percent at these stations. There was not any correlation between flow discharge and sediment concentration or sediment size distribution characteristics.
Conclusion: In most of the samples, there was not any relationship between the flow discharge and particle size distribution of suspended sediment which emphasize on the non-hydraulic nature of sediment transport and the effects of different factors including sediment sources, the season, transport energy, rainfall erosivity, soil erodibility and deposition process. Generally, the size of sediment particles at Mobarakabad station was coarser than Nokhaleh station. This could be due to the type of soil erosion which is different at upstream and downstream. In upstream regions, mainly because of severity of topography and vegetation cover including forest and rangeland, the occurrence of gully erosion and landslide is higher in comparison with surface soil erosion. But in downstream especially in paddy fields, the soil erosion type is mainly splash and sheet erosion. Also the cultivation practices including plowing and paddling of the field usually provides fine particles entering to the river. In addition, the river profile is very gentle at the plain before the Nokhaleh station which resulted in deposition of coarser particles.
