پديد آورندگان :
زمان زاده، سيدمحمد نويسنده استاد يار دانشكده جغرافيا، دانشگاه تهران , , احمدي ، مهدي نويسنده مركز تحقيقات فن آوري هاي زيست محيطي، دانشگاه علوم پزشكي جندي شاپور اهواز ,
چكيده لاتين :
Introduction
In arid and semiarid regions such as Kahur plain of Lamerd in the south of Fars province, inhomogeneous distribution of vegetative cover and land use mal-management besides factors such as the geology, soil and climate conditions, increased spatial changes and resulted in significant changes in physical and hydrological characteristics of land surface. This type of erosion is developed in vast areas of Kahur plain of Lamerd in the south of Fars province resulting in severe soil erosion and prohibition of agricultural activities (Ahmadi, 2006, Ghodusi, 2003, Nazari samani et al., 2009). The major consequences of soil erosion include soil denudation, sedimentation behind dams, destruction of agricultural lands and vegetative cover (Evens et al., 2010). One of the most important factors in soil erosion is aggregates stability and influential factors in their stability. It is represented that soils with robust aggregates and percentage of large aggregates is high in them are stable soils so the most erodible soils usually contain 40 to 60% silt (Richter et al., 1966, Top et al. 1997). It is represented that stability increasing effect of clay is obtained when sodium contents is low in the soil (Kamper and Koch, 1996). The studies indicated that increase in sodium adsorption ratio (SAR) results in Critical Coagulation Concentration, i.e. increase in sodium adsorption ratio results in increasing necessary electrolyte concentration (EC) for flocculation of clays (Abu-Sharar et al., 1987). In these aggregates after irrigation and precipitation, the concentration of electrolyte comes below critical point and the first stage of aggregation generation, i.e. flocculation of clays, and domain generation is conversely affectedand the clays are dispersed. Some experiments also show that gypsum contents along with sodium in marly and clay bearing formations is a major factor in sensitivity of soils to piping process and gully formation (Morgan, 2005). The presence of weak sediments such as shale, salty and gypsiferous marls with a percent of silt and silty and clayey sediments in Tertiary and Quaternary creates a suitable condition for concentrated gully framework in many parts of the world especially in Iran (Soofi, 2003).
The study area
The study area is located in the south of Fars province some 30 km north of Persian Gulf in rout from Lamerd to Ashkenan. The Kahur plain is a hill apron plain composed of fluvial sediments of Cenozoic situated between two aniclines with a trend of NW-SE. the area of this plain is 1972.62 hectar and gully affected area is about 116.16 hectar.the mean altitude of the plain is 485 m above sea level with a minimum altitude of 380 m and a maximum of 1080 m above sea level. The plain has a very gentle profile with very low topographic difference. All gullies are formed in areas with less than 3% slope.the average precipitation is 211.5 mm and annual evaporation ranges between 2867 to 3704 mm and the mean annual temperature is 24.2 C. The climate of the region is considered as dry desert in De Marton classification and vegetative cover is very sparse.
Methodology
The gullies were selected according to their general characteristics and appearance. Totally six gullies almost with a constant distance from each other from the lower part of the area to the upper part in which the gully erosion gradually increased were selected. Arial photographs with a scale of 1:40000 taken in 1992 were used to limit the boundary of gullies then GPS was used to locate each gully. Arc Gis software was used to prepare the necessary maps. The volume of erosion in each gully was calculated at three intervals of 25, 59 and 75% of gully length from the gully head. To calculate the volume in each section the depth, upper part width and the lower part width measured.
Totally 4 sediment samples (400 grams each) were taken (2 samples from gully head and 2 from 50% from the head) from the depth of 30 and 60 cm to carry out the necessary sedimentological and physic-chemical properties (acidity, EC, gypsum contents by Acetone method, sodium concentration by flamephotometry and total carbonate by calcimetry, organic matter contents by wet oxidation, texture through hydrometry and calcium and magnesium concentration by titration method.
Results and Discussion
The volume of produced sediment in gullies 1 to 6 was measured in the study area. It shows an increasing trend from gully 1 to 6. To illustrate the soil textures quantitatively, according to soil texture classification. The biggest number (weight) is assigned to the lightest and the smallest one to the heaviest soil. The heaviest weight is for the depth of 30-60 cm of 50% section from the gully head and the smallest weight belongs to the depth of up to 30 cm in the gully head. According to the graphs drawn in most of gullies soil texture gets heavier from surface toward the depth of 60 cm due to the increase in clay contents in comparison to silt and as a result decrease in cementation of aggregates. This decreases the stability of soil texture. Consequently the more soil gets lighter, the more gully erosion happens.
EC usually has an increasing trend in most of gullies from surface to the depth of 60 cm. The observations show that there is a direct relationship between increasing EC and soil erosion, but in the study area the contrast happens, so that with decreasing EC gully development increases.
SAR amount shows an increase in the upper part of the soil. This increase in amount of sodium with respect to calcium and magnesium and due to the hydrated ion radius of sodium cations, the sodium ions can adsorb much water and result in clay expansion and decreasing permeability of soil which results in soil erosion (Jha and Kapat, 2009).
Results of TNV experiments show that most of samples have above 40% carbonate. Comparison of TNV changes in the soil samples from surface to the depth with erodability of soil shows a relatively direct relationship between them. The pH of the soil in the studied area shows a small change (7.6 to 8.5) because of abundance of basic ions in the region. This basic nature of the soil helps in aggregates disintegration and ease of soil grains transportation.
The organic matter content of the soil in all samples is low with a decreasing trend from surface to depth, this is due to concentration of organic matter in the surface. In the studied area no meaningful relationship was found between organic matter content and gully development.
The amount of gypsum in the samples was very low, but an indirect relationship between the amount of gypsum and development of gullies is clearly seen. The studies show the positive effect of gypsum in elimination of sodium effect in the soils (Williams et al, 1981; Jafari Ardakani et al., 1997) which results in increasing aggregate strength.
Conclusion
The results of this study show that there is a completely reverse relationship between EC, SAR and gypsum contents with gully development in the study area. The direct relationship between the latter point is seen with texture of the soil. Sensitivity of the soils to gully erosion with respect to TNV, OM and pH in the whole area is very high. The statistical analysis of the factors represented that the major sediment generating factors and development of gullies are drainage basin characteristics above gully heads and the size of soil particles. Because of high amount of sodium in lower parts of the gullies it is not possible to remediate soil with adding organic matter, so it is better first chemical treatments such as adding and replacement of two valent cations e.g. calcium is applied and then soil management through modification of vegetative cover and grazing are carried out. Since run off is high in the upper part of the gullies, through earth dam construction in the upper end of gullies, the run-off is collected and can be used in development of suitable vegetative cover.
