نوع مقاله : مقاله پژوهشی
1 دانشیار ژئومورفولوژی، دانشگاه خوارزمی تهران.
2 دانشیار پژوهشکده حفاظت خاک و آبخیزداری، سازمان تحقیقات، آموزش و ترویج کشاورزی، تهران، ایران.
3 دانشجوی دکتری ژئومورفولوژی، دانشگاه خوارزمی تهران.
عنوان مقاله [English]
The studied area is located in the southeast of Fars, Iran, about 30 km north of the Persian Gulf coastline. Mehran river basin forms a fairly rough plain and gentle slope which has occurred as a result of alluvial deposits in the Cenozoic era, and is located between the two branches of the altar with the direction of northwest – southeast following southern Zagros folding process. The area of study includes 9 geological formations which are from Fars, Bangestan and Khami groups. The types of studied rocks in the area are mostly alluvial deposits, limestone, marl, sandstone, shale and gypsum. The area of the basin is about 3772 square kilometers (377190 hectares). The cut-off trench area is about 131 square kilometers (13100 hectares). Approximately 97.12% (12723 hectares) of the gullies occurred at a gradient of 0-5%. The average annual precipitation is 2311.5 mm. Based on De martonne‘s Method, the region's climate has been labelled as a dry desert.
First, lithology layers, slope and land use in the whole basin were combined together in the Arc-GIS software environment to provide homogeneous units. Then, homogeneous units in lands affected by gully erosion have been extracted from the homogeneous units of the whole basin including 4 units. Considering the percentage of gullies in homogeneous units, in unit number 3 where there are more than 95% of the gullies, six sample gullies have been chosen while in other units, 2 samples of the gullies were chosen. The volume of gully erosion for each gully was determined by measuring the cross sections of sample gullies in the desert. To determine the effect of soil physical and chemical properties in the form of gullies, a sample was chosen from the forehead sections, the center, and the outlet point of each gully- concluding a total number of 36 samples. Eleven effective variables, including clay percentage, silt percentage, sand percentage, E.C. P.H, Na concentration, total concentrations of Ca + Mg, O.C percentage, TNV percent, gypsum/ plaster content and SAR were measured for each soil sample. Characteristics related to the area and slope, bare soil percent, vegetation percentage, pebbles and litter in the upstream basin were measured for all gullies. Other features for each gully sample include characteristics of the cross-sectional shape, the shape of the walls, the forehead plan, cross section of forehead of gully, and the type of gully use, which were considered and recorded. Multivariate analysis of variance, one-way analysis of variance and Post Hoc Test in SPSS software were used to analyze the data and test the differences in various homogeneous units.
Results and discussion
Interpretation and conclusion of results being conducted by morphometry, soil science and soil morphology indicate the fact that the value of variables related to EC, plaster and SAR measured in units 1 and 2, compared to units 3 and 4, are very low. On the other hand, better vegetation in units 1 and 2 increases the amount of soil O.C compared to units 3 and 4. As a result, the combination of these factors leads to better soil adhesion and soil penetration. Total available features on the soil of units 1 and 2 have led to formation of gullies with almost angular shapes. In fact, due to the better soil adhesion which is affected by its properties, during rainfall , the deposits separated from the various components of the gully in these units – especially from the walls and forehead gully – are so massive that this process has caused the walls very vertical and U-shaped. A fairly proper vegetation in the upper basin of these gullies has increased the relative reinforcing of O.C in the forehead of this gully with low plaster content, low SAR, and E.C which indicates that a lack of salinity and salts in the soil of this gully causes the formation of cavernous and vertical foreheads. However, the high values of E.C. variables, SAR and plaster in the soil are factors that have prevented vegetation from growing in the range of units 3 and 4 such that the surface of the ground in these units is very bare and the lands occupied by these gullies enjoy the minimum amount of O.C. The combination of these factors in the lands of units 3 and 4 caused the soil to have very low permeability and to be sensitive to erosion- especially tunnel or dissolution -during rainfall. Due to the presence of high salts, vegetation loss and low O.C, the soil of the area is diffused and the processes caused the gullies in these lands to have a milder shape such that their various components, including the walls and the forehead, are formed in an oblique fashion. Due to the lack of adhesion of soil grains, erosion from different parts of the gully is not massive, it is often made by dissolution and superficial erosion.
The results of the Lambda Wikel Test for comparing soil properties in different units indicate that totally there was a significant difference between soil properties in different units. )ّ F= 2/41 , P <0/01) . Based on the investigations carried out in this study, it can be drawn that amount of deposits production due to morphological characteristics, which is a function of the physical and chemical properties of the soil in units 1 and 2, is higher than units 3 and 4. The distinction of the morphology of the gullies in units 1 and 2 compared to 3 and 4 is related to the amount of E.C., SAR, plaster and variables with regard to the surface cover of the earth. While the degree of variation of variables is negligible, other variables are the same in all units.