تاکنون روشهای زیادی جهت برآورد میزان شدت بیابانزایی در نقاط مختلف دنیا ارائه شده است. در همین راستا به دلیل منطبق نبودن مدلهای ارائه شده توسط کارشناسان در سایر کشورها با شرایط اقلیمی کشورمان در سال 1375 مدل تجربی IRIFR.E.A توسط محققان کشورمان تدوین و ارائه گشته است. در این تحقیق سعی شده تا با استفاده از مدل نام برده شده میزان تخریب اراضی در سطحی برابر با 48219 هکتار برآورد و نقشهی خطر بیابانزایی منطقه ترسیم شود، برای این کار ابتدا با بررسی شرایط منطقه بر اساس بازدیدهای میدانی، عوامل مؤثر بر روند تخریب اراضی شناسایی شده است. این عوامل شامل سازند زمینشناسی منطقه، شکل اراضی و پستی و بلندی، وضعیت باد و شرایط خاص اقلیمی، خاک و پوشش سطحی، پوشش گیاهی، فرسایش بادی، رطوبت خاک، نهشتههای بادی و عملیات مدیریتی میباشد. با استفاده از جداول ارزیابی مدل مورد استفاده، میزان تأثیر هر عامل بر روند بیابان شدن منطقه مورد ارزیابی قرار گرفت. در همین راستا نتایج به دست آمده از این تحقیق نشان میدهد منطقهی مورد مطالعه در طبقههای بیابانزایی متوسط، شدید و خیلی شدید قرار گرفته است. همچنین نتایج نشان میدهند از کل منطقهی مورد مطالعه 4/59 درصد معادل 28642 هکتار در طبقه بیابانزایی شدید، 6/40 درصد 19577 هکتار در طبقه بیابانزایی خیلی شدید قرار دارد. همچنین بر اساس نتایج به دست آمده مشخص شد از میان عوامل مؤثر بر فرآیند بیابانزایی مدیریت کاربری اراضی و اقلیم با کسب بیشترین میزان امتیاز به عنوان مهمترین عوامل مؤثر بر فرآیند بیابانزایی شناخته شدند.
Evaluation and Zoning of Severity and Danger of Desertification in the Some Parts of Sistan Plain and Hamoon Lake Bed Using IRIFR Model
Dr. Ahmad Pahlavanravi
Assistant Professor of Geomorphology
University of Zabol
According to Glantz,s theory, desertification term has about one hundred definitions(Glantz, 1977) . The common point in all these definitions is the severe demolition of the environment due to desertification..Some of these definitions are: ecosystem degradation (Reining, 1978), degradation of plant species (Le Houerou, 1975), decrease of ecosystem production and a decrease in biological production. Each of these definitions shows the ecosystem change from favorable conditions to unfavorable conditions and decrease of biological production. In the Environment and Development conference of the United Nations held in Riodojaniro in 1992, desertification was defined as: land degradation and demolition of dry , semi dry and dry semi humid areas as the result of human activities and climatic changes. At present desertification is a serious problem plagued many countries of the world. Considering and paying attention to desertification for a country like Iran which 80 million hectares of which is covered by dry and semi dry areas, is a necessary and inevitable issue.12 million hectares of this area is covered by running sands , 6 million of which is formed of active sand hills (Refahi , 2006) . Wind erosion due to wind blowing usually happens on bare areas with no surface vegetation cover. Wind is said to horizontal movements of the atmosphere. Wind is subject to pressure, therefore changes of pressure produces different winds in the atmosphere. At the low levels of the atmosphere, molecules are near to each other, the contacts are great and the pressure is high, therefore the wind blows in the direction which has the highest pressure (Moghimi 2006). Soil erosion is a phenomenon, in which soil is displaced due to environmental factors like water, wind, gravity force and so on and after carrying, is deposited in another area. The under study area is considered as one of the driest areas in the world and wind erosion is actively salient in this area and each year, makes a great deal of soil out of access. In the areas with no control on erosion, the soils are gradually eroded and lose their fertility. Erosion, not only weakens the soil and desolates the farms and makes great and irreparable damages, but also through sedimentation of materials in water ways, dam reservoirs, ports and decreasing their impounding capacity, makes Rstages. Such situation is visible in the study area which is a large part of Sistan plain and Hamoon Lake. Wind erosion is saliently active in this region. During wind erosion, the particles are moved by three methods of creeping, jumping and suspension. Particles mostly move by jumping movements. Diameters of jumping particles are usually between 0.05 up 0.5mm. Jumping movements of particles are usually happened at the limit of 0.1up to 0.15mm (Refahi 2006).increase of soil erosion, in addition to the soil features and nature depends greatly on the jumping particles. So that the particles which are moved by jumping method, when contacting with the ground surface, move the resting particles of the ground surface. During this process, the particles which their threshold speed is more than the wind rate will move.
In general, the effective factors on intensifying the wind erosion in the area under study can be divided in to two human and environmental factors. Environmental factors are mainly rooted in the structure of climatic changes, geology, pedology and geomorphology, and human factors (management) can be used for uncontrolled grazing of livestock, clearing plants, changing forest and pasture lands to agricultural lands and non controlled and inappropriate use of lands. Although for specifying the priorities , it is sometimes required to use the statistics and comparing the under study parameters in particular time periods , but until achieving this methods , it is possible to use the information, experiences and expressions of the people who have obtained during the long times of living in this area.With respect to the above said items, in this research for estimating the severity of desertification, IRIFR.E model, presented by the Iranian researches ( Ekhtesassi and Ahmadi) has been used. IRIFR model like PSIAC model, evaluates the role and effect of nine factors in wind erosion and the resulting sedimentation amount. A score is given to each factor depending on its severity and weakness and its effect on desertification. By adding up the above scores, degradation severity of geomorphologic facies, whish is the basis of studies for soil erosion, will be estimated. The following table shows the nine factors effective on intensifying the wind erosion(able 1-)
Table-1: Effective factors on soil erosion and sediment generation based on IRIFR model
AEffective factors on soil erosion and sediment generation based on IRIFR model
Wind speed and condition
Soil and land cover
Vegetation cover density
Surface erosion effects
Type and dispersion of wind sedimentations
Land use planning and management
Discussion and Results
With respect to the field studies, the land use map was designed which includes nine facies as the following figure ( figure No. 10). Specifications of geomorphologic facies has been presented in the following table (table No. 12).
Evaluation and Zoning of Severity and Danger of Desertification in the ...
Fig-1: working units maps obtained from the study area using aerial photo processing,
Topographic and geology maps and the visits.
Table-12 Units,types and Geomorphologic facies types were identified in the study area
Name and code of Geomorphologic Unit
Name and code of Geomorphologic type
Name and code of Geomorphologic Facies
Obsolete undesirable agricultural lands
Salty and bloated land
dry earth crust without vegetation
dry earth crust with cane
Hard and dry earth crust with Tamarix tree
In continue, after specifying the homogenous units (land use map), the considered factors in each unit has been evaluated based on the designed table and the results obtained from evaluation and scoring the under study indices in this study have been presented in the following table(table 13) . Finally with respect to the obtained results from pervious stages, the map for current condition of desertification is being obtained by using IRIFR model in the under study area and based on the following figure (figure No.2)
The results of this research indicate this fact that desertification has been occurred actively in the area and has an increasing trend and also show that the under study area is placed at desertification class of medium, severe and very severe and from all of the under study area, 26% equal 22791 hectares is placed in medium desertification class, 44.3% equal to 38832.6 hectares in severe desertification class and 29.7% equal to 26034.4 hectares in very severe desertification class . Regarding the obtained results from this research and performed visits of the area, it is possible to prevent the progress of desertification process in the areas which are placed in low critical places through performing biological and mechanical plans, otherwise, due to special conditions dominant on the area, the condition of these areas becomes more critical and severe. The areas of severe class with an area of more than 38832.6 hectares, have dedicated itself most part of the area which includes humid area, Kavir lake , dry and hard earth crust without any vegetation cover together with cane. In case of lack of control, this phenomenon is transferred to other areas and makes the situation of the area more critical. After providing the land sensitivity to wind erosion map, each class limit in all the area is calculated by using digit system. The calculation and relative percentage of wind erosion classes in the study area has been presented in the following table.
Table 14. Percentage of the area related to each level of desertification class in the study area
IRIFR score Limit
Erosion Intensity classes
Region Area in Hectares
Estimation of sediment yield resulting from wind erosion in the study area by using IRIFR model:
To evaluate sediment yield potential of wind erosion in the study area by using IRIFR model, the relation presented between sedimentation degree and amount of sedimentation production as below, was used:
QS = 41[EXP (0.05R)]
Qs is annual sediment yield, tone per Square kilometer
R is Sediment yield degree (using total score factors affecting wind erosion)
For obtaining the R sedimentation degree, in each of geomorphologic facies and all of the study area from the total score, the effective factors on wind erosion is extracted and by using the above relation, the sedimentation potential resulting from wind erosion has been estimated. Results of evaluating sedimentation amount of the area has been presented in the following table.
Table No.15: results of evaluating sedimentation amount at the study area
Evaluation and Zoning of Severity and Danger of Desertification in the ...
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