تعیین حساسیت رسوبات دریاچه‌ای به فرسایش بادی و نقش آن در ایجاد گردوغبار (مطالعه موردی: دریاچه خشک‌شده پریشان، استان فارس)

هدف از این پژوهش بررسی رسوبات دریاچه پریشان و نقش آن در وقوع پدیده گردوغبار می باشد. بدین ‌منظور 24 نمونه خاک سطحی از بستر دریاچه و 9 نمونه خاک از اراضی کشاورزی اطراف آن جمع‌آوری و خصوصیات فیزیکی و شیمیایی نمونه‌ها اندازه گیری شد. دانه بندی نمونه‌ها به روش الک خشک با 5 طبقه روزنه ای، بافت خاک به روش هیدرومتری بایکاس تعیین و درصد رطوبت اشباع، درصد گچ، درصد آهک، سدیم تبادلی، کلسیم و منیزیم تبادلی، ph و ec برای هر یک از نمونه‌های خاک اندازه‌گیری شد. نتایج نشان داد بافت خاک نمونه‌های کشاورزی و دریاچه تقریباً مشابه بوده و در تمامی نمونه ها ماسه بیشترین درصد، و به طور میانگین 62% مواد تخریبی رسوبات دریاچه را تشکیل می‌دهد. درصد رطوبت اشباع در تمامی نمونه ها بیشتر از 50 و قلیائیت نمونه های سطح دریاچه بیشتر از نمونه های کشاورزی است. مورفومتری نمونه‌های برداشت‌شده از سطح دریاچه و زمین های کشاورزی با استفاده از نرم افزار g.r.graph تعیین گردید. مقدار میانه و میانگین تقریباً مشابه هم بوده که علت آن نزدیکی به منشا و مسافت حمل‌ونقل کمتر می‌باشد. ذرات دارای جورشدگی خیلی ضعیف، نامرتب و چولگی زیاد به سمت ذرات درشت‌دانه می باشند. عوامل موثر در فرسایش بادی شامل فرسایندگی باد و فرسایش‌پذیری خاک می‌باشد، بررسی نتایج آزمایشگاه خاک نشان می دهد که خاک منطقه نسبت به فرسایش پذیری در برابر باد مقاوم است. نکته مهمی که از بررسی بادهای منطقه حاصل می شود این است که سرعت بادهای غالب منطقه با بیشترین فراوانی کمتر از 3/5 متر بر ثانیه است و نقش موثری در جابه جایی ذرات خاک از سطح دریاچه ندارد.

Determination of lake sediments susceptibility to wind erosion and its role in dust formation (Parishan lake, Fars province)

Extended abstractIntroductionSoil erosion is a widespread environmental problem threatening environmental sustainability. It is one of the significant factors in both water and wind erosion. Understanding the physical and chemical characteristics of lakes and wetlands sediments is essential to study sedimentology and erosion. Drying up the wetlands around the world is one of the worst experiences of human life that has had many adverse effects. Large wetlands are drying because of changes in water use patterns, due to landuse changes and water constraints due to climate changes.MethodologySystematic random sampling was carried out from lake sediments. Then the texture of deposits was determined using triangle soil texture. The velocity and direction of the dominant and erosive winds were determined by drawing the wind and storm rose. Thirtythree soil samples were taken from the lake surface and the agricultural lands around the lake by random sampling from 030 cm depth, and the location of each sample was recorded using GPS. Sorting was done by dry sift technique. GR graph 3.0 software was used to determine sedimentological indices such as Median Diameter, Median, Kurtesis, and Skewness of the samples.According to Kazeroon synoptic station’s wind data, the wind condition of the area was studied in the form of a wind rose. The erosion rate threshold of the lake surface sediments is needed to calculate the storm rose. For this purpose, wind data from the study area and sampling were used to determine the wind erosion threshold velocity, based on soil properties and experimental formula.ResultsThe soil texture results showed that the sand had the highest percentage of material degradation in all samples, and the average sand content of lake sediments was about 62%. Clay and silt are less abundant, respectively. Soil texture in the study area was almost similar to agricultural land and the lake area, only with slight changes in texture due to increasing treatment intervals. According to the soil chemical analysis results, the electrical conductivity of all samples taken from the study area was less than four dSm1 indicating that the soil was ordinary. Soil sodium content was low, thereby increasing aggregate resistance to dissemination and making the soil more resistant. In all samples, saturated moisture content was more than 50%, having high moisture content, resulting in fine particle adhesion.Based on the Kazerun station results, the frequency numbers of distribution of the annual wind class show that 50.97% of the winds are calm, and 49% of the winds have direction and velocity. The direction of the prevailing wind is westward. The Kazerun station annual storm results (20062016) show that 99.8% of the winds are calm, and only 0.2% of the winds have direction and velocity. This value is not visible in the flowchart diagram of the storm rose, so they can be ignored.Discussion ConclusionsParishan Lake is one of several lakes in Iran that has dried up in recent years. In this study, we tried to show whether this event as a dust storm source, can be an environmental threat for residents living in urban and rural areas around the lake. The results showed that the sand had the highest percentage of material degradation in all samples. Therefore, due to the low abundance of silt particles in Parishan Lake’s sediments, lake sediments cannot be considered a potential origin for Iran’s dust storm. According to the samples taken from the Lake Surface and agricultural land, the calculated morphometric values showed that the particles have irregular Kurtesis and a high skewness towards coarse grains. The results of soil physical and chemical property analysis show that the soil is resistant to wind erosion. The important thing to conclude from the wind rose is that the prevailing winds of the region are located in the classes of velocities less than 3.5 m/s, which do not play a major role in generating dust storms, which are more abundant in the basin. The problem does not threaten the rural areas around the lake and the city of Kazerun in terms of dust. If the dust is observed a few days a year, it may be due to external sources.