بررسي تأثير كاربري اراضي بر غلظت گرد و غبار و ميزان هدر رفت خاك در مناطق بياباني (مطالعه موردي: عين خوش ـ دهلران، ايلام)

An Investigation of Land-Use Effect on Dust Concentration and Soil Loss in Desert Areas: A Case of Ein Khosh-Dehloran, Ilam

يكي از مهم­ترين بحران‌هاي زيست‌محيطي در مناطق خشك و نيمه‌خشك، پديده­ي مخرب بيابان‌زايي و فرسايش بادي است. وقوع طوفان‌هاي گرد و غبار ناشي از فرسايش بادي، فرآيندي است كه باعث تخريب سرزمين مي‌شود و مي‌تواند به‌عنوان يك شاخص بيابان‌زايي قابل توجه قرار گيرد. اين پژوهش به‌منظور بررسي ميزان هدررفت خاك و غلظت گرد و غبار در كاربري‌هاي مختلف با استفاده از دستگاه‌هاي تونل باد وpro Microdust اجرا شده­است. نتايج نشان داد شدت فرسايش بادي و ميزان انتشار گرد و غبار با افزايش سرعت باد، از 2 به 16 متر بر ثانيه افزايش مي‌يابد. همچنين با توجه به نتايج مشخص شد ميزان اين پارامترها در اثر فرسايش بادي در كاربري‌هاي مختلف، متفاوت است و در هر دو سري نمونه‌هاي دست‌خورده و دست‌نخورده، بيشترين و كمترين ميزان فرسايش بادي و غلظت گرد و غبار به ترتيب در اراضي تپه ماسه‌اي، مرتع، جنگل دست كاشت كهور، كشاورزي و جنگل طبيعي كنار مشاهده شد. از طرفي در كاربري‌هاي مختلف، ميانگين هدررفت خاك به ميزان mg/cm2/min 03/0 و ميانگين ذرات معلق به ميزان mg/m3 27/0 در نمونه‌هاي دست‌خورده بيش از دست‌نخورده بود؛ زيرا در اثر نمونه‌برداري از ساختمان خاك به هم خورده، پايداري بين خاكدانه‌ها از بين مي­رود و خاك به‌راحتي در معرض بادبردگي قرار مي‌گيرد. همچنين در اثر به هم خوردگي خاك، جرم مخصوص ظاهري نيز تغيير مي‌كند؛ به گونه­اي كه با افزايش جرم مخصوص ظاهري، كيفيت خاك كاهش مي‌يابد. درنهايت با توجه به نتايج همبستگي و مؤلفه‌هاي Pc1 ـ كه تأثير كنترلي بيشتري بر فرسايش بادي دارد ـ مي‌توان نتيجه گرفت مهمترين عواملي كه در توليد گرد و غبار و فرسايش‌پذيري بادي خاك تأثير دارند شامل رطوبت، بافت و ماده آلي خاك است و عواملي مانند نسبت جذب سديم، شوري، تخلخل، منيزيم و آهك نيز اهميت كمتري دارد.

1- Introduction The occurrence of dust storms caused by wind erosion is a process that causes the destruction of land and can also be considered as a desertification indicator (Xu, 2006). Generally, the formation of a dust storm depends on three factors: the presence of strong winds, a sensitive surface to wind erosion and unstable weather conditions (Xia and Yang, 1996). Humans play an important role in the formation of dust storms through changing land use in sensitive areas to sand storms. In the arid and semi-arid areas covered with tiny and unstable materials, land degradation may occur as a result of land use change caused by over-grazing and farming. As a result, wind erosion increases, leading to more sand storms (Xu, 2006). 2- Methodology For this research, first, using satellite imagery and land use map, the land uses in the study area that included the Ziziphus Nummularia natural forest, planted Prosopis Juliflora forests, agricultural land, sand dunes and rangelands were determined. After observing and accurately identifying the area, one week after the first rainfall, 15 undisturbed soil samples and 15 disturbed soil samples were collected from the area. After transferring the samples to the laboratory, the samples were exposed to dry air and then some physical and chemical properties of the soil were measured. Wind tunnel was used to determine the soil loss in different land uses, also the dust density determined using a Microdust pro device, which was installed in the outlet of the wind tunnel. This device measures dust concentrations in milligrams per cubic meter. So soil samples were simulated in 4 wind speeds including 2, 9, 16 m/s and wind erosion threshold velocity for 5 minutes. To determine the wind erosion threshold velocity, soil samples were placed in the wind tunnel. Then, by adjusting the wind speed that was possible by the inverter and using the accelerometer, the wind speed erosion threshold was measured in different land uses. In this way, the velocity has slowly increased, and the first particle that began to move was considered as the wind erosion threshold. In this study, the erosion rate was calculated from the ratio of the weight or volume of eroded soil to the sample surface. So, to determine the amount of soil loss, at the end of each experiment, the amount of sediment accumulated in the sediment trap was collected and weighed, and the soil loss was calculated based on the amount of soil erosion in grams per cm2 per minute. 3- Results Based on the results, there was no significant relationship between the dust concentration in the undisturbed and disturbed samples, but there was a significant relationship between soil loss in the undisturbed and disturbed samples. Comparison of the mean of suspended particles and the amount of soil loss in the both sample groups showed that the lowest and highest amount of suspended particles and soil losses was related to the Ziziphus Nummularia natural forest and sand dunes, respectively. According to the correlation results, there was a positive and significant correlation between wind erosion and SAR parameter (P <0.01), but there was a negative and significant correlation between the erosion and OC, Silt, SP and CS (P <0.01). Also, erosion had a negative correlation with EC, Mg, P and had a positive correlation with sand (P<0.05). The Principal Components Analysis (PCA) showed that three main components of wind erosion controller were Pc1, Pc2 and Pc3, whose quota were about 48.7%, 21% and 9.7%, respectively. 4- Discussion & Conclusions The results showed that by increasing the wind speed from 2 to 16 m/s, the intensity of wind erosion and dust concentration increased, but the amount of these parameters was various in different land uses. As in both sampling methods, these parameters had decreased from sandy hill, pasture land, planted Prosopis Juliflora forests, agriculture and Ziziphus Nummularia natural forest, respectively. In general, it can be said that in different land uses, the amount of soil loss and dust concentration in disturbed samples was more than undisturbed samples. Actually, since the soil structure is broken up during the sampling, the stability between the soil particles is lost and the soil is easily exposed to wind erosion. Also, due to the corrosion of the soil, the bulk density varies. As the bulk density increases, the soil quality will decrease (Harris et al, 1996). Finally, it was found that Pc1 components had more control over wind erosion. The components of Pc1 include EC, organic matter, Mg, lime, silt, saturation moisture content, porosity and compressive strength. These parameters have an effect on wind erosion, and cause erosion to be further reduced.