تعيين مناسب ترين اندازه پلات براي نمونه برداري از سنگ‎فرش بيابان

Determining Optimum Plot Area to Measure Desert Pavement (Case Study: Khezrabad, Yazd)

سنگ‎فرش بياباني يكي از عوامل محافظت‎كننده خاك سطحي در مقابل فرسايش بادي در مناطق بياباني است. درصد تراكم سنگ‎فرش و قطر ذرات، يا به‎نوعي دانه بندي ذرات، از مهم‎ترين عوامل تعيين كننده ميزان حفاظت خاك توسط اين ذرات است. نمونه برداري هاي سنگ‎فرش بيابان، به‎طور معمول در عرصه و به‎صورت تخميني يا با استفاده از خط كش و پلات انجام مي گيرد؛ ولي در هيچ يك از منابع، ابعاد استانداردي براي پلات در نمونه‎برداري از سنگ‎فرش بيابان ارايه نشده و حتي روشي نيز براي تعيين آن معرفي نشده است. در اين مطالعه با استفاده از يك روش نوآورانه، ابعاد بهينه پلات براي نمونه برداري از سنگ‎فرش بيابان در انواع مختلف دشت سر تعيين شده است. اين روش برمبناي افزايش ابعاد پلات و ترسيم منحني هاي دانه بندي متعدد در پلات هاي با ابعاد مختلف و مقايسه آنها با يكديگر استوار است. نتايج اين پژوهش نشان مي دهد، چنانچه هدف از نمونه برداري از سنگ‎فرش بيابان در دشت سر لخت باشد، ابعاد 40×40 سانتي‎متر، در دشت سر اپانداژ، ابعاد 30×30 سانتي‎متر و در دشت سر لخت، ابعاد 20×20سانتي‎متر، مناسب ترين ابعاد پلات در نمونه‎برداري از سنگ‎فرش بيابان در دشت سر پوشيده است. چنانچه نمونه‎برداري از سنگ‎فرش بيابان به‎صورت كلي مطرح باشد، ابعاد مناسب پلات، 40 ×40 سانتي‎متر است.

Introduction This research focuses on desert pavement. There are several tremendous methods and classification which presented for plain classification. One of The natural resource based classification which is mostly using in Iran, classified plains into three types, Erosional Pediment, Denudation Pediment Aggradational Piedmont. The qualitative and quantitative factors to differentiate them from each other are also used appropriately. Desert pavement condition is an important factor in classifying pediment. However, there is no standard method to measure the desert pavement. In this study, a method was developed for the determination of optimum plot area. The result showed that the best plot area for Erosional Pediment is 40×40, for Denudation Pediment is 30×30 and for Aggradational Piedmont is 20×20. Then 40×40 cm plot can be use for all plain to measure desert pavement. Desert pavement plays as a significant shield over surface soil in wind erosion. If the density and diameter of the desert pavement particles are greater, then its protection function would be larger. The diameter of desert pavement diminishes with the distance from mountain area. Well away from the mountains decreases the diameter of desert pavement particle. Methodology The Khezrabad Basin is located in the central Iran; in Yazd Province; and lies between 51?41ʹ 08" to 54? 13ʹ 33" east longitudes and 31?45ʹ07" to 32? 09ʹ 30" north latitudes). It has an area of 129318. Square plots are used for sampling of the desert pavement. So far it was not identified an optimized plot size for sampling of the desert pavement. However, in vegetation studies, there is a method namely minimum plot area which determines the optimized plot size in each vegetation type. But there is no proper method to demonstrate the optimized plot size in desert pavement sampling. In this study, we presented a new method to identify an optimized plot size in the desert pavement sampling. We also demonstrated an optimized plot size for sampling the desert pavement in different types of plains deserts .The geomorphological map of the targeted area was prepared using visual-digital interpretation. This map was used to determine the potential sampling points. The sampling method was classified randomly, next three points were selected randomly in each geomorphic type, in this research, plots with size of 20×20 cm were used for sampling and grading tests were performed on the samples. The plot sizes increased during sampling. Next step, plot with size of 30×30 was used and finally increased up to 40×40 and 50×50 sizes. Plot size increasing continued until it was not performed any differences in grading curve. Consequently an optimized plot size was achieved when there were not significant changes in grading curve even with increasing plot sizes. Plot Dimensional in which the Increase of level, doesnʹt not change gradation curves that will be the optimal dimensions for pavement sampling in this study, ASTM method was used to perform grading test and plot grading curve as well. Cumulative granulometry curve was exhibited by weighting different diameter particles and it was performed for different plot sizes. Aggregate gradation samples curve were drawn in the plots with different dimensions. This methodology was performed for all random sampling points. Results and Discussion The results revealed that in bare plain, increasing plot sizes up to 40× 40 can effectively change grading curve but it does not make any change afterward. The optimized plot size at Denudation Pediment are 20×20 and 30×30, respectively. Further in aggradational Piedmont, plot with size of 20 ×20 cm, was appropriate and with increasing size of plot, is not created any changes in the grading curve. Conclusion The results revealed that plot size of 40×40 is an appropriate size for sampling in desert pavement and erosional Pediment. Moreover sizes of 30×30 and 20×20 or less can be appropriate optimized plots in Denudation Pediment and aggradational piedmont, respectively. Finally plot size of 40×40 can be used in any types of plain areas.