چکیده:
تحقیق حاضر با هدف بررسی اثر 9 تیمار از ترکیبهای مختلف پوشش گیاهی شامل غالبیت گرامینه با ارتفاع کم (T1)، ترکیب بوتهای متراکم با گرامینه (T2)، بوتهای با ارتفاع کم و پراکنش متوسط (T3)، بوتهای پراکنده با ارتفاع عمدتاً کم و متوسط (T4)، ترکیب بوتهای پراکنده با گرامینه (T5)، بوتهای متراکم در بالادست (T6)، بوتهای با ارتفاع کم و پراکنش بسیار کم (T7)، بوتهای متراکم با پوشش تقریباً یکنواخت (T8) و بدون پوشش گیاهی (شاهد) (T9) بر مؤلفههای رواناب و رسوب انجام شد. علاوه بر این، اثر درصدهای مختلف (صفر، 40 و 60-40) پوشش گیاهی بر تغییرات مؤلفههای رواناب و رسوب نیز بررسی شد. تیمارهای مطالعاتی با سه تکرار در قالب طرح بلوکهای کاملاً تصادفی و در کرتهای صحرایی به ابعاد 2×1 متر و شیب تقریبی 15-12 درصد، در بارندگی با شدت 32 میلیمتر بر ساعت با تداوم زمانی 18 دقیقه بررسی شد. نتایج تحقیق حاضر نشان داد که اثر ترکیبهای مختلف پوشش گیاهی بر مؤلفههای رواناب و رسوب، معنیدار (0001/0>p-value) بود؛ به طوری که حداکثر زمان شروع رواناب (33/1388 ثانیه) در تیمار T4 و حداقل میزان رواناب (41/0 لیتر) و ضریب رواناب (14/2 درصد) در تیمار T2 بهترتیب با 98+، 82- و 82- درصد تغییر نسبت به تیمار شاهد بهدست آمد. همچنین حداقل هدررفت خاک برابر با 30/1 گرم، در تیمار T2 و حداقل غلظت رسوب برابر با 6 گرم بر لیتر، در تیمار T8 بهترتیب با 86- و 69- درصد تغییر نسبت به تیمار شاهد بود. تجزیه و تحلیل آماری مربوط به اثر درصدهای مختلف پوشش گیاهی نیز نشان داد که بین میانگین زمان شروع رواناب و غلظت رسوب (001/0>p-value)، اختلاف معنیدار و بین میانگین مقدار و ضریب رواناب (73/0p-value) و هدررفت خاک (15/0p-value)، اختلاف غیرمعنیدار وجود دارد.
1- Introduction
Today, the increasing population and, consequently, the demand for agricultural products have caused the natural cover of land, especially forests and pastures to be destroyed by humans at an alarming rate to become agricultural land; even in many areas due to lack of water and nutrients, vegetation growth is limited. Declining vegetation worldwide due to human activities such as overgrazing and deforestation reduces permeability and consequently increases runoff and can reduce soil particle adhesion and predisposes fertile soil particles to erosion. Soil erosion in managed ecosystems such as crops, pastures, or forests, as well as in natural ecosystems leads to extensive damage. It also reduces the infiltration capacity due to runoff and reduces soil organic matter and thus valuable soil nutrients. At the same time, it significantly reduces the diversity of plant and animal species. To this end, controlling soil erosion is one of the most important goals in water conservation and management programs. Vegetation can be a very important tool to control water erosion and regenerate the ecosystem. Vegetation reduces the shear stress by increasing the roughness and decreasing the water flow velocity, and the hydraulic resistance created by the vegetation causes the absorption and deposition of suspended sediments. Vegetation and its associated factors on a long-term scale also play an important role in modifying the hydrological properties and soil erodibility and sediment load. The role of vegetation in reducing runoff and soil erosion in different studies has been proven. However, the effects of different vegetation compositions have not been studied extensively in runoff and soil erosion control. Accordingly, the present study was planned to investigate the effect of different compositions of graminea and bushes with different coverage percentages on runoff and sediment components.
2- Methodology
The study area is part of the natural rangelands located in the surroundings of the University of Mohaghegh Ardabili, Ardabil, NW of Iran. A total of nine treatments from different vegetation compositions including low-height graminea predominance (T1), the composition of dense bushes with graminea (T2), bushes with low-height and medium-distribution (T3), sparse bushes mostly with low and medium height (T4), the composition of sparse bushes with graminea (T5), dense bushes in upper parts (T6), low-height bushes with very low distribution (T7), dense bushes with almost uniform distribution (T8), and no vegetation cover (control) (T9) were selected. In addition, the effect of different percentages (zero, 2 with a slope of 12-15% were installed. Study treatments with three replications were designed in a completely randomized block with help of field plots with dimensions of 2*1 m and an approximate slope of 12-15%. This study was performed using a rainfall simulator with an intensity of 32 mm h-1 and a duration of 18 min. The plots were placed in a rectangular in the direction of the slope, using 15 cm high metal sheets, five cm of which were sunk into the soil so that the generated runoff did not seep out of the plots. Totally, five components including time to runoff, runoff volume, runoff coefficient, soil loss, and sediment concentration were measured for each plot.
3- Results
The results showed that the effect of different vegetation compositions on runoff and sediment components was significant (p-value -1 in treatment T8 with -86 and -69% change compared to the control treatment, respectively. Statistical analysis of the effect of different vegetation percent also showed that there was a significant difference between the mean time to runoff and sediment concentration (p-value
4- Discussion & Conclusions
The runoff threshold in different compositions and percentages of vegetation has a significant difference compared to the control treatment. Vegetation in both groups of