تاثیر پوشش گیاه یونجه بر رواناب تولیدی در مقیاس کرت‌های فرسایشی با استفاده از ‏‏‏‏شبیه‌‏ساز‎‎ باران

پوشش ‏گیاهی‎ یکی از عوامل مهم در کاهش میزان رواناب به واسطه ربایش قطرات باران توسط تاج پوشش و کاهش انرژی جنبشی آن شده که این امر نشان دهنده نقش بالای ‏‏پوشش‏ گیاهی‎ در حفاظت خاک است. این پژوهش با هدف بررسی نقش ‏‏پوشش ‏گیاهی‎ در مهار میزان رواناب با استفاده از ‏‏‏‏شبیه‏ ساز‎‎ باران انجام شده است. در این پژوهش از سه سطح ‏‏پوشش ‏گیاهی‎ (35، 65 و 90 درصد) و یک تیمار شاهد (بدون پوشش) در سه تکرار تحت بارندگی با شدت 90 ‏‏میلی ‎متر‎ بر ساعت و دوام 70 دقیقه‌ای استفاده شد. با توجه به نتایج به دست آمده با افزایش سطح پوشش از صفر به 35، 65 و 90 درصد، حجم رواناب به طور متوسط به ترتیب 40/77، 51/02 و 59/49 درصد کاهش یافت. همچنین ضریب رواناب در تیمارهای مذکور به ترتیب 40، 41 و 72 درصد کاهش یافته است. نتایج حاصل از بررسی نمودار تغییرات زمانی رواناب نیز نشان داد که در انواع سطح پوشش‌‌ها میزان رواناب در دقایق ابتدایی آزمایش با شیب بالایی افزایش یافته و با نزدیک شدن به زمان‏ها‎ی میانی و انتهایی به یک مقدار ثابت رسیده‌است. این یافته‌‌ها حاکی از آن بود که با نزدیک شدن به انتهای آزمایش و اشباع شدن خاک کارایی کاهش رواناب در انواع سطح پوشش‌ها کمتر شده است. نتایج حاصل از آزمون تجزیه واریانس یک طرفه نیز نشان داد که مقادیر حجم رواناب در تیمار شاهد با تیمارهای مختلف ‏‏پوشش ‏گیاهی‎ دارای اختلاف ‏معنادار‎ی در سطح یک درصد هستند. در یک نتیجه‏ گیری نهایی می‏ توان بیان نمود که هر سه سطح ‏‏پوشش‏ گیاهی‎ (35، 65 و 90 درصد) عملکرد مناسبی در کاهش رواناب دارند. لذا برآورد میزان رواناب تولیدی در درصدهای مختلف برای شناخت چگونگی کنترل فرسایش خاک و سیل‌خیزی و همچنین مشخص کردن بهینه‌ترین درصد سطح پوشش از اهمیت ویژه‌ای برخوردار است.

the effect of alfalfa cover on runoff generation at the scale of erosive plots using a rainfall simulation

introductionvegetation is a universal and effective method for water and soil conservation. the significant implications of soil loss necessitate the adoption of appropriate soil conservation practices and soil management solutions according to the regional climate. investigation of the effects of soil cover on the soil erosion process can provide the information necessary for the success of soil and water conservation practices. in general, vegetation cover can mitigate runoff by increasing water infiltration, enhancing sediment trapping, reducing the kinetic energy of raindrop impact, increasing surface roughness, decreasing flow velocity, and controlling runoff hydraulic properties. the impact of vegetation cover on runoff generation has been studied at different spatial scales, from experimental to watershed scales, and most studies have shown the effectiveness of vegetation cover in reducing surface runoff. in arid regions, it may be difficult and economically infeasible to cover the entire surface area of a soil conservation project with vegetation. therefore, the question arises as to how much of the soil surface should be covered to have an optimal impact on runoff generation. the main objective of this study was to evaluate the effects of different ratios of alfalfa coverage on runoff generation under simulated rainfall conditions. materials and methodsa completely randomized design was used with treatments based on the ratio of alfalfa coverage (0, 35, 65, and 90 %) under artificial rainfall conditions. the texture of the studied soil was sandy loam and collected from the surface horizon of farmlands in kashan. all experimental plots were 2 m long, 0.55 m wide, and 0.35 meters deep with a metal runoff collector. alfalfa seeds were planted at a spacing of 15×15 cm in the 35 % plot, 10×10 cm in the 65 % plot, and five in 10 cm in the 90 % plot. all plots were subjected to a simulated rainfall with an intensity of 90 mm hr-1 for 70 min. to achieve uniform antecedent moisture conditions, all plots were subjected to a 10-minute long simulated rainfall at an intensity of 90mm h-1 exactly one hour before the experiment.during the experiment, the runoff and sediment samples were gathered using the metal collector (placed at the plot outlet) at 5-minute intervals. in order to investigate the changes in runoff reduction efficiency in different treatments and different testing periods, statistical criteria of runoff reduction efficiency were used. the kolmogorov–smirnov, one-way anova, and duncan’s tests were used for data analysis. results and discussionthe results indicated that the plant cover treatments were most effective in reducing runoff during the first 15 minutes of the experiments. however, their effectiveness decreased as the experiments progressed due to the soil’s inability to pass water at a higher rate once the infiltration reached its final rate. the coverage ratio treatments (0, 35, 65, and 90 %) showed a significant difference in runoff generation (p <0.01). the bare plots had the highest average runoff generation and were classified into a separate group (a). the plots with 35, 65, and 90 % coverage ratios had average runoff rates of 0.329 mm min-1, 0.222 mm min-1, and 0.112 mm min-1, respectively, and were placed in separate groups as well. in the bare soil treatments, the high amount of rainfall drops on the soil surface without vegetation interception exceeded the infiltration rate and subsequently produced runoff. additionally, soil sealing and crusting processes due to raindrop impacts could reduce infiltration and generate higher runoff. the effectiveness of runoff reduction was also evaluated at 35, 65, and 90 % alfalfa cover. the results showed that 65 and 90 % cover significantly reduced runoff production, but no significant difference was observed between the 35 and 65 % levels. conclusionin this study, we evaluated the influence of alfalfa coverage ratios on runoff generation in experimental conditions subjected to artificial rainfall. we found that higher coverage of alfalfa was associated with a delay and reduction in runoff generation. while all the examined vegetation cover percentages demonstrated an acceptable performance in reducing runoff and sediment. furthermore, we were able to determine a practical threshold for coverage ratio, which could result in a significant decrease in surface runoff production. we found that a minimum of 65% alfalfa coverage was necessary to achieve this reduction. this threshold could be considered a beneficial criterion for soil and water conservation practices, especially in arid areas where dense vegetation establishment can be difficult and more expensive. this study is also particularly important for dry and semi-arid regions that face challenges of drought and soil erosion. the use of alfalfa as a vegetative cover can help to conserve soil moisture, reduce erosion, and improve water quality in these areas. in addition, vegetation plays a crucial role in soil and water conservation, and alfalfa, being a perennial plant with a deep root system, is a suitable option for erosion and runoff control in various regions, especially in dry and semi-arid areas.