بررسي تغييرات مكاني – زماني پتانسيل جدايش ذرات خاك در فرسايش شياري مطالعه موردي؛ زمين‌هاي ديم‌زار منطقه دشمن‌زياري استان فارس

Evaluation of Spatial-Temporal Variation of Soil Detachment Rate Potential in Rill Erosion, Case study: Doshmanziari Rainfed Lands, Fars province

فرسایش خاك به عنوان یكی از عوامل اصلی تخریب زمین معرفی شده و آگاهی و شناخت از فرایندهای پیچیده وقوع و توسعه آن مورد توجه بسیاری از پژوهشگران قرار گرفته است. این پژوهش به‌منظور بررسی اثرات عوامل هیدرولیكی از قبیل تنش برشی و قدرت جریان بر تغییرات مكانی- زمانی جدایش ذرات خاك صورت پذیرفته است. محدوده مورد بررسی در زمین‌های با شیب میانگین شیب (56/22 درصد) زیر كشت دیم گندم در 60 كیلومتری غرب شیراز واقع شده است. به‌منظور ایجاد شیارهای 10 متری با عرض و عمق 30/0 متر، شخم رایج از بالا به پایین شیب انجام و ریخت شناسی شیار در فاصله‌های یك متری با اندازه‌گیری شیب و مقطع عرضی اندازه‌گیری شد.‌ بر روی شیارها دبی‌های 10، 15 و 20 لیتر بر دقیقه به مدت 10 دقیقه اعمال گردید. غلظت‌های رسوب در سه فاصله مساوی زمانی و مكانی اندازه‌گیری و از این مقادیر به‌منظور محاسبه نرخ جدایش ذرات در زمان‌ها و مقاطع مختلف استفاده گردید. نتایج نشان داد كه تغییرات تنش برشی و قدرت جریان در طول شیار بر نرخ جدایش ذرات اثر می‌گذارد. بررسی میانگین نرخ جدایش ذرات خاك نشان داد كه اختلاف معنی‌داری (05/0 >P) بین مقطع اولیه شیار با بازه‌های طولی دنباله رو آن وجود دارد. همچنین نرخ جدایش ذرات خاك به‌طور عمده در بازه زمانی پایانی آزمایش صورت پذیرفته است. این مطالعه نشان داد كه اختلاف معنی‌داری بین میانگین جدایش ذرات خاك در دبی‌های متفاوت اعمال شده وجود ندارد.

Introduction: Soil erosion by water is one of the most widespread forms of land degradation and it has caused many undesirable consequences in last decades. On steep slopes, rill erosion is the most important type of erosion, which produces sediment and rill flow. It can be also considered as a vehicle for transporting soil particles detached from upland areas. Recent studies indicate that soil detachment rates are significantly influenced by land use. It is also known that there is a major difference between detachment rates of disturbed and natural soils (Zhang et al., 2003). Plowing rills especially in steep slopes increases sediment production. Sun et al. (2013) reported that the contribution of rill erosion in hill slope lands in china was more than 70%, which was approximately 50% of total soil erosion. In addition, measured soil loss is statistically related to hydraulic indicators such as slope, water depth, flow velocity, flow shear stress and stream power (Knapen et al., 2007). This study aims to evaluate the effects of hydraulic variables (shear stress and stream power) on spatial-temporal soil detachment rate. The focus is on the plowing rills in hillslope areas under wheat dry farming cultivation. Materials and Methods: The study area is located in hilly slopes with the slope of 22.56% under dry farming wheat cultivation at 60 km of west of Shiraz, Iran. Top-down conventional plowing was carried out in order to create 10 meters furrows. Slope and cross sections of rills were measured throughout the experiment at 1 m intervals by rill-meter. Water was added to the top of the rills for 10 minutes and inflow rates were 10, 15 and 20 L min-1. Hydraulic parameters such as shear stress and stream power were calculated measuring rill morphology and water depth. Flow velocity and hydraulic radius along the different rill experiments were also calculated. Sediment concentrations were measured in three equal regular time and distance intervals (measurement points (MPs)), they were considered to calculate sediment detachment rate in different times and sections of each rill experiment for spatial and temporal soil detachment rate evaluation. One-way analysis of variance (ANOVA) was employed to test the significance of differences of sediment detachment rate among different treatments. Results and Discussion: The results showed that the maximum values of shear stress and stream power were 14.07 Pa and 10.29 Wm-2 and the minimum values were 7.41 and 2.77 respectively. This research also indicated that changes in longitudinal profile of these hydraulic parameters along the rills affected the soil detachment rate values. Obtained average, minimum and maximum of the soil detachment rate were determined as 0.09, 0.02 and 0.22 kgm-2s-1, respectively. Due to Detachment-Transport Coupling mechanism, there was a significant difference between the initial and following MPs (P <0.05) of soil detachment rate along the rills. This means that in the following MPs, the sediment load from rill upside could affect the capacity of flow to detach sediment from the rill bed. Accordingly, after sediment loading till sediment transport capacity, runoff energy is capable to transport the sediment. Soil particles detachment consequently is reduced or stopped. Temporal investigation identified that there was enough time for undercutting and falling rill bank, which provides available sediment in final stage of experiment for transportation. As a result, the contribution of soil detachment rate was higher than initial stages of the experiment. This study also showed that there is no significant difference among soil detachment rates in applied inflow rates. This result can be attributed to the important role of rill bed geometry when the flow depth is low. In other words, the initial inflow rate (10 L min-1) is strong enough to detach soil particles and move them to the downstream. Under these conditions it could be inferred that high local turbulence plays the main role for soil detachment, which inversely is proportional to increasing flow sediment load. Conclusion: This research showed that there was a significant difference between the initial MP and following MPs of soil particle detachment rate along the experimental rills. In addition, the soil detachment rate contribution in final time slot was more than the initial time stage in different treatments. There was no significant difference among soil detachment rates under varied inflow rates.