بررسي نقش شيب و ويژگي هاي خاك در ايجاد فرسايش شياري در دامنه ها (مطالعه موردي: حوزه آبخيز تهم چاي، شمال غرب زنجان)

The role of slope steepness and soil properties in rill erosion in the hillslopes (A case study: Taham Chai catchment, NW Zanjan)

سابقه و هدف: فرسايش شياري يكي از مهمترين اشكال فرسايش خاك در دامنه ها است كه به دليل تمركز رواناب سطحي به وجود مي آيد. به طور معمول شيارها به عنوان مسيرهاي كوچك و موقت جريان متمركز هستند كه به عنوان منبع مهم رسوب و تحويل رسوب در دامنه ها نقش ايفاء مي كنند. گسترش شيارها اغلب در اراضي مرتعي تحت چراي شديد كه ظرفيت نگهداشت آب آن ها پايين بوده و يا در اراضي تازه شخم خورده كه ساختمان آن ها از بين رفته است، رخ مي دهد. فرسايش شياري بخش بزرگي از فرسايش آبراهه اي را در اين عرصه ها به ويژه در مناطق نيمه خشك در بر مي گيرد. فرسايش شياري در دامنه ها مي تواند تحت تاثير عوامل مختلف از جمله خصوصيات توپوگرافي، وضعيت پوشش گياهي، شرايط بارندگي، ويژگي هاي خاك و روش هاي مديريتي قرار گيرد. شناخت عوامل موثر بر گسترش فرسايش شياري براي برنامه ريزي مناسب جهت كاهش گسترش اين نوع فرسايش ضروري است. اين پژوهش تلاشي براي يافتن عوامل موثر بر گسترش شيار در دامنه هاي مرتعي بود. مواد و روش ها: براي اين منظور تعداد ده دامنه با شيب متفاوت و تحت فرسايش شياري و فاقد پوشش گياهي در حوزه آبخيز تهم چاي در شمال غربي زنجان انتخاب شد و تمام مشخصات شيارها همراه با برخي ويژگي هاي خاك در آن ها اندازه گيري شدند. تفاوت مشخصه هاي شيار، درجه شيب و ويژگي هاي خاك بين دامنه ها مورد ارزيابي قرار گرفت. تاثير تندي شيب و ويژگي هاي خاك بر فرسايش شياري با استفاه از روش ماتريس همبستگي تعيين شد. از روش رگرسيون خطي چندگانه براي ارائه رابطه اي براي برآورد فرسايش شياري در دامنه هاي منطقه استفاده شد. يافته ها: بر اساس نتايج، غير از طول شيار، ساير ويژگي هاي شيار تفاوت معني دار بين دامنه ها داشتند. همچنين تفاوتي معني دار بين خاك شيارها از نظر شن، سيلت، رس، هدايت هيدروليكي اشباع و درصد سديم تبادلي وجود داشت. مساحت مقطع عرضي شيار از 01/ تا 0/29 متر مربع با ميانگين 0/07 مترمربع بين دامنه ها متغير بود. اين متغير به عنوان شاخص مهم شيار براي توصيف گسترش فرسايش شياري در دامنه ها تشخيص داده شد. اين شاخص، هم بستگي معني دار با تندي شيب، سيلت، رس و هدايت هيدروليكي اشباع خاك داشت. مقدار مساحت مقطع عرضي شيار در دامنه هايي كه داراي تندي شيب بالاتري بوده يا در دامنه هايي كه داراي خاكي با ذرات سيلت و رس بيشتري بودند، بالاتر بود. تجزيه رگرسيوني خطي چندگانه نشان داد كه سطح مقطع عرضي شيار رابطه اي معني دار با تندي شيب و درصد سيلت خاك دارد (

Background and Objectives: Rill erosion is one of the most important soil erosion types in the hillslopes which occurs due to concentration of surface runoff. In general, rills are a small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Rill development is common in overgrazed land where soil water retention capacity is diminished and also in freshly cultivated soil where the soil structure has been loosened. Rill erosion can be a large portion of the channel erosion in these areas, particularly in semi-arid regions. Rill development in the hillslopes can be affected by different factors including topographic characteristics, vegetation cover condition, rainfall characteristics, soil properties and management practices. Knowledge of factors affecting rill erosion development is necessary to control this erosion in the hillslopes. Materials and Methods: This study was conducted to find factors influencing rill development in semi-arid rangelands in the TahamChai catchment, north west of Zanjan, Iran. Toward this, ten sparse hillslope as affected by rill erosion were selected and all rill characteristics along with some soil properties were determined in the rills. Rill characteristics, slope steepness and soil properties were compared among the hillslopes. Effects of slope steepness and soil properties on the rill erosion were recognized using the correlation matrix method. Multiple linear regression analysis was used to develop an equation for estimating rill erosion in the hillslopes. Results: Results indicated that all rills characteristic expect length were significantly differed among the hillslopes. Significant differences were found among the hillslope soils in sand, silt, clay, saturated hydraulic conductivity and exchangeable sodium percentage. Rill cross section area varied from 0.01 to 0.29 m2 with an average of 0.07 m2 in the hillslopes. It was recognized to be the most important rill characteristic to describe rill erosion development in the hillslopes. Significant correlations between this rill characteristic were observed and slope steepness, sand, silt, clay and saturated hydraulic conductivity. Rill cross section area was the highest where either the hillslopes tend to have higher slope steepness or they have higher percentage of silt and clay. Multiple linear regression analysis appeared that rill cross section area in the rangeland hillslopes was significantly related to slope steepness and silt (R2=0.38, P<0.01). In fact, about 38% of variation of rill erosion in the hillslopes could be explained by the developed equation. Slope steepness was recognized as the major topographic factor which explained about 20% of rill erosion variation in the hillslopes. Conclusion: With regarding to the importance of slope steepness and silt in the rill erosion development, maintaining vegetation cover through preventing over-grazing in the hillslopes of the area where slope steepness is higher and the soil is sensitive to water erosion processes is very essential. According to the developed equation there are also other variables which may control rill development in the hillslopes. Land shape, soil profile characteristics and land surface cover can be introduced as unknown variables which can be investigated in the next studies in order to develop a reliable model to prediction of rill erosion in the hillslopes.