پهنه‎بندي حسّاسيّت اراضي به فرسايش خندقي در حوضه‎ي زوّاريان استان قم

Zoning of Land Sensitivity to Gully Erosion in Zavvarian Basin, Qom Province

يكي از انواع فرسايش آبي كه موجب تخريب اراضي و برهم‎خوردن تعادل در پهنه‌هاي منابع طبيعي و اراضي كشاورزي مي‎شود و خسارات سنگيني به‎بار مي‎آورد، پديده‎ي فرسايش خندقي است. براي كنترل اين پديده، شناخت عوامل موثّر و پهنه‌بندي حساسيت اراضي نسبت به آن، اهميت ويژه‌اي دارد. تخريب اراضي، بر‌هم خوردن منظر زمين و تعادل اكولوژيك و به مخاطره افتادن منابع زيستي در اين مناطق، ازجمله موارد ديگري است كه پژوهش درباره‎ي فرسايش، به‎ويژه فرسايش خندقي در اراضي استان قم را گريزناپذير و الزامي مي‎كند. در همين راستا، حوضه‎ي آبخيز زوّاريان در استان قم، براي انجام پژوهش انتخاب شده است. روش‌هاي مختلفي براي پهنه‌بندي حسّاسيّت اراضي به فرسايش خندقي وجود دارد. در اين پژوهش از شاخص همپوشاني براي پهنه‌بندي در منطقه استفاده شد. براي تهيّه‎ي نقشه‎ي حسّاسيّت فرسايش خندقي از عكس‎هاي هوايي، تصاوير ماهواره‎ايETM+ و براي عمليّات ميداني از GPS استفاده شد. بررسي‌ها نشان داد كه جنس زمين، شيب، منابع و قابليّت اراضي، جهت شيب و كاربري اراضي، عوامل اصلي موثّر در وقوع فرسايش خندقي هستند. نتايج حاصل از اين پژوهش نشان داد كه واحدهاي سنگ‎شناسي Q2 (تراس‎هاي جوان و مخروط‎افكنه‌هاي درشت‎دانه ـ دوره‎ي كواترنري) 41/94 درصد و تيپ اراضي دشتي و تپّه‎ماهوري 96/99 درصد از مناطق تحت تاثير فرسايش خندقي را به خود اختصاص داده‌اند، درضمن طبقه‎ي شيب 0-10 درصد (87/84 درصد)، جهت شيب شمال، مشرق و جنوب‎شرقي با (06/69 درصد) و كاربري مراتع متوسّط و زمين‌هاي كشاورزي(85/98 درصد) داراي بيشترين گسترش مناطق تحت تاثير فرسايش خندقي در حوضه‎ي آبخيز مورد مطالعه هستند.

Introduction The most important form of soil erosion is gully erosion which reduce production amount, can restrict land use seriously and can be a serious danger for fences, roads and different structures. A gully is a channel or small valley with relatively steep slopes which occurs after heavy rainfall or by water erosion caused by snow melting. The gully erosion includes an area of 1085/80 acres of Zavvarian basin and close to 7.77 percent of arable lands of the case study area. Thus it is necessary to recognize effective factors in formation and development of the gullies and also to do the zoning of land sensitivity to gully erosion in order to optimize the management of the risk. This study aimed to identify the gully erosion effective factors, growth and expansion of this type of erosion and finally review and analysis of these factors. Methodology In this study, library method, site visit and conceptual models have been used. First, maps of the gullies have been prepared by using aerial photographs, satellite imagery, topographic maps, geological maps and field operations. For this research, after digitizing geological layers, elements, land slope, land use and aspect in GIS and by using ILWIS software, the variables have been classified and then the associated factors with gullies have been studied. The next step is to determine the percentage of gully area in each class, each weighting factor and weighted maps were prepared and then they have been combined together. Finally by using overlay index and matching with existing condition, zoning of land sensitivity to gully erosion have been done and results and recommendations were presented. Results and Discussion Assessing the relationship between slope and aspect in gully erosion affected areas show that the most relative abundance of gully erosion is in the lands with slope of zero to 20 percent and also there is the most gully frequency in the slopes with direction to north, east and southeast. Lands with south direction, flat shape and northwest direction are ranked second in terms of relative frequency formation and gully erosion spread. In terms of Lithology, the most percent of gully formation belongs to young terraces and coarse alluvial fans of quaternary period and the lowest ones belong to old terraces and long alluvial fans of quaternary period. About 69.69 percent of the gullies are in plain lands, 30.27 % are in hills and about 0.04 of the gullies are placed in mountain lands of 1. The most amounts of the gullies (98.85 %) are in the average rangelands and agricultural lands and the lowest amounts (1.5 %) are in the gardens and residential areas. About 1085.80 hectares of the area have been affected by gully erosion. Conclusion In conclusion, the natural conditions of Zavarian basin such as geomorphology and geology has prepared an appropriate condition for gully erosion occurrence. The studies shows that the land type, sources and land capabilities, slope aspect and land use are the main factors in gully erosion occurrence. The results of this study show that lithologic unit of Q2 (young coarse alluvial fans of quaternary period) has allocated 94.41 percent and the plain land type and hills have allocated 99.96 % of the affected area. Additionally, most of the affected areas by gully erosion in the studied catchment are the areas with slope of 0-10 percent (84.87 %), aspect of north, east and southeast (69.06 %) and average rangelands and agricultural lands (98.85 %).