تعیین آستانه‏ های اقلیمی و توپوگرافیکی برای شروع فرسایش خندقی در شیب‏ های نواحی نیمه ‏خشک (مطالعة موردی: حوضة اوجان‏چای، دامنه‏ های شرقی کوهستان سهند)

In semi arid regions,gully and rill erosion are main types of water erosion.In recent time these erosion is very intensive ,due to climatic changes.The surfaces of slopes is disturbance, due to creation new channels or make loose old channel by incision runoff .When channel made on slope ,slope materials are delivered into rivers in short time, that these materials are cultivated soils .Surface erosion and soils delivery into flow can be damages to cultivated area .Drainage networks are experience intensive liner erosion that are located on eastern slopes of Sahand ,special Ojan chay basin ,as a semi arid region .The storm occurrence is creating runoff that have erosion power, just before plants is covering slopes surface. Deep channels that there are created on up slopes and foot slopes are suggested which erosion is very accreted .Range cultivated lands on slopes is play important role on quick concentrated runoff in study area. Soils is delivery into water at lower sites in during storm ,in these conditions.

In study area ,liner erosion is intensive ,because surface material ,topographical property and climatic condition is suitable for liner erosion. Anthropologic factor is play main role on gully and rill erosion. In this paper for threshold liner erosion start determination, is used climatic data, topographic maps(1 :50000) and EXCELL ،SPSS وARCVIEW .Because that climatic factors is play main role on liner erosion ,is take care of in this paper .HTK is climatic coefficient that is used. HTK=ΣR/Σt.10,HTK is climatic threshold liner erosion , ΣR,is precipitation , Σt.10,is temperature (upper 10).Slope and area are play main role in rate of erosion. To attention of this role ,threshold of erosion is estimated by use of two parameters(A≈6As/10000 and r =SAs0.4(6/10000)0.4› 0.025 ).In final is zoned from an liner erosion ,all part of study area by using of statistical methods and by data that they gathering from study area . Gully and rill erosion are main reasons for increase and delivery of sediment into river in semi arid area .These forms of erosion are delivery huge of slope materials into water flow .Store of these material on foot slopes and washing they by floods ,is reason for increase of sediment into water flow in Ojan chay basin. Soils are deep in study area .These soils that liner erosion is active ,without salty in somewhere and are salty in other site .Study on soil horizons is suggested that silt and clay percent is high on upper part of horizon .These texture is sensitive to opposed to water erosion. The presence of sand on upper part of horizon is cause for gully walls instability.

In study area ,slope ,surface materials and area of upper site are main parameters on slope disturbance .Thresholds of erosion on slope is determined by use of these parameters. Yearly average of precipitation is decrease and this is one reasons for suitable site for gully erosion. The results of study show that HTK is high in some months .This suggested that slopes is advanced to erosion in these months .This result show that runoff have power erosion in all parts of Ojan basin and soil is lose by water erosion. Field study show that when slopes is gentle , gully and once slopes is steep ,rills are created on slopes. Up walls of gully is very instable .Investigation slope of by using of A coefficient show that potential of erosion is high in many parts of study area .Liner erosion is main problem on slope of study area. Ojan chay basin is important farmer land that is main site for dry and water farming in Sahand mountain 3qu1cai8l8. This suggested that slopes is advanced to erosion in these months .This result show that runoff have power erosion in all parts of Ojan basin and soil is lose by water erosion. Field study show that when slopes is gentle , gully and once slopes is steep ,rills are created on slopes. Up walls of gully is very instable Liner erosion is main problem on slope of study area. Ojan chay basin is important farmer land that is main site for dry and water farming in Sahand mountain. In this study area lose of soil and disturbance of farmland by runoff ,have prejudicial for farmland and is increased sediment. The results of study show that this basin have high potential for liner erosion . The main reason for intensive liner erosion is quick concentration runoff, as natural or anthropologic .These cause is active on slope of study area and delivered high volume of soils into lower channel .Walls of gullies is unstable and many materials delivery in to water flow .Development of gully is very quick .For these reasons ,cause of gully creation must be investigated and slope management . Liner erosion is main problem on slope of study area. Ojan chay basin is important farmer land that is main site for dry and water farming in Sahand mountain. In this study area lose of soil and disturbance of farmland by runoff ,have prejudicial for farmland and is increased sediment. The results of study show that this basin have high potential for liner erosion . The main reason for intensive liner erosion is quick concentration runoff, as natural or anthropologic .These cause is active on slope of study area and delivered high volume of soils into lower channel .Walls of gullies is unstable and many materials delivery in to water flow .Development of gully is very quick .For these reasons ,cause of gully creation must be investigated and slope management .Liner erosion is main problem on slope of study area. Ojan chay basin is important farmer land that is main site for dry and water farming in Sahand mountain. In this study area lose of soil and disturbance of farmland by runoff ,have prejudicial for farmland and is increased sediment. The results of study show that this basin have high potential for liner erosion . The main reason for intensive liner erosion is quick concentration runoff, as natural or anthropologic .These cause is active on slope of study area and delivered high volume of soils into lower channel .Walls of gullies is unstable and many materials delivery in to water flow .Development of gully is very quick .For these reasons ,cause of gully creation must be investigated and slope management .