**2. Loess deposits**

For the first time in the world in 1823, a scientist, Leonhard from Heidelberg University, found a sample of yellow silty loam in an area called Harlas near Heidelberg and due to prevalence of local name, called it Loess and introduced it to the world. People of the area used the term "Loesch" for a soft soil which is probably cognate with English word "Loose", meaning soft. In 1911, Oberochov studied these sediments in more detail. This Russian geologist is actually pioneer in recognition of loess and loess- like sediments (Rozoycki, 1991).

Due to the structural nature and abundance of terrigenous particles (silt- size grains) and chemical salts, usually loesses are erodible and cause in- site and off- site damages(Pashaee,

Therefore, investigation of effective factors on sediment production of loesses and identification of most important factors on each erosion feature for proposing more effective measures for erosion control in region where dry- farming and range lands are widespread and play an important role in the economy of Golestan Province are very helpful. Due to the fact that loess deposits are widespread in Golestan Province, especially in Gorganrood Catchment which are exploited improperly (dry- farming in sloped lands and overgrazing of rangelands), the amount of yearly erosion in some subcatchments of Gorganrood Catchment reaches about 20 t/h/y of which a major part is from surface and gully erosions

Although scattered studies about erosion and sediment production of loesses in Iran have been performed, but comprehensive studies from the view point of effective factors in sediment production, prioritization of each factor and determination of the amount of their effects, have not been performed yet. The objective of this research is determining the effective factors on erosion of loesses on surface and rill erosion features. For this purpose, portable rainfall simulator with 1 m2 plot was used. Rainfall Simulator has two advantages. First, the speed of performance of research projects increases notably, because waiting for the occurrence of rainfall is not needed. Second, by controlling some of the most important rainfall parameters, the results of study are usually more accurate and reliable. By using rainfall simulator, analogy, reasoning and statistical relationships of the results obtained from natural rainfall close to considered conditions are not necessary and similar rainfalls

The results from the study about the effective factors on erodibility of non- loessic areas cannot be generalized for loessic regions including northern part of Iran. Therefore, it is necessary that research and investigations would be performed in this region with respect to particular status and conditions of the area, in order to be able to identify the main effective factors on erosion and sediment production of loesses in predominant erosion features of the region and subsequently, with applying suitable measures, the erosion of these deposits

The research has been performed with the objective of determination of effective factors on erosion and sediment production of loesses and proposing regression models between sediment production of loesses and some of their physical and chemical characteristics, using field rainfall simulator in semi- arid climatic condition of Sarab Drainage Basin on surface and rill erosion features. With regard to the prevailing conditions of the area, it is tried to determine the most important effective factors on erosion and the amount of their

For the first time in the world in 1823, a scientist, Leonhard from Heidelberg University, found a sample of yellow silty loam in an area called Harlas near Heidelberg and due to prevalence of local name, called it Loess and introduced it to the world. People of the area used the term "Loesch" for a soft soil which is probably cognate with English word "Loose", meaning soft. In 1911, Oberochov studied these sediments in more detail. This Russian geologist is actually pioneer in recognition of loess and loess- like sediments (Rozoycki,

(Golestan Province Watershed Management, 2003).

can be simulated in order to obtain repeatable results (Refahi, 2001).

1998).

would be controlled.

effectiveness.

1991).

**2. Loess deposits** 

Despite of many studies which have been performed on loesses, a comprehensive and acceptable, international definition of loesses does not exist yet which can be due to complex formation, physical characteristics and diversity of loesses on the surface of the earth. Therefore, many researchers express loess according to their own objective and subjective. Loess and loess- like deposits cover about %10 of the earth surface. The main areas covered by loesses are: China, Central Asia, east and central Europe, parts of North America, Argentina and New Zealand (Rozoycki, 1991).

One of the effective sedimentary processes in Quaternary of Iran is combined glacialaeolian processes which formed loess deposits. Loess deposits of Golestan Province have the area of more than 320000 ha, of which 220000 ha are located in Gorganrood Catchment (Khaje, 2003). By researching paleosoles of Iran, Kehl et al. (2005) proposed that the formation of paleosoles formed in interglacial and more arid periods after interglacial phases of Quaternary period.

According to the definition, typical loess or real loess is a porous, non-layering, yellow and homogenous sediment which has undergone a little diagenesis and its particles are mostly coarse-grain silt. Different perspectives have caused naming loess different terms such as rock, sediment, formation or soil.

Loesses which have been transported and deposited in a new place by different processes, mainly water, have been called "Reworked Loess". Some of the most typical differences of these loesses with true loesses are that reworked loesses are heterogeneous and layered. Usually these deposits have undergone erosion and sedimentation phases and fine to coarse gravels may also be found in them. Paleosoles which are present as interlayes in some loesses are red horizons which have been formed due to climatic changes in Quaternary and formation of warm and humid climatic conditions.

Mineralogical composition of most of loesses includes: Quartz, Feldspar, Calcite, Dolomite, Mica and Clay minerals. Due to chemical weathering of ferroginous minerals and formation of iron oxide and hydroxide, the color of loesses is usually yellow and rarely brown (Okhravi and Amini, 2001). They usually form highly sloped even vertical walls. Due to the fact that particles of loesses are angular, their porosity is high, but due to the abundance of silt particles, low permeability and absence of cement between the particles, they are usually erodible and produce high amount of runoff. Loessic deposits of the studied area are usually relatively resistant in arid conditions but in contact with water, they are highly unstable and erodible. These deposits usually have considerable thicknesses that can be observed in roadcuts (Picture 1).
