**13. Results**

#### **13.1 Erosion features of loesses**

In the studied area, different erosional features with different intensities are present. Water erosion is the main erosion processes of the area and terraces of wind erosion were not observed. Erosional features of the area are described below (Figure 7):

Investigation of Effective Factors on Runoff Generation and

Picture 6. Surface erosion in rangelands (route to Islamabad Village)

Picture 7. Surface erosion in dry-farming land (south of Dahane Village)

are increased.

Due to the presence of excess livestock, overgrazing and entrance of livestock to rangeland in inappropriate times, tracks appear as notable and visible lines (microterrace) on slopes (Picture 8). Due to the pressure of over population of livestock, soil is compacted and losses its infiltration capacity and vegetation growth, therefore, runoff and consequently erosion

Sediment Yield of Loess Deposits Using Rainfall Simulator 131

Fig. 7. Map of erosional features of studied area

#### a. Surface Erosion

Due to abundance of silt-size particles and lack of cement between particles, loesses are usually friable and shortage of vegetation causes particle susceptibility to rain drop impact and surface runoff which appear as surface erosion throughout the studied area, This feature is one of the most prominent erosional features in the area, is seen both in rangelands and dry- farming lands and play important role in sediment production of the area. During rainfalls, sediment-laden water flows are observed at the foot of slopes. With regard to the amount of surface flow, slope, vegetation cover of soil, intensity of surface erosion are different in different parts of the area. Surface erosion in rangeland is seen as the appearance of roots (Picture 6) and in cultivated land as light sports or shortage of crops in growing seasons in some parts of agricultural lands (Picture 7). This kind of erosion acts slowly but finally most of the fertilized surface layer of soil is lost and the underlying loess sediment is appeared. Transported materials, from surface erosion are usually fine clay and organic matter.

Due to abundance of silt-size particles and lack of cement between particles, loesses are usually friable and shortage of vegetation causes particle susceptibility to rain drop impact and surface runoff which appear as surface erosion throughout the studied area, This feature is one of the most prominent erosional features in the area, is seen both in rangelands and dry- farming lands and play important role in sediment production of the area. During rainfalls, sediment-laden water flows are observed at the foot of slopes. With regard to the amount of surface flow, slope, vegetation cover of soil, intensity of surface erosion are different in different parts of the area. Surface erosion in rangeland is seen as the appearance of roots (Picture 6) and in cultivated land as light sports or shortage of crops in growing seasons in some parts of agricultural lands (Picture 7). This kind of erosion acts slowly but finally most of the fertilized surface layer of soil is lost and the underlying loess sediment is appeared. Transported materials, from surface erosion are usually fine clay and

Fig. 7. Map of erosional features of studied area

a. Surface Erosion

organic matter.

Picture 6. Surface erosion in rangelands (route to Islamabad Village)

Picture 7. Surface erosion in dry-farming land (south of Dahane Village)

Due to the presence of excess livestock, overgrazing and entrance of livestock to rangeland in inappropriate times, tracks appear as notable and visible lines (microterrace) on slopes (Picture 8). Due to the pressure of over population of livestock, soil is compacted and losses its infiltration capacity and vegetation growth, therefore, runoff and consequently erosion are increased.

Investigation of Effective Factors on Runoff Generation and

c. Gully erosion

reaches 10 m/y.

shown in the picture with arrow.

d. Bank Erosion

Sediment Yield of Loess Deposits Using Rainfall Simulator 133

Contrast to rill erosion, gully erosion is the prominent erosion feature in the studied area, which in addition to soil loss, causes major damages to agricultural lands, roads, houses and other establishments. Gully erosion starts as headcuts in most parts of the area and it happens suddenly. In some parts, stepped gullies are observed which in time change to single gully. In addition to the nature of loesses and their erodibility, degradation of ranges and agricultural lands, increase runoff volume which causes formation of big gullies(Picture 10). The walls of gullies are falling, collapsing and retreating which cause increase of the area and volume of gullies and intensification of damages. In some parts of gullies, vegetation growth has stabilized the substrate. In regard to the gathered information from residences and local experts, the rate of gully advancement in the area is different and may

Picture 10. Gully erosion (Gharnagh Village). Advancing headcut toward the village is

undercut due to impact of water flow and nature of loesses and falls down in blocks.

In the studied area, especially in low-slope regions, meandering processes (Picture 11), mainly along the main drainages, have caused bank erosion (Picture 12) which vanishes agricultural lands adjacent to drainages each year. The exterior part of the curve of river is

Picture 8. Microterrace on rangelands (Route to Oghchi Village)

b. Rill erosion

Rill erosion is not usually seen on loesses of the area and is mostly limited to sloped cultivated lands (Picture 9), because water is concentrated in rills resulting from ploughing and flows down the slope with higher velocity. The depths of rills may be 20-40 centimeters, but with later ploughing, there is usually no sign of rills on the slopes.

Picture 9. Rill erosion in dry – farming lands due to ploughing in the direction of slope (Route to Roshan Dareh Village)

#### c. Gully erosion

132 Soil Erosion Studies

Rill erosion is not usually seen on loesses of the area and is mostly limited to sloped cultivated lands (Picture 9), because water is concentrated in rills resulting from ploughing and flows down the slope with higher velocity. The depths of rills may be 20-40 centimeters,

Picture 9. Rill erosion in dry – farming lands due to ploughing in the direction of slope

Picture 8. Microterrace on rangelands (Route to Oghchi Village)

but with later ploughing, there is usually no sign of rills on the slopes.

b. Rill erosion

(Route to Roshan Dareh Village)

Contrast to rill erosion, gully erosion is the prominent erosion feature in the studied area, which in addition to soil loss, causes major damages to agricultural lands, roads, houses and other establishments. Gully erosion starts as headcuts in most parts of the area and it happens suddenly. In some parts, stepped gullies are observed which in time change to single gully. In addition to the nature of loesses and their erodibility, degradation of ranges and agricultural lands, increase runoff volume which causes formation of big gullies(Picture 10). The walls of gullies are falling, collapsing and retreating which cause increase of the area and volume of gullies and intensification of damages. In some parts of gullies, vegetation growth has stabilized the substrate. In regard to the gathered information from residences and local experts, the rate of gully advancement in the area is different and may reaches 10 m/y.

Picture 10. Gully erosion (Gharnagh Village). Advancing headcut toward the village is shown in the picture with arrow.

#### d. Bank Erosion

In the studied area, especially in low-slope regions, meandering processes (Picture 11), mainly along the main drainages, have caused bank erosion (Picture 12) which vanishes agricultural lands adjacent to drainages each year. The exterior part of the curve of river is undercut due to impact of water flow and nature of loesses and falls down in blocks.

Investigation of Effective Factors on Runoff Generation and

Picture 13. Landslide in the studied area (Rout to Oghchi Village)

Picture 14. Tunnel erosion or piping in loesses (Route to Oghchi Village)

f. Tunnel Erosion

collapse and gullies are developed.

Sediment Yield of Loess Deposits Using Rainfall Simulator 135

One of the erosional features seen in the area is tunneling or piping (Picture 14) which after spread of the dimension of tunnels and collapse of their roofs, gullies are formed. Porosity and chemical characteristics of the loesses and also hydraulic slope for flow of water in loesses are the major factors in development of tunnel erosion. This erosion is intensified with the animal activities such as mice. Due to animal activities, holes are formed at the surface. The holes are connected in the subsurface to form channels in which water flows and transports soil and sediment and their dimensions are increased. Finally their roof

Picture 11. Meander (near Tamer Village). Flow direction is shown with arrow in the picture

Picture 12. Bank erosion (adjacent to Haji Beik Village)

e. Mass Movements and Landsliding

Due to semi-arid climatic conditions and low rainfall, mass movements are not frequent in the area and are formed after bank erosion along drainages (Picture 13). After undercutting and degradation of walls, loess slope slides as rotational slides to river-bed, due to high slope and lack of cement between loess particles.

Picture 13. Landslide in the studied area (Rout to Oghchi Village)

f. Tunnel Erosion

134 Soil Erosion Studies

Picture 11. Meander (near Tamer Village). Flow direction is shown with arrow in the picture

Due to semi-arid climatic conditions and low rainfall, mass movements are not frequent in the area and are formed after bank erosion along drainages (Picture 13). After undercutting and degradation of walls, loess slope slides as rotational slides to river-bed, due to high

Picture 12. Bank erosion (adjacent to Haji Beik Village)

slope and lack of cement between loess particles.

e. Mass Movements and Landsliding

One of the erosional features seen in the area is tunneling or piping (Picture 14) which after spread of the dimension of tunnels and collapse of their roofs, gullies are formed. Porosity and chemical characteristics of the loesses and also hydraulic slope for flow of water in loesses are the major factors in development of tunnel erosion. This erosion is intensified with the animal activities such as mice. Due to animal activities, holes are formed at the surface. The holes are connected in the subsurface to form channels in which water flows and transports soil and sediment and their dimensions are increased. Finally their roof collapse and gullies are developed.

Picture 14. Tunnel erosion or piping in loesses (Route to Oghchi Village)

Investigation of Effective Factors on Runoff Generation and

Fig. 9. Landuse map of the studied area

runoff and sediment were collected and measured.

**17. Results of field and laboratory analyses** 

**16. Land units** 

anion is not present.

Sediment Yield of Loess Deposits Using Rainfall Simulator 137

Figure 10 shows the land units of the area which are 23 units and more than 300 polygons. Each land unit is assigned letters and numbers. The first letter of the name of land unit is land use (A for dry-farming cultivation and R for rangelands). The second letter is for erosional features (SE surface erosion, RE rill erosion and SRE for surface and rill erosion) and latest number is for slope class (1 for class 0-%10, 2 for 10-%20, 3 for 20-%30 and 4 for more than 30 percent). Among the polygons, the ones which are accessible, having the largest area and are good indicators of similar polygons, were chosen (Figure 11), Rainfall simulator analyses were performed on each one with three repetitions and the resulted

Table 4 shows the mean and variation coefficient of all measured variables including physical and chemical properties of the samples and variables measured in the field. The amount of electrical conductivity is usually bellow 3 ms/cm, the abundance of silt-size particles is more than %55 is completely evident and in some samples reaches %70. In contrast, abundance of clay particles is not more than %30 and the mean amount of sand in all samples is less than %20. Gypsum is not present in many samples or is little. Carbonate
