Soil and water erosion

About soil and water erosion: definition and types of erosion

1. Fundamentals of Soil and Water Conservation Engineering (AGL308) Mr. Rohitashv Nagar Assistant Professor Department of Agronomy School of Agricultural Sciences Career Point University, Kota (Raj.)

2. Lecture-1 Soil and Water Erosion: definition and types of erosion

3. Soil Erosion: Soil Erosion is the process of detachment of soil particles from the parent body and transportation of the detached soil particles by wind and water . The agents causing erosion are wind and water.

4. A.Types of Water Erosion 1.Sheet Erosion: In the initial stages of soil erosion, rain drops churn the top soil and along with runoff the muddy water moves away from the field. It is uniform removal of top soil in thin layer from the field.

5. 2.Rill Erosion: When runoff starts, channelisation begins and erosion is no longer uniform, incisions are formed on the ground and erosion is more apparent than sheet erosion. This is the second stage of erosion.

6. 3. Gully Erosion: Unchecked rills result in increased channelisation of runoff gullies are formed when channelisation runoff from vast sloping land is sufficient in volume and velocity to cut deep and wide channels. Gullies are the most spectacular symptoms of erosion.

7. 4.Ravines Erosion: These are manifestation of a prolonged process of gully erosion, typically found in the large expanses of deep alluvial soils. They are deep and wide gullies and their formation indicates very advanced stage of gully erosion.

8. 5.Landslides: Sliding down of large chunk of soil due to steep slopes is called landslides. Landslides occur in mountain slopes when the slope exceeds 20% and width 6m .

9. 6.Stream-bank Erosion: Small streams, rivulets, torrents (hill streams), are subjected to stream-bank erosion due to obstruction to their flow.

10. B. Types of wind erosion 1. Saltation: Saltation is the first stage of movement of soil particles in series of jumps. The soil particles moved by saltation are between 0.1 to 0.5 mm in diameter. Due to the effect of wind , soil particles jump up vertically into air and rise to a height of 30 to 60 cm and fall through the air. Generally 50 to 75 % of the weight of soil lost by wind erosion is carried in saltation.

11. 2. Surface creep: Rolling of coarse grains, larger than 0.5 to 3 mm in diameter and too heavy to be lifted, by wind along the surface creep. The particles are pushed along the surface of the ground, rolling and not jumping. Generally, 5 to 25 % weight of the soil lost by wind erosion is carried in surface creep.

12. 3. Suspension: Floating of fine dust particles through the air is known as suspension. The particles are smaller than 0.1 mm in diameter . Generally 3 to 4 % of the weight of soil lost by wind erosion is carried in suspension.

13. Factors affecting soil and water erosion 1. Rainfall : Rainfall influences both the process of detachment and transportation . Amount, intensity, duration and distribution of rainfall influence runoff and erosion. High intensity rainfall of long duration causes severe erosion.

14. 2.Vegetation: Among the different factors that influence erosion, vegetative cover is more important. The impact of rain drops is absorbed by vegetation present on the soil surface and therefore, there is no break-down of soil aggregates. In addition, the plant roots bind the soil particles. Due to addition of organic matter by vegetation, stable aggregates are formed which are resistant to break-down.

15. 3. Soil: Erosivity is the capacity of agents causing erosion , while erodibility is the susceptibility of soil to erosion. Soil characteristics have considerable influence on soil erodibility. These are topography, physical, chemical and biological properties of soil. The degree of slope and the length of slope determine the amount of runoff and extent of erosion. The soil physical properties like soil structure and texture influence both detachability and transport of soil particles . Soil with stable aggregates are resistant to detachment.

16. 4. Man and Beast: Man and beast accelerate erosion by extractive farming and excessive grazing. Practices like cultivation on steep, cultivation along the slope, cutting and burning the forests lead to heavy erosion. Excessive grazing destroys all vegetation, leaving the land bare for ravages of erosion.

17. Soil Loss Equation Based on the mechanism and factors influencing soil erosion, an universal soil loss equation (USLE) was developed. It is useful for predicting soil loss due to water erosion. A= RKLSCP A = Predicted soil loss (t/ac/year) (ac=acre=0.4 ha) R = Rainfall and runoff factor K = Soil erodibility L = Slope length

18. S= Slope gradient or steepness C= Soil cover and management P= Erosion control practice

19. Problem-1: Estimate soil loss from fields with 4 % slope and 30.5 m length of slope which was cleanly tilled and fallowed. The R factor for the region is 150 and soil erodibility factor is 0.33. R = 150 L.S for 4 % slope and 30.5 m length = 0.4 (from table 11.1) K = soil erodibility factor = 0.33 C = 1 since there is no plant cover P = 1 since there is no soil conservation measure

20. A = (150) (0.33) (0.400) (1.0) (1.0) = 19.8 t/ac/year. Or The soil has more than the permissible limit of 11 t/ha/year

21. 19.8 x 2.5 = 49.5 t/ha/year.