**4.3 Rill erosion**

*Soil Moisture Importance*

**4. Water erosion**

**4.1 Splash erosion**

**4.2 Sheet erosion**

splash and sheet erosion only.

Water and wind are two key agents that degrade soils through various kinds of erosion processes. Globally, around 1100 mha is affected by water erosion (56% of the total degraded land) and around 28% of the total degraded land area is affected by wind erosion [19]. Runoff removes the soil particles from sloping and bare lands while the wind blows away loose and detached soil particles from unprotected lands. Other processes of land degradation are soil compaction, waterlogging, acidification, alkalinization, and salinization depends on parent material, climatic conditions, and crop management practices. In this chapter, we will discuss about the soil erosion by water, different types, processes, factors, and management.

Worldwide, water erosion is the most severe type of soil erosion. In this form of erosion, detachment, and transportation of soil particles from their parental source take place by water through the action of rainfall, runoff, hailstorm, and irrigation. Water erosion is a prevailing form of erosion in humid and sub-humid agro-ecosystems. It also creates the problem in arid and semiarid regions, characterized by an intensive rainstorm and scanty vegetation cover. Water erosion comprises three basic phases, i.e., detachment, transportation, and deposition. Rainfall is one of the major factors which causes the movement and detachment of soil particles. The detached soil particles seal the open-ended and water-conducting soil pores, reduce water infiltration, and cause runoff. The first two phases determine the quantity of soil to be eroded and the third phase determines the distribution of the eroded material along the landscape. If there is no dispersion and transport of soil particles, there will be no deposition. Hence, detachment and transport of soil particles are the primary processes of soil erosion. Understanding the mechanisms and extent of water erosion is crucial to manage and develop erosion control practices. Splash, sheet, rill and gully erosion are main forms of soil erosion by water (**Figure 1**). The other forms of water erosion are ravine formation, slip, tunnel, stream bank, and coastal erosion [20, 21]. The different forms of water erosion are described below:

Splash erosion is the first form of soil erosion by water. Falling raindrops on the soil surface break the soil aggregates and disperse and splash soil particles from their source, known as splash erosion. The process of splash erosion involves raindrop impact on soil particles, a splash of soil particles, and the formation of craters [22]. The raindrops falling on soil surface act like a small bomb which disintegrates soil particles and forms cavities of contrasting shapes and sizes. The depth of craters is equal to the depth of raindrop penetration which is a function of raindrop velocity, size, and shape. In this form, soil particles can move only a few centimeters away from their source.

This is the next phase to splash erosion, which promptly initiates sheet erosion. The fertile topsoil surface is removed uniformly as a thin layer from the entire sloping surface area of the field by runoff water. Sheet erosion is a function of particle detachment, rainfall intensity, and land slope. The shallow flow of runoff water causes this type of soil erosion in which small rills are formed. This is the most common and severe form of soil erosion from an agricultural point of view as it removes the nutrient-rich top layer of soil. Out of total soil erosion, nearly 70% is caused by

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It describes the flow of runoff water loaded with soil particles and organic matter in finger-like small channels, known as rill erosion. This is the advanced form of sheet erosion for soil loss. Water flow in small channels erodes soil at a faster rate than sheet erosion. Rill erosion is the second most common form of water erosion. These rills can be easily managed by tillage operations but can cause higher soil loss during intensive rainfall. The key factors that cause rill erosion are soil erodibility, land slope, runoff transport capacity, and hydraulic shear of water flow.

#### **4.4 Gully erosion**

Gully erosion is the advanced form of rill erosion. When the volume and velocity of concentrated runoff water increase, the rills become deep and broad and forms gullies. The gullies are linear incision channels with 0.3 m width and 0.3 m depth. Concentrated runoff flow is a primary factor for gully formation. Continuous gully erosion results in the removal of the entire soil profile. The extreme form of gully erosion may results in failure of crops, expose plant roots, reduce the groundwater level, and adversely affects landscape stability. It can cut apart the fields and aggravate the non-point source pollution (e.g., sediment, chemicals) to nearby water bodies. Gullies cannot be corrected by usual tillage operations. The dominant

factors affecting gully erosion are shear stress of flowing water and critical shear stress of the soil. The further erosion of gullies results in ravines formation. Based on the size, depth, and drainage area, gullies can be classified as:


## **4.5 Ravine formation**

It is referred to as a network of deep and narrow gullies that flows parallel to each other while linking with the river system. Mismanagement and non-judicious use of land result in enlargement of rills and gullies and eventually lead to ravine formation. Abrupt changes in elevation of the river bed and the adjoining land surface, deep and permeable soil with high erodibility, sparse vegetation, and backflow of river water during the recession period causes severe bank erosion which consequently results in ravine formation.

## **4.6 Tunnel erosion**

It is the sub-soil erosion through runoff flow in channels while surface soil remains intact. Tunnel erosion is also known as pipe erosion and commonly occurs in arid and semiarid regions where the soil permeability for water varied with the soil profile. The further widening and deepening of tunnels form large gullies which degrade the productive agricultural lands. Soil with erodible characteristics, having sodic B horizon and stable A horizon are highly prone to tunnel erosion. Runoff flow through natural cracks and animal burrows initiates tunnel formation by infiltrating thorough dispersible subsoil layers. Seepage, lateral flow, and interflow are key indicators of tunnel erosion. It alters the geomorphic and hydrologic characteristics of the affected areas. Management practices for tunnel erosion are ripping, contour farming, vegetation including trees and deep-rooted grasses with proper fertilization and liming, consolidation of surface soil, and diversion of concentrated runoff.

#### **4.7 Slip erosion or landslip erosion**

It is the downward and outward movement of slope forming materials composed of natural rocks and debris from sloppy lands. It is also known as mudslide or mass erosion. This type of erosion mostly occurs in hilly regions having watersaturated soils slips down the hillside or mountain slope. Banks along highways, streams, and ocean fronts are often subject to landslides. The large masses of land slip down which destroy the vegetation and degrade the productivity of lands. The slope can be stabilized through developments of diversion drains, contour trenches, crib structures, geotextiles, kutta—crate structures, and retaining walls.

#### **4.8 Stream bank erosion**

The scouring of soil material from the stream bed and cutting of stream bank by the action of flowing water is known as stream bank erosion. Streams and rivers change their direction of flow by cutting the bed from one side and depositing the sediment to the other side of the stream. Flash floods enhanced the stream bank

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*Soil and Water Conservation Measures for Agricultural Sustainability*

erosion which is more destructive. Stream and gully erosion are relatively comparable. Primarily, stream bank erosion predominantly occurs at the lower end water

Sea level is incessantly rising due which can increase the frequency of occurrence of natural disasters like the tsunami in the coastal areas in the future. Such natural hazards produce strong water waves which can severely erode the seaside areas. It is projected that the erosion rate will be higher in coastal regions in the coming years. The anthropogenic activities leading to coastal erosion are port construction,

The universal soil loss equation (USLE) was given by Wischmeier and Smith

*A* = *RKL SCP* (1)

Among the above-listed factors, vegetation and to some extent soil can be managed to reduce the rate of the soil erosion but the climatic and topographic factors, except slope length, are not manageable. Primarily, soil loss through erosion is a function of erosivity of raindrops and erodibility of the soil which can be math-

where Erosivity is the potential of rainfall to cause erosion under given soil type and climatic condition; Erodibility is the vulnerability or susceptibility of the soil to erosion which depends on soil bio-physico-chemical properties, and land use and crop management practice. Sandy soils can be easily detached while well aggregated clayey soils are more resistant to erosion than sandy soils. When clay particles detached they can be easily removed by runoff due to their smaller size. Silt soils are

The accelerated soil erosion significantly influences the soil quality, agricultural production and nutritional quality [25]. Higher soil erosion results in the removal of fertile topsoil along with nutrients which leads to reduced agronomic yield, land degradation, and terrain deformation [25–27]. The main causal factors affecting the rate of soil erosion are parent material, soil texture, slope steepness, plant cover, tillage, and climate [13]. According to an estimate of existing soil loss data, the mean annual rate of soil erosion in our country is approximately 16.4 ton ha<sup>−</sup><sup>1</sup>

Erosion = *f* (Erosivity,Erodibility) (2)

which

 year<sup>−</sup><sup>1</sup> );

tributaries which have a relatively flat slope and continuous flow of water.

destruction of mangroves, and beach and river bed mining [23].

**4.10 Universal soil loss equation for water erosion**

(1978) based on the soil erosion causing factors [24].

*R*, rainfall erosivity factor; *K*, soil erodibility factor *L*, slope-length factor *S*, slope-steepness factor;

ematically expressed as follows:

the most erodible type of soil [9].

**5. Impact of soil erosion on agriculture**

*C*, cover and management factor; *P*, support practice factor.

where A, mean annual soil loss (metric tons hectare<sup>−</sup><sup>1</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.92895*

**4.9 Coastal erosion**

erosion which is more destructive. Stream and gully erosion are relatively comparable. Primarily, stream bank erosion predominantly occurs at the lower end water tributaries which have a relatively flat slope and continuous flow of water.
