Soil and Water Conservation, what is Wind erosion, what is Soil erosion, what is Water erosion

Soil and Water Conservation

Part-1

Soil and water are two important natural resources and the basic needs for agricultural production. During the last century it has been observed that the pressure of increasing population has led to degradation of these natural resources. In other words increase in agricultural production to feed the increasing population is only possible if there sufficient fertile land and water are available for farming. In India, out of 328 million hectares of geographical area, 68 million hectares are critically degraded while 107 million hectares are severely eroded. That's why soil and water should be given first priority from the conservation point of view and appropriate methods should be used to ensure their sustainability and future availability. Status of global land degradation is shown in Fig. 1.1.

 

Fig. 1.1. Global soil degradation map.

(Source: UNEP, International Soil Reference and Information Centre (ISRIC), World Atlas of Desertification, 1997).

 Water conservation is the use and management of water for the good of all users. Water is abundant throughout the earth, yet only three percent of all water is fresh water, and less than seven-tenths of freshwater is usable. Much of the usable water is utilized for irrigation. Detailed analysis will show that in about fifteen years, about two-thirds of the world’s population will be living in some sort of water shortage. Water is used in nearly every aspect of life. There are multiple domestic, industrial and agricultural uses. Water conservation is rapidly becoming a hot topic, yet many people do not realize the importance of soil conservation.

 Soil conservation is defined as the control of soil erosion in order to maintain agricultural productivity. Soil erosion is often the effect of many natural causes, such as water and wind. There are also human factors which increase the rate of soil erosion such as construction, cultivation and other activities. Some may argue that since it is a natural process, soil erosion is not harmful. The truth is that with the removal of the top layer of soil, the organic matter and nutrients are also removed.

 

Ø   What is Soil Erosion?

The uppermost weathered and disintegrated layer of the earth’s crust is referred to as soil. The soil layer is composed of mineral and organic matter and is capable of sustaining plant life. The soil depth is less in some places and more at other places and may vary from practically nil to several meters. The soil layer is continuously exposed to the actions of atmosphere. Wind and water in motion are two main agencies which act on the soil layer and dislodge the soil particles and transport them. The loosening of the soil from its place and its transportation from one place to another is known as soil erosion.

Hence soil erosion is defined as a process of detachment, transportation and deposition of soil particles (sediment).

 

Importance of Soil Conservation

·        To prevent erosion of bare soil, it is important to maintain a vegetation cover, especially in the most vulnerable areas e.g. those with steep slopes, in a dry season or periods of very heavy rainfall.

·        Where intensive cultivation takes place, farmers should follow crop rotation in order to prevent the soil becoming exhausted of organic matters and other soil building agents.

·        Construction of highways and urbanization should be restricted to areas of lower agricultural potential. With extractive industries, a pledge must be secured to restore the land to its former condition before permission for quarries or mines is granted.

 

Ø   Causes of Soil Erosion

No single unique cause can be held responsible for soil erosion or assumed as the main cause for this problem. There are many underlying factors responsible for this process, some induced by nature and others by human being. The main causes of soil erosion can be enumerated as:

 

(1) Destruction of Natural Protective Cover by

(i) indiscriminate cutting of trees,

(ii) overgrazing of the vegetative cover and

(iii) forest fires.

 

(2) Improper Use of the Land

(i) keeping the land barren subjecting it to the action of rain and wind,

(ii) growing of crops that accelerate soil erosion,

(iii) removal of organic matter and plant nutrients by injudicious cropping patterns,

(iv) cultivation along the land slope, and

(v) faulty methos of irrigation.

 

 Types of Soil Erosion

 

1)       According to Origin: Soil erosion can broadly be categorized into two type’s i.e. geologic erosion and accelerated erosion.

 Geological Erosion: Under natural undisturbed conditions equilibrium is established between the climate of a place and the vegetative cover that protects the soil layer. Vegetative covers like trees and forests retard the transportation of soil material and act as a check against excessive erosion. A certain amount of erosion, however, does take place even under the natural cover. This erosion, called geologic erosion, is a slow process and is compensated by the formation of soil under the natural weathering process. Its effect is not of much consequence so far as agricultural lands are concerned.

 Accelerated Erosion: When land is put under cultivation, the natural balance existing between the soil, its vegetation cover and climate is disturbed. Under such condition, the removal of surface soil due to natural agencies takes places at faster rate than it can be built by the soil formation process. Erosion occurring under these conditions is referred to as accelerated erosion. Its rates are higher than geological erosion. Accelerated erosion depletes soil fertility in agricultural land.

 According to Erosion Agents: Soil erosion is broadly categorized into different types depending on the agent which triggers the erosion activity. Mentioned below are the four main types of soil erosion.

(1) Water Erosion: Water erosion is seen in many parts of the world. In fact, running water is the most common agent of soil erosion. This includes rivers which erode the river basin, rainwater which erodes various landforms, and the sea waves which erode the coastal areas. Water erodes and transports soil particles from higher altitude and deposits them in low lying areas. Water erosion may further be classified, based on different actions of water responsible for erosion, as : (i) raindrop erosion, (ii) sheet erosion, (iii) rill erosion, (iv) gully erosion, (v) stream bank erosion, and (vi) slip erosion.

 

(2) Wind Erosion: Wind erosion is most often witnessed in dry areas wherein strong winds brush against various landforms, cutting through them and loosening the soil particles, which are lifted and transported towards the direction in which the wind blows. The best example of wind erosion is sand dunes and mushroom rocks structures, typically found in deserts.

 

(3) Glacial Erosion: Glacial erosion, also referred to as ice erosion, is common in cold regions at high altitudes. When soil comes in contact with large moving glaciers, it sticks to the base of these glaciers. This is eventually transported with the glaciers, and as they start melting it is deposited in the course of the moving chunks of ice.

 

(4) Gravitational Erosion: Although gravitational erosion is not as common a phenomenon as water erosion, it can cause huge damage to natural, as well as man-made structures. It is basically the mass movement of soil due to gravitational force. The best examples of this are landslides and slumps. While landslides and slumps happen within seconds, phenomena such as soil creep take a longer period for occurrence.

 

 Agents of Soil Erosion

Soil erosion is the detachment of soil from its original location and transportation to a new location. Mainly water is responsible for this erosion although in many locations wind, glaciers are also the agents causing soil erosion. Water in the form of rain, flood and runoff badly affects the soil.

 

Factors Affecting Soil Erosion

·        Climatic Factor

·        Temperature

·        Topographical Factors

·        Soil

·        Vegetation

·        Biological Factors of Soil Erosion

 

Ø   Mechanics of Soil Erosion

Soil erosion is initiated by detachment of soil particles due to action of rain. The detached particles are transported by erosion agents from one place to another and finally get settled at some place leading to soil erosion process. Different soil erosion processes are shown in Fig.

                

Fig. Process of water erosion by the impact of raindrops.

 Mechanics of soil erosion due to water and wind is discussed below.

 Mechanics of Water Erosion

There are three steps for accelerated erosion by water:

i) Detachment or loosening of soil particles caused by flowing water, freezing and thawing of the top soil, and/or the impact of falling raindrops,

ii) Transportation of soil particles by floating, rolling, dragging, and/or splashing and

iii) Deposition of transported particles at some places of lower elevation. Rain enhances the translocation of soil through the process of splashing as shown in Fig Individual raindrops detach soil aggregates and redeposit them as particles. The dispersed particles may then plug soil pores, reducing water intake (infiltration). Once the soil dries, these particles develop into a crust at the soil surface and runoff is further increased.

 

 Mechanics of Wind Erosion

Wind erosion occurs where soil is exposed to the dislodging force of wind. The intensity of wind erosion varies with surface roughness, slope and types of cover on the soil surface and wind velocity, duration and angle of incidence. Fine soil particles can be carried to great heights and for (maybe) hundreds of kilometers. The overall occurrence of wind erosion could be described in three different phases. These are initiation of movement, transportation and deposition.

                                        

                                                    Wind Erosion, Estimation and Control

 Wind Erosion

Wind erosion is a serious environmental problem. It is in no way less severe than water erosion. High velocity winds strike the bare lands (having no cover), with increasing force. Fine, loose and light soil particles blown from the land surface are taken miles and miles away and thereby, causing a great damage to the crop productivity. It is a common phenomenon occurring mostly in flat, bare areas; dry, sandy soils; or anywhere the soil is loose, dry and finely granulated and where high velocity wind blows. Wind erosion, in India, is commonly observed in arid and semi-arid areas where the precipitation is inadequate, e.g. Rajasthan and some parts of Gujarat, Punjab and Haryana.

 

 

Fig. An Illustration of Wind Erosion.

Mechanics of Wind Erosion

 The overall occurrence of wind erosion could be described in three distinct phases. These are:

1. Initiation of Movement

2. Transportation

3. Deposition.

 

1.      Initiation of Movement: The soil particles are first detached from their place by the impact and cutting action of wind. These detached particles are then ready for movement by the wind forces. After this initiation of movement, soil particles are moved or transported by distinct mechanisms.

 Transportation: The transportation of the soil particles are of three distinct types and occur depending upon size of the soil particles. Suspension, saltation, and surface creep are the three types of soil movement or transport which occur during wind erosion. While soil can be blown away at virtually any height, the majority (over 93%) of soil movement/transportation takes place at or within one meter height from land surface. These transportation mechanisms of soil particles due to wind are shown in Fig.

                 

Fig. Mechanics of Wind Erosion.

 Suspension: Soil moved by suspension is the most spectacular and easiest to recognize among the three forms of movement. The soil particles of less than 0.1 mm size are subjected to suspension and around 3 to 40 % of soil weights are carried by the suspension method of soil transport under the wind erosion.

 

Saltation: The major fraction of soil moved by the wind is through the process of saltation. Saltation movement is caused by the pressure of the wind on soil particles as well as by the collision of a particle with other particles. Soil particles (0.1 to 0.5 mm) move in a series of bounces and/or jumps. Depending on soil type, about 50 to 75% of the total weight of soil is carried in saltation.

 

Surface Creep: The large particles which are too heavy to be lifted into the air are moved through a process called surface creep. In this process, the particles are rolled across the surface after coming into contact with the soil particles in saltation. In this process the largest of the erosive particles having diameters between 0.5 to 2 mm are transported and around 5 to 25% of the total soil weights are carried in this fashion.

The mass of soil moved can be related to the influencing parameters by the following equation:

 

Quantity of soil moved (V – Vth)³ / D^0.5

Where V = wind velocity, Vth = threshold velocity, and D = particle diameter.

 

3.      Deposition: Deposition of soil particles occurs when the gravitational force is greater than the forces holding the particle in the air. This generally happens when there is a decrease in the wind velocity caused by vegetative or other physical barriers like ditches or benches. Raindrops may also take dust out of air.

 

Ø  Wind Erosion Control

 

A suitable surface soil texture is the best key to wind erosion protection. Properly managed crop residues, carefully timed soil tillage, and accurately placed crop strips and crop barriers can all effectively reduce wind erosion. Proper land use and adaptation of adequate moisture conservation practices are the main tools which help in wind erosion control.

Three basic methods can be used to control wind erosion:

·        Maintain Vegetative Cover (Vegetative Measures)

·        Roughen the Soil Surface by Tillage Practices (Tillage Practices or may be called Tillage Measures)

·        Mechanical or Structural Measures (Mechanical Measures)

 

1.      Vegetative Measures

Vegetative measures can be used to roughen the whole surface and prevent any soil movement. The aim is to keep the soil rough and ridged to either prevent any movement initially or to quickly trap bouncing soil particles in the depressions of the rough surface. A cover crop with sufficient growth will provide soil erosion protection during the cropping season. It is one of the most effective and economical means to reduce the effect of wind on the soil. It not only retards the velocity near the ground surface but also holds the soil against tractive force of wind thereby helping in reduction of soil erosion.

Vegetative measures can be of two types:

·        Temporary Measures

·        Permanent Measures

 

2.      Tillage Practices

The tillage practices, such as ploughing are importantly adopted for controlling wind erosion. These practices should be carried out before the start of wind erosion. Ploughing before the rainfall helps in moisture conservation. Ploughing, especially with a disc plough is also helpful in development of rough soil surface which in turn reduces the impact of erosive wind velocity. Both the above effects are helpful in controlling the wind erosion.

The common tillage practices used for wind erosion control are as under:

·        Primary and Secondary Tillage

·        Use of Crop Residues

·        Strip Cropping

 

3.      Mechanical Measures

This method consists of some mechanical obstacles, constructed across the prevailing wind, to reduce the impact of blowing wind on the soil surface. These obstacles may be fences, walls, stone packing etc., either in the nature of semi-permeable or permeable barriers. Generally, in practice two types of mechanical measures are adopted to control the wind erosion;

i) Wind breaks and

ii) Shelter belts.

 

Wind Breaks

This is a permanent vegetative measure which helps in the reduction of wind erosion. It is most effective vegetative measure used for controlling severe wind erosion. The term wind break is defined as any type of barrier either mechanical or vegetative used for protecting the areas like building apartments, orchards or farmsteads etc. from blowing winds.

A further use for "windbreaks" or "wind fences" is for reducing wind speeds over erodible areas such as open fields, industrial stockpiles, and dusty industrial operations. As erosion is proportional to the cube of wind speed, a reduction in wind speed by 1/2 (for example) will reduce erosion by over 80%. The largest one of these windbreaks is located in Oman (28 m high by 3.5 km long) and was created by Mike Robinson from Weather Solve Structures.

 

Shelter Belts

A shelterbelt is a longer barrier than the wind break, is installed by using more than two rows, usually at right angle to the direction of prevailing winds. The rows of belt can be developed by using shrubs and trees. It is mainly used for the conservation of soil moisture and for the protection of field crops, against severe wind erosion.

Woodruff and Zingg (1952) developed the following relationship between the distance of full protection (d) and the height (h) of wind break or shelter belt.

Where, d is the distance of full protection (m),

h is the height of the wind barrier (wind break or shelter belt) (m),

v_m is the minimum wind velocity at 15 m height required (m/s),

V_m = 9.6 m/s (for smooth, bare soil surface)

v is the actual velocity at 15 m height, and

θ is the angle of deviation.

This relationship (equation) is valid only for wind velocities below 18 m/s.

 

Problem: 1 Determine the spacing between wind breaks that are 15 m high. 5 year return period wind velocity at 15 m height is 15.6 m/s and the wind direction deviates 10° from the perpendicular to the field strip. Assume a smooth, bare soil surface and a fully protected field.

 

Solution:

Given: h = 15 m

V = 15.6 m/s

θ = 10°

Vm = 9.6 m/s (for smooth, bare soil surface)

Spacing = distance of full protection by a windbreak,

 
 Thus, the spacing between windbreaks = 154.54 m.

 Problem: 2 Determine the full protection strip width for field strip cropping if the crop in the adjacent strip is wheat, 0.9 m tall, and the wind velocity at 15 m height is 8.9 m/sec at 0° with the field strip.

Solution:

Given: h = 0.9 m

v = 8.9 m/s

θ = 0°

Assuming v_m = 8.9 m/sec (Because theoretical v_m = 9.6 m/sec which is greater than the prevailing wind velocity). Since the field conditions are not specified taking v_m = v.

Thus, strip width = 15.30 m.

 

Ø   Estimation of Soil Loss

However, to estimate soil erosion, empirical and process based models (equations) are used. Universal Soil Loss Equation (USLE) is an empirical equation. It estimates the average annual mass of soil loss per unit area as a function of most of the major factors affecting sheet and rill erosions.

 The Universal Soil Loss Equation (USLE)

The USLE is an erosion prediction model for estimating long term averages of soil erosion from sheet and rill erosions from a specified land under specified conditions (Wischmeier and Smith, 1978).

Where, A = soil loss per unit area in unit time, t ha^-1 yr ^-1

R = rainfall erosivity factor which is the number of rainfall erosion index units for a particular location

K = soil erodibility factor

L= slope length factor

S = slope steepness factor

C = cover management factor

P = support practice factor

 

1.      Rainfall Erosivity Factor (R)

It refers to the rainfall erosion index, which expresses the ability of rainfall to erode the soil particles from an unprotected field. The rainfall erosion index unit (EI30) of a storm is estimated as:

Where, KE = kinetic energy of storm in metric tons /ha-cm, expressed as

Where, I = rainfall intensity in cm/h, and

Ι30 = maximum 30 minutes rainfall intensity of the storm.

 

2.      Soil Erodibility Factor (K)

The soil erodibility factor (K) in the USLE relates to the rate at which different soils erode. Under the conditions of equal slope, rainfall, vegetative covers. The formula used for estimating K is as follows:

Where, K = soil erodibility factor, A₀ = observed soil loss, S = slope factor, and ΣEI = total rainfall erosivity index.

 Table-Values of K for Several Stations (Source: K. Subramanya, 2008)

 

3.       Topographic Factor (LS)

The two factors L and S are usually combined into one factor LS called topographic factor. This factor is defined as the ratio of soil loss from a field having specific steepness and length of slope (i.e., 9% slope and 22.13 m length) to the soil loss from a continuous fallow land.

Where, L = field slope length in feet and S = percent land slope.

 

Crop Management Factor (C)

The crop management factor C may be defined as the expected ratio of soil loss from a cropped land under specific crop to the soil loss from a continuous fallow land, provided that the soil type, slope and rainfall conditions are identical.

 Table-Values of Crop Management Factors for Different Stations in India (Source: K Subramanya, 2008)

 

 

5.      Support Practice Factor (P)

This factor is the ratio of soil loss with a support practice to that with straight row farming up and down the slope. The conservation practice consists of mainly contouring, terracing and strip cropping.

Table-Different Values of Support Practice Factor (P) for Some Indian Locations (Source: K. Subramanya, 2008)