The methods used for controlling soil erosion through crops or vegetation and through agronomical practices are known as biological methods. The methods discussed herein are applicable both for controlling water and wind erosion.
The biological methods for controlling water erosion consist of:
(1) Contour cultivation,
(2) Cropping systems, and
Contour cultivation means carrying out agricultural operations like planting, tillage and inter-cultivation very nearly on the contour. Contour cultivation reduces the velocity of overland flow and retards soil erosion. In some cases, after the inter-cultivation operations a ridge and furrow system or the contour develops and offers greater resistance to surface runoff.
Crops like maize, sorghum, pearl, millet which are normally grown in rows are ideally suited for contour cultivation. To layout the system in the field, guidelines are marked across the slope using a Dumpy level or even a hand level. All the subsequent agricultural operations are carried out making use of the guidelines.
Strip cropping means growing different crops in alternate strips across the slope such that they serve as vegetative barriers to erosion. The alternate strips consist of close growing erosion resisting crops to erosion permitting crops like row crops.
To achieve the best results, strip cropping is to be done in combination with other farming practices, like good crop rotations, contour cultivation etc.
There are four types of strip cropping systems.
(1) Contour strip cropping,
(2) Field strip cropping,
(3) Buffer strip cropping, and
(4) Wind strip cropping.
Contour strip cropping means growing alternate strips of erosion permitting and erosion resisting crops along the contour. Depending upon the topography the widths of the strips will vary.
In field strip cropping the strips are laid across the slope in uniform width without taking into consideration the exact contours. This method is useful on regular slopes and with the soils of high infiltration rates.
In wind strip cropping the crop strips are laid out at right angles to the direction of the prevailing winds irrespective of the direction of the land slope. The objective here is to control wind erosion rather than water erosion.
In buffer strip cropping permanent strips of grasses are located either in badly eroded areas or in areas that do not fit into a regular rotation.
Fig. 22.2 shows the types of strip cropping-
The widths of the strips of erosion resisting and erosion permitting crops depend upon several factors like slope soil texture, rainfall characteristics, type of crops, etc. These widths are determined based largely on the local conditions.
In general, the steeper the slope the greater is the width of the erosion resisting crop and smaller the width of erosion permitting crop. Table 22.1 gives typical values.
However, strip cropping on a large scale has not been adopted in India because of small size holdings.
2. Cropping Systems:
Cropping system refers to a sequence of crops grown on a given area over a period of time. The cropping system for an area is designed to achieve objectives like maintenance of soil fertility, protecting the soil from erosion and making the best use of available soil moisture. Crop rotations, strip cropping, intercropping and crop mixtures are terms used for describing different cropping systems.
Crop rotations for a given area serve one or more of the following purposes.
These are –
(1) Prevention or control of soil erosion,
(2) Building up of soil fertility,
(3) Building up the organic matter and thus improving the physical conditions of the soil, and
(4) Control of weeds.
Crop rotations for an area can be chosen in such a way that during the rainy period there is a vegetative cover over the soil surface. A vegetative cover controls splash erosion by intercepting the rain drops, and absorbing their energy. It also helps in maintaining the infiltration rate of the soil as on bare soil the beating action of rainfall breaks down the clods and forms a tight layer.
Legumes when included in the crop rotation help in maintaining soil fertility. Crops like groundnut, soyabean, green gram, chickpea etc., are legumes commonly used in crop rotations.
Inclusion of suitable green manuring crops in crop rotations is also beneficial for adding organic matter to the soil. Crop rotations are developed to suit particular soil and climatic conditions and hence differ from region to region.
Mixed cropping is the system of growing more than one crop together on the same land. Crops may be grown as homogeneous mixtures by mixing seeds of various crops before sowing. Different crops may be sown in separate rows according to a pattern in which case it is referred to as intercropping.
The benefits of mixed cropping are as follows:
1. Mixed crops varying in root system help in better utilization of plant nutrients in the profile.
2. Mixed crops requiring different spacing for growth, help in better utilization of space and profile moisture.
3. Objectives like erosion control by growing erosion resisting and erosion permitting crops, reducing wind affects by growing tall and short crops, etc., can be achieved.
Crop mixtures usually consist of-
(i) Tall and short crops,
(ii) Shallow rooted and deep rooted crops,
(iii) Early maturing and late maturing crops,
(iv) Wide spaced and close spaced crops,
(v) Legumes and non-legumes, and
(vi) Erosion permitting and erosion resisting crops.
The mixed cropping patterns have to be chosen depending upon the soil and climatic conditions.
Experiments at the International Crops Research Institute at Hyderabad in India have shown that for the deep black soils of that region the intercropping systems useful were –
(1) Maize intercropped with pigeon pea, and
(2) Sorghum intercropped with pigeon pea.
Tillage practices effect water conservation and erosion control.
The engineering measures consist of some land surface modification for retention and disposal of rainfall.
The engineering measures adopted are –
(1) Contour and graded bunding,
(2) Broad based terraces,
(3) Bed and furrow system, and
(4) Ridge and furrow system.
Suitable water disposal forms a part of all these methods.
Bunding is constructing small embankments or bunds across the slope of the land. The bunds are referred to as contour bunds when they are constructed on the contour and graded bunds when a grade is provided to them.
Terracing is a method of modifying the land surface for erosion control and water conservation. Terraces are broadly classified as the bench terraces and the broad base terraces.
Just like contour and graded bunds, broad base terraces are also classified as the level or the ridge type terrace (when no grade is provided) and the graded or channel type terrace (when a grade is provided).
Bunds or broad base terraces, decrease the length of slope and thus reduce erosion. A series of terraces divide the area into small strips and help in retaining the rainfall falling between them.
Contour bunds or level terraces are useful in low rainfall areas. They retain most of the rainfall and also control soil erosion. In areas of medium and high rainfall graded bunds or graded terraces are used. They safely dispose the excess runoff. Thus they help in controlling soil erosion and give longer time for the rainwater to infiltrate into the soil.
The bunds and the broad base terraces perform the same functions. The difference between them is that the broad base terraces are cultivated and cropped, whereas the bunds are not put under crop. Bunds are put under some permanent vegetation like grasses for their protection.
Contour and graded bunding have been extensively used in India for protection of agricultural lands. Broad base terraces have been similarly adopted in the USA. The broad base terraces need careful maintenance year after year.
The farmer in several Asian countries is unable to provide this maintenance. Coupled to this, the relatively small holdings is another reason for not adopting the broad base terraces.
In general, bunding is suitable for lands having slopes from 2 per cent to 10 per cent. Soil erosion in areas with less than 2 per cent is controlled using ‘biological measures’. It is advisable to adopt bench terraces in lands beyond 10 per cent slope as in such areas the bunds if adopted will have to be put a very close intervals.
In soil conservation practices soils are grouped as follows:
1. Shallow soils when the soil depth is less than 25 cm.
2. Medium soils when the soil depth is 25 cm to 45 cm.
3. Medium deep soils when the soil depth is 45 cm to 90 cm.
4. Deep soils when the soil depth is beyond 90 cm (when the clay content is above 30 per cent, these are termed as deep heavy clay soils).
Experience of bunding in India in the past several years indicated that bunding in shallow, medium and medium-deep soils is reasonably successful. Bunds constructed in deep black soils are not successful. This is due to the nature of the soil which cracks during the hot weather.
These cracks develop both in the bunds as well as all along the field. The flow of water in these cracks at times causes breaches of the bunds and consequent damage to the lands. Broad bed and furrow system is a method suitable for managing rainwater in black soils.
The choice of contour or graded bunds and the grade to be given to the bunds depends upon the rainfall, soil conditions and the type of the outlets. In low rainfall areas (annual rainfall < 500 mm) where moisture is the limiting factor for crop production, bunds on the contour may be adopted. In heavy and medium rainfall areas, the grade of the bunds may be 0.2 to 0.3% towards the outlet.
Spacing of the Bunds:
In spacing the bunds principles to be adopted are –
(1) The seepage zone below the upper bund should meet the saturation zone of the lower bund,
(2) The bunds should check the water at a point when the water attains erosive velocity, and
(3) The bund should not disturb agricultural operations.
On a 3 per cent slope, calculate the horizontal spacing of bunds in medium rainfall zone and the length of bunds per hectare.
Grassed waterways are open channels protected with suitable grasses. These are constructed along the slope and act as outlet for the terraces or graded bunds. As surplussing arrangement from bunded areas, they occupy land area as compared to waste weirs. Waterways are safer to dispose the water as compared to waste weirs as in waste weir the water flows over cropped land and likely to cause damage. Grassed waterways are also used to safely convey runoff from ponds.
The design of the grassed waterways is similar to the design of irrigation channels. The design of the grassed waterway consists in determining the size and shape of the waterway.
Size of Waterway:
The size of the waterway depends upon the expected runoff. A 10 year recurrence interval is used to calculate the maximum expected runoff to the waterway: As the catchment area of the waterway increases towards the outlet, the expected runoff is calculated for different reaches of the waterway and used for design purposes.
Shape of Waterway:
The shape of the waterway depends upon the field conditions and the type of the construction equipment used. The three common shapes adopted are trapezoidal, triangular and parabolic shapes. In course of time due to flow of water and sediment deposition the waterways assume an irregular shape nearing the parabolic shape.
If farm machinery is to cross the waterways parabolic shape or trapezoidal shape with very flat side slopes are preferred. The geometric characteristics of the different shapes of the waterways are given in Fig. 22.11. These are useful for design purposes.
Velocity of Flow:
The velocity of flow through the grassed waterways is dependent upon the ability of the vegetation in the channel to resist erosion. Even though different types of grasses have different capabilities to resist erosion an average of 1.5 m per second to 2 m per second are the average velocities used for design purposes. It may be noted that the average velocity of flow is higher than the actual velocity in contact with the bed of the channel. Fig. 22.12 shows the velocity distribution in a grass-lined channel.
Design of Cross-Section:
The design of the cross-section is done using Eq. 22.9 for finding the area required and then using the Manning’s formula for checking the velocity. A trial procedure is adopted for the purpose. For the required cross-sectional area, the dimensions of the channel sections are assumed. Using the hydraulic properties of the assumed section, the average velocity of flow through the channel cross-section is calculated using the Manning’s formula –
The coefficient of roughness is to be selected depending upon the existing and proposed vegetation to be established in the bed of the channel. If the velocity calculated by the Manning’s formula exceeds the permissible velocity, the cross-section is suitably altered and the velocity is calculated again. The process is repeated till the desired velocity is obtained with selected cross-section.
Design a grassed waterway of parabolic shape to carry a flow of 2.6 m3/s down a slope of 3 per cent. The waterway has a good stand of grass and a velocity of 1.75 m/s. can be allowed. Assume the value of n in Manning’s formula as 0.04.
Construction of Waterway:
It is advantageous to construct the waterways at least one season before the bunding. It will give time for the grasses to get established in the waterways. The area marked for the waterway and unnecessary vegetation like shrubs etc. is removed. The area is then ploughed if necessary and smoothened. Establishment of the grass is done either by seeding or sodding technique.
Maintenance of the waterways is important for their proper operation. Removal of weeds, filling of the patches with grass and proper cutting of the grass are of the common maintenance operations.
In bunded areas improved agricultural practices should be adopted so as to obtain the full benefits if bunding. Use of better variety of seeds, use of fertilizers or manures, adoption of plant protection measures and suitable crop rotations are part of the improved practices.
Contour cultivation wherein all the agricultural operations like ploughing, planting and harrowing be done on the contour to be adopted. The bunds serve as guidelines for the contour cultivation. The bunds may be planted with some vegetation for their protection. Regular maintenance of the bunds and the surplussing arrangements need to be done.
The bed and furrow system combines an element of erosion control with surface drainage. The beds could be of different dimensions to suit the cropping patterns adopted.
The broad beds and furrows have been found to be suitable for managing the deep black soils in India where surface drainage during the monsoon period is a problem. The beds function as “minibunds” at a grade and they help in reducing the velocity of surface runoff and increase the infiltration opportunity time. The excess water is removed through the furrows. A permanent bed and furrow system provides water control for in situ soil and water conservation.
The other advantages are –
(1) No land is taken out of production,
(2) The beds can easily be maintained by minimal tillage,
(3) The surface soil of beds dries more quickly between early monsoon showers than does the surface soil on flat cultivated areas, thus facilitating early planting on beds, and
(4) Soils on beds are found to remain friable throughout the cropping season.
The layout of the beds and furrows has to be done similar to that of the layout of graded bunds. A grade of 0.4 to 0.8 was found to be satisfactory for the deep black soils.
The bed and the furrow system can conveniently be adjusted within the existing field boundaries. To form the bed and furrows the land is tilled and using levelling instrument guidelines are marked. Narrow beds and furrows can be made using either animal drawn or tractor drawn ridgers.
For forming the broad beds and furrows, an animal drawn wheeled tool carrier is convenient equipment. This is referred as the “Tropicultor” and a range of attachments arc available for different operations like ridging, cultivation and bed formation.
The broad bed and furrows once formed are only to be maintained and the necessary tillage operations are carried out on the beds only. It may become necessary after some years to cultivate the entire field and prepare the beds again.
Diversion drains or diversions are channels constructed across the slope for the purpose of intercepting surface runoff and conveying the same to a safe outlet. Diversion drains are generally located at the bottom of the hills and above the agricultural lands. Diversion drains are also used at the gully heads or at the upstream of bunded or terraced areas to intercept the surface runoff. At times diversion drains are also used to divert surface runoff away from farm buildings. Fig. 22.16 shows the cross-section of a diversion drain and the spoil bank.
The design of the diversion drains consists in determining the area of cross-section for the given catchment area and slope. The velocity of flow should be safe for the type of vegetation which is likely to be established in the channel.
The general principles of design are exactly same as in case of vegetated water courses. While the vegetated waterways are along the slope, the diversion drains are laid across the slope.
The catchment area at the outlet of the diversion drain is determined from the existing maps or actual surveys. The maximum rate of runoff expected for a 10-year frequency is determined. The cross-section of the channel to handle this runoff is determined as in case of water ways.
In the layout of the diversion drains if the grade of the bed of the drain has an abrupt change simple drop can be used. Stones if available at the site can conveniently be used for such drop. The spoil bank of the diversion drain should be planted with suitable vegetation for stability and protection of the embankment.
Agroforestry is a land use system that helps to enhance productivity and ensure sustainability by combining annual food crops and woody perennial plants under either a rotational or intercropping arrangement. Under this system, the food crops supply the dietary needs, whereas the trees help to stabilize slopes, reduce erosion and meet the needs of fuel wood, small timber, fertilizer (in the form of green manure) and fodder.
The agroforestry systems for a given area will depend on the local situation like land characteristics, adaptability of the species, and farmers requirements.
The following different systems are in vogue:
1. Alley Cropping:
This is an agroforestry system in which food crops are grown in alleys, formed by contour hedgerows of trees or shrubs. The hedgerows are preferably established from native trees or shrubs of some economic value.
These trees are periodically pruned to prevent shading of the food crops. The shrubs and trees also act as windbreaks, facilitate nutrient recycling, suppress weed growth, decrease runoff, and reduce soil erosion. The prunings can be used as fodder for livestock, or as green manure, or sometimes as vegetative mulches.
2. Silvipastoral System:
This is the practice of simultaneously growing grasses and legumes in association with trees and shrubs on the same land. It meets the twin needs of firewood and forage including grazing for the animals.
3. Agrisilviculture System:
This refers to the system when trees are grown in conjunction with crops keeping in view the green manuring benefits that could be obtained from the trees.
4. Agrihorticulture System:
This refers to the system when horticultural crops are planted in conjunction with food crops.
5. Farm Forestry:
This refers to the trees planted on farm boundaries, embankments of irrigation channels, road sides, and other such vacant areas on the farms.
The overall effects of agroforestry in a given area need to be considered before the systems are established. Agroforestry could affect crop production under certain conditions. The competitive effect for moisture, nutrients, light in a mixed crop and tree culture and the problem of pests, diseases and birds need to be considered before adopting agroforestry systems.