How to Conserve Soil and Water? | Soil Management

The soil is always under natural/man forced threat in terms of erosion either due to water or wind actions. To check the occurrence of soil erosion/soil loss various measures are followed. For better effectiveness of soil loss control measures, their proper design and construction need to the very important.

The design is carried out based on the few pre requisite informations. In general, the following informations are taken into consideration as prerequisites for adoption/construction of soil & water conservation measures/structures to promote better agriculture for increasing the crop yield, and thus, to achieve maximum benefits from the land, existing.

1. Physiography of the Area:

It includes the size, shape, relief, drainage etc. of the area to be undertaken for implementation of soil conservation measures.

The description of these features are made, as under:

i. Size:

The size of catchment area is essential to evaluate the peak runoff rate and runoff volume generated in that, which are essentially required to take action against water erosion, and also to design erosion control structures such as gully control structures, bunding, terracing etc., and the grassed water ways to carry out the runoff from the catchment. The peak runoff rate or its volume, both depend on the catchment area.

ii. Shape:

Generally, the narrow and round shapes are counted as the main shape of watershed in soil and water conservation field, because variation is runoff rates in these shaped watersheds is more pronounced, as result the measures taken against them are also varied, accordingly. For example – the narrow watersheds have greater time of concentration, resulting into lower peak runoff rate than the round shaped watersheds of same size.

iii. Relief:

It is defined as the elevation difference between any reference point of the catchment and the outlet.

Its knowledge has importance for following points:

i. For computation of time of concentration.

ii. To compute the size of pump required for lifting the water.

iii. For use in Cook’s method to compute the peak runoff rate of the area, etc.

iv. Mean Elevation:

The knowledge of mean elevation (reduced to mean sea level) is essential for planning of developmental programmes such as cropping practices, afforestation, horticultural plantation, utilization of different sources of energy, use of conservation measures etc.

v. Land Slope:

The land slope decides the application of soil conservation measures on a particular land. For soil conservation the installation/use of any measure is governed by the runoff rate. The land slope affects it, significantly. The greater is the land slope, higher will be the runoff velocity and vice-versa.

The experimental evidence revealed that, the erosive capacity of runoff to erode the soil particles from the land surface varies in the proportion of land slope. The land slope can be evaluated by using topographic map of the area, and applying following formula –

S = [(M.N./A ) x 100] … (10.1)

In which, S is the land slope (%); M is the total length of all contour lines within the watershed (m); N is denoted as contour interval (m) and A is the watershed area (m²).

Based on the steepness of land slope the planning of soil conservation measures is performed, either they are agronomical or mechanical measures.

vi. Drainage:

This characteristics of watershed is assessed to predict the susceptibility of soil to erosion, runoff pattern, sedimentation, location of erosion control structures etc. The drainage capability of watershed is predicted in terms of drainage density, which affects the runoff pattern of the area.

The drainage density is expressed as –

D_d = Total length of all drains existing in the watershed (km)/Area of watershed (km²).

Drainage Pattern:

The drainage pattern refers to the pattern of water courses and their tributaries in the watershed. The land slope, lithology and structure are the main factors, which affect the pattern of land drainage. In addition, the distribution and attitude of the rocks and their arrangement also affect the drainage pattern of the area. The parameters “drainage texture” and drainage pattern are essential to interpretate the geomorphic features of the watershed, and understand the land forms evolution.

The drainage texture is concerned, it is classified into following three classes:

(a) Fine drainage texture

(b) Medium drainage texture; and

(c) Coarse drainage texture.

The fine drainage textures of dendritic pattern are characterized by the following features:

i. Rock formations are impervious and slow permeable.

ii. Soil depth is mere.

iii. Soils are heavy and slow permeable.

iv. These are facing the problem of severe erosion; even at some places formation of gully is also observed.

The medium drainage texture is observed in rock formations, involve following characteristics:

i. Presence of fractures and joints.

ii. Moderate permeability.

iii. Soils are moderately deep and medium texture.

iv. Drainage patterns are observed in the form of radial, braided and pinnate.

The coarser drainage textures involve high hydraulic conductivity. The rectangular and angulate drainage patterns are associated to this drainage texture.

The other features of this class of drainage texture are as follows:

i. Hydraulic conductivity is high.

ii. Soils are generally shallow.

iii. Soil texture is coarser.

iv. Soil erosion is mainly on steep slopes.

In the areas, where trellis, annular, rectangular and annular drainage patterns are observed due to pervious and resistant nature of the rock formation, the erosion hazard is relatively less. On the other hand, the areas covered under braided drainage patterns are affected by moderate to severe erosion hazards.

Similarly, in that area, where dendritic drainage pattern is found, the erosion hazard varies from severe to very severe. In brief, the drainage pattern provides a guideline for locating the vulnerable areas, which are in need of soil & water conservation measures, to install.

vii. Soils:

The description of soil series and their properties has prime importance for adoption of soil conservation measures.

A soil series furnishes following informations:

i. About the nature of soil.

ii. Different soil forming processes taking place in the region.

iii. Fertility level of soil.

The soil properties are concerned, the followings are mainly taken into consideration:

i. Texture of the soil.

ii. Depth of soil.

iii. Depth and occurrence of hard or kankar pan below the soil surface.

iv. Stoniness.

v. Soil permeability.

vi. Soil acidity and alkalinity.

In India the majority of soil series have been evaluated and all existing soil series have been grouped into four different hydrological soil groups starting from group A to group D.

The soil grouping is based on the following four main soil characteristics:

(a) Effective depth, which is defined as the soil depth up to which the plant roots are penetrated, either for abstraction of water or nutrients.

(b) Average clay content present in the entire thickness of soil profile.

(c) Infiltration characteristics of soil; and

(d) Permeability status.

The four different hydrological soil groups are described as under:

a. Hydrologic Soil Group-A:

The soils of this group have low runoff potential, as they have high infiltration rate even when thoroughly wetted.

The other features are as follows:

i. Soil depth is more.

ii. Well to excessively drained sand and gravel.

iii. Have high rate of water transmission.

b. Hydrologic Soil Group-B:

This group of soil involves moderately low runoff potential. The infiltration rate is moderate even when thoroughly wetted.

The other characteristics are as follows:

i. The soil depth varies from moderately deep to deep.

ii. Drainage of soil varies from moderately well to well.

iii. Soil texture ranges from fine to moderately coarse.

iv. Rate of water transmission is moderate.

c. Hydrologic Soil Group-C:

These soils are characterized by having moderately high runoff potential.

The other important features of this group soils are mentioned below:

i. Infiltration rate is very slow even when thoroughly wetted.

ii. Involves moderately deep to deep soil depth.

iii. Soil drainage varies from moderately well to well.

iv. Soil texture varies from moderately fine to moderately coarse.

v. Have moderate rate of water transmission.

d. Hydrologic Soil Group-D:

Group-D soils have very high runoff potential than the other soil groups.

The main characteristics are mentioned as under:

i. Have very slow infiltration rate even when thoroughly wetted.

ii. Involve mainly clay soils with high swelling potential.

iii. Soils have high water-table in permanent form.

iv. Soils have clay pan or clay layer close to the ground surface.

v. Shallow soil depth, nearly impervious materials.

The classification of infiltration rate and permeability of the soil is given in following table:

viii. Vegetative Cover:

It acts as cloth for covering the soil surface and also making the soil able to fight against erosion, either due to water or wind. The impact of vegetations on hydrological happenings is very dominating. It dissipates the direct impact of falling raindrops over the soil surface; and thus reduces the splash erosion.

Similarly, there are several other effects of vegetative covers are also observed, as the following:

i. It reduces the breaking of soil particles into finer during intense storm, as a result the pore spaces of lop soil surface are not blocked, thereby the intake rate of water is maintained within the limit. Greater intake rate decreases the runoff and soil loss.

ii. Vegetation creates an obstruction in flow path of surface water, resulting into some additional water to get move down into lower soil profiles. This happening not only increase the soil moisture, but also reduces the runoff magnitude and erosion, too.

iii. Vegetation makes the soil surface rough; by this effect the flow velocity of surface water is reduced, and thus the scouring of soil particles or erosion get reduced.

iv. The root system of vegetation holds the soil particles very tightly, which causes difficult to detach and transport the soil particles.

v. Along roots, the water gets a direct path to move downward into lower soil profiles. It also causes reduction in net water available on the ground surface to run away from its original place.

vi. Vegetation acts as a screen to filter the suspended loads of flowing water leading to the reservoirs, canals or some others. And thus, it plays an important role to control the bad happening of silt deposition into these structures.

vii. Flood moderation, runoff recycling, pisciculture etc. are also performed by vegetations as a most economical means.

ix. Land Use:

For planning and reorganization of land use as per land use capability classification to get sustained production, the informations on existing land use and practices followed by the farmers of region is essentially needed.

The land use data includes following collections:

(A) Agriculture:

(a) Rainfall areas –

(i) Area in hectares

(ii) Crops raised and their yields

(iii) Rotations adopted

(iv) Package of management practices

(b) Irrigation areas –

(i) Area in hectares

(ii) Crops raised and their yields

(iii) Rotations adopted

(iv) Package of management practices

(B) Forests:

(a) Major forest types –

(i) Area in hectares

(ii) Species and composition

(iii) Pure or mixed

(iv) Canopy density 0 to 1

(b) Composition –

(i) Upper storey of each forest type

(ii) Middle storey

(iii) Lower storey

(iv) Ground cover

(v) Injuries to which the crop is liable

(c) System –

(i) Main silvicultural system and management

(ii) Effects on the crop.

(d) Forest products –

(i) Timber

(ii) Fuel

(iii) Fodder

(iv) Other minor products.

(e) Form forestry –

(i) Planting pattern

(ii) Species used

(iii) Stocking per unit area

(iv) Average height and diameter.

(C) Horticulture:

(i) Area engaged under fruit orchards.

(ii) Species used

(iii) Variety type

(iv) Stocking, viz., good, fair or poor

(v) Average age of the trees (old)

(vi) Middle aged (young)

(vii) Bearing time (years)

(viii) Average annual yield

(ix) Average annual income

(D) Pastures and Grass Lands:

(i) Area engaged

(ii) Nature and type

(iii) Floral composition

(iv) Stocking or condition (viz., good, fair and poor)

(v) Carrying capacity

(vi) Management

(vii) Income.

(E) Other Miscellaneous Information:

It includes the roads, buildings, water courses, farm ponds etc.

(F) Existing Mechanical Structures:

(i) Cheek dams

(ii) Retaining walls

(iii) Bridges etc.

(G) Land Use Map:

The present land use map is required for study of existing cropping system followed in the area, and to make change in the pattern as per thrust of the area.

Various types of land use are shown by following colour symbols on land use map:

x. Land Capability Classes:

In this head, the data on land capability classes and area under each class, brief description of each class and main hazard by which land is affected, are collected. A detail inventory is also prepared showing the columns of land capability classes, sub classes and total area under each of them. The field surveys are carried out for the entire watershed to collect the data on land slope, soil and parameters causing the soil erosion. A self-contained map is also sketched based on the collected data.

2. Rainfall:

It includes average depth of rainfall, rainfall intensity-duration-return period relationship, rainfall distribution of the area etc., to compute, which are required for design of soil and water conservation structures, runoff disposal structures and also for planning of the Hood control structures.

Average Rainfall Depth:

The precipitation measured by the rain gauge represents the depth of rainfall occurred at that particular geographical point only, not the areal rainfall. For determining the areal precipitation, various rain gauges are required to install for measurement of rainfall covering the entire area, and by taking the mean of their values the areal precipitation is computed. The knowledge of areal precipitation plays an important role for computing the runoff, by using the empirical equations relating rainfall and catchment area.

The authenticity of rainfall data recorded for determining the areal precipitation is dependent on the following main points:

i. Distance between rain gauge and center of the representative area.

ii. Topographical characteristics of the area.

iii. Size of the watershed.

iv. Characteristics of local storm-pattern.

v. Nature of the rainfall.

3. Intensity-Duration-Return Period Relationship:

The general form of intensity-duration-return period relationship, derived after analysing the rainfall characteristics of several rain gauge stations is given as under –

Where,

I = rainfall intensity (cm/h)

T = return period (years)

t = storm duration (h) or the time of concentration.

K, a, b, n are constant; their value varies from location to location. The values of K, a, b and n are given in appendix-F for different zones of India. Using above equation, the value of rainfall intensity (I) can be computed, provided that the K, a, b, n, T and t are known.

The values of T and t are determined as under –

i. Return Period (T):

It is the reciprocal of rainfall frequency, i.e. if p is the rainfall frequency (plotting position) then T is given by –

T = 1/p … (10.9)

The probability or frequency of any rain event is calculated by the following formula (Weibull’s equation):

P (%) = [m/(n+1)] x 100 … (10.10)

Where,

m = rank number assigned to the rain event, which is done after arranging the rainfall events in descending order by their magnitude.

n = total number of observations.

However, from Table 10.1 the values of ‘T’ can also be have for the soil and water conservation structures taken into consideration for installation purposes.

ii. Storm Duration (t):

The duration up to which particular rainfall event is continued is called storm duration. It can be any time, may also be as the time of concentration of the watershed, which can be computed by using the following equation –

t = 0.01947. K_f^0.77 … (10.11)

In which, K_f is given by following equation –

Where,

L = maximum length of travel (m)

H = elevation difference between remotest point and outlet of the watershed

The value of ‘t’ can also be computed as follows:

t = Distance between remotest point and outlet of the watershed (m)/Average flow velocity (m/s) x 3600

The value of ‘t’ is obtained in the unit of “hour”. The recommended average velocities for use to determine ‘t’ are given in Table 10.2.

4. Design Peak Runoff Rate:

It is an important calculation for designing the soil & water conservation structures, either they are gully control structures or water storage structures.

5. Flood/Drought:

The flood and drought are two natural consequences, which cannot be eliminated but their effects can be minimized. In India it has been estimated that about 9.0 m.ha lands are affected by flood occurrence each year, causing damage to the crops, house hold properties and lives.

Therefore, it seems to be very essential to examine their frequency and probable occurrence time, so that a warning can be broadcasted to make the population alert. The flood forecasting is done by several methods; the probability analysis is one of the important methods, used for it.

The effect of flood can be controlled to a large extent by moderating it, which can be done by adopting following measures:

i. Land management using soil and water conservation measures

ii. Afforestation

iii. Head water erosion control practices

iv. Downstream flood protection work to reduce the flood peaks

v. Measures to maintain the capacity of drainage system of the area

Drought is just an opposite consequence to that of the flood. In flood, the areas concerned are submerged in the water, while in drought occurrence there is acute scarcity of water over the land surface. In extreme conditions the water-table is so-lowered that the wells/pump sets are failed to discharge the water, as a result not only the crops are suffered, but the human being also face a crucial problem for drinking water.

In India about 260 mha lands are subject to droughts of varying intensity. The severances of drought effects has been classified in different classes; the states falling in respective classes, are given as under –

Socio-Economic Informations:

Socio-economic informations are essential to evaluate suitability of the soil conservation or land reclamation projects in respect of social benefit. The success of these projects depends on socio-economic factors.

The impact of sociological happenings on the project works is studied in following three different phases:

(a) Analysis of human milieu and of sociological and organizational structures.

(b) Definition of norms, standards and manners and means for altering the milieu for immediate application; and

(c) Analysis of changes made in the original situation, effected.

Data Collection:

Data on Sociological and Demographic Features:

i. Total population of the area

ii. Social stratification

iii. Lines of authority and political organization existing in the village

iv. Population structure (sex, age, ethnic group location, occupations etc.)

v. Organized population shifts or migration

vi. Occupations and economic activities

Data on Land Tenure Structure:

i. Land holdings

ii. Manner of land tenure

iii. Methods of acquisition transmitted and alienation of land titles or rights

iv. Relation between land rights and social, political and religious systems

v. Farming practices, i.e. either farming done directly by the land owners or on private basis etc.

Data on Farm Structure:

i. Farm size and its main features like area and number of farm workers engaged.

ii. Organization of work

iii. Traditional crops, cultivation techniques etc.

Data on Attitudes and Behaviour:

i. Impact of project on farmers of the area

ii. Understanding of farmers and views on social as well as economic progress.

6. Storm Pattern:

It is studied by deriving the rainfall intensity histogram that is drawn between rainfall intensity and cumulative time. A rainfall intensity histogram is illustrated in Fig. 10.4, which is prepared by plotting the data shown in Table 10.3.

Based on the time of occurrence of intense rainfall, the storm patterns are classified as under:

1. Advance pattern

2. Delayed pattern

3. Intermediate pattern; and

4. Uniform pattern

If an intense rainfall takes place at the beginning of the storm, when soil has high capacity to absorb the water, then storm pattern is referred as advance pattern. In this case, the runoff potential of the storm is being less, whereas in opposite case, the formed storm pattern is called delayed pattern.

In delayed pattern, the runoff potential of storm is being more, as most of the soils have already been saturated with the previous rainfall, and when an intense rainfall takes place the water accumulates over the soil surface and begins to move either through the rills or inter-linked depressions towards stream/channel, immediately.

The uniform pattern refers to the occurrence of intense rainfall throughout the storm period at uniform rate. In this rainfall pattern, initially the rain water is consumed for wetting the soil but after reaching at saturation stage, the runoff yield increases proportionally to the rainfall duration and its rate.

In intermediate pattern, the intense rainfall occurs at middle of the storm period. The runoff potential of the soil is in intermediate range.

7. Stream Flow Rate:

The data of stream flow rate has importance in the field of soil and water conservation engineering. The design of water conservation structures like reservoirs, ponds etc. is carried out mainly on the basis of peak flow rate of connecting river/stream. Likewise, the design and installation of river training works and stream bank erosion control practices are also performed based on the stream flow rates.

The discharge of a stream is determined based on the measurement of stream flow velocity and flow depth at the gauging point. The stream section used for gauging the flow, is called gauging station or sometimes control section. Permanent control section is always preferred for measurement, because at such sections the stream flow characteristics remain un-change for longer duration.

The stream flow rating curves (discharge V_S gauge height) of such section are valid for longer duration; not required to modify them.

The relationship between discharge (Q) and gauge height (G) is given by the following equation, called stage-discharge relationship or rating curve –

Q = C_r (G – a)^β … (10.31)

Where,

C_r and β = constants of rating curve

G = gauge height

a = constant, is the gauge height corresponding to zero discharge

To determine the stream discharge using above equation only gauge reading (G) is required to measure. The other values such as C_r, a and β are already available for deriving the rating curve of given gauging station.

However, if they are not available, then they can be determined as under:

Rating Curve Constant C_r and β:

These are determined by using the least-square-error method. Considering equation 10.31,

Q = C_r (G – a)^β

Taking the log both side,

log Q = β log (G – a) + log C_r

or Y = β X + b … (10.32)

In which

Y = log Q (dependent variable)

X = log (G – a) (independent variable)

and b = logC_r

or C_r = Anti log b … (10.33)

For a best-fit curve of X and Y, the values of β and C_r are given as under –

In which, N is the number of observations.

The correlation coefficient (r) is also computed to determine the correlation between X and Y variables. The formula for ‘r’ is given as under –

If

r = 1.0, then there is perfect correlation between X and Y

= between 0.6 and 1.0, considered as good correlation.

In present case, the Q gets increase with increase in (G – a), so there is +ve value of ‘r’.

Constant ‘a’:

It is also known as the stage for zero discharge, is a hypothetical parameter, cannot be measured in the field.

However, the following methods can be used for predicting it:

i. Method (1) – Steps:

1. Plot Q V_S G data on arithmetic scale.

2. Draw a smooth curve by eye judgment through the plotted points.

3. Select three different discharge points Q₁, Q₂ and Q₃ on the curve in such a way, that –

Q₁/Q₂ = Q₂/Q₃

4. Also, read the values of gauge readings G₁, G₂ and G₃, corresponding to Q₁, Q₂ and Q₃.

5. Use following formula to determine ‘a’

ii. Method (2) – Steps:

1. Plot Q V_S G data on arithmetic scale and draw a best fit curve.

2. Extrapolate the curve by eye judgment.

3. Find the gauge reading (G) corresponding to Q = 0. This value of G represents the ‘a’.

4. Again, to verify the value of ‘a’, plot Q V_s (G – a) on arithmetic scale. If plotting-yields a straight line then ‘a’ value obtained above is assumed to be correct. Otherwise,

5. Select another value in the neighborhood of previous value, and by trial and error method determine ‘a’ which should give a straight line between Q V_S(G – a).

iii. Method (3) – Steps:

Wisler and Brater (1965) have reported a method, called Running’s method to predict the ‘a’.

The procedure is given as under:

1. Plot Q V_S G data on an arithmetic scale, and draw a smooth curve through the plotted points.

2. Select, three points on the drawn curve, in such a way that the discharge values at these points are in geometric progression, i.e.

Q₁/Q₂ = Q₂/Q₃

In which, Q₁, Q₂ and Q₃ are the discharge values at three selected points A, B and C, respectively on the curve.

3. Draw vertical lines from the points A and B.

4. Also, draw horizontal lines from points B and C. They cut at points D and E as shown in Fig. 10.9.

5. Now, draw straight lines from points D and E, and A and B, which intersect at point F.

6. Find the ordinate at point F; it is the value of gauge height at zero discharge (a).