Land Suitability for Irrigation and Land Irrigability Classification

LAND SUITABILITY FOR IRRIGATION AND LAND

IRRIGABILITY CLASSIFICATION

Selection of Soils

The selection of lands for irrigation encompasses factors of climate, topography, drainage

physical and chemical characteristics of soil. The suitability of soils for different crops in relation

to the proposed irrigation system planning depends upon following consideration:

Climate

Climate exerts important influences on the selection of lands for irrigation. Soils tend to

show a strong geographical correlation with climate, especially at the global scale. Energy and

precipitation strongly influence physical and chemical reactions on parent material. Climate also

determines vegetation cover which in turn influences soil development. Precipitation also affects

horizon development factors like the translocation of dissolved ions through the soil. As time

passes, climate tends to be a prime influence on soil properties while the influence of parent

material is less.

Climate affects both vegetative production and the activity of organisms. Hot, dry desert

regions have sparse vegetation and hence limited organic material available for the soil. The lack of

precipitation inhibits chemical weathering leading to coarse textured soil in arid regions. Bacterial

activity is limited by the cold temperatures in the tundra causing organic matter to build up. In the

warm and wet tropics, bacterial activity proceeds at a rapid rate, thoroughly decomposing leaf

litter. Under the lush tropical forest vegetation, available nutrients are rapidly taken back up by the

trees. The high annual precipitation also flushes some organic material from the soil. These factors

combine to create soils lacking much organic matter in their upper horizons.

The character of the soil, drainage conditions, distribution of native vegetation, and crop

adaptation are strongly related to climate. To a lesser extent climate also influences the relief of the

land surface. These factors affect needs of the area, type of plan formulated, design of facilities,

and economic impacts from irrigation. As a result, the lands selected to be irrigated will have

different characteristics and qualities in different climate settings.

The climatic influences on the farm input-output relationship have considerable effect on

the quality of lands included in irrigation projects. In areas producing high income crops such as

citrus, vegetables, and cotton (Gossypium herbaceum), expenditures as high as $ 550/- / acre can be

made to develop the farm for irrigation. In areas producing general field crops similar expenditures

would probably not exceed $ 250, while areas growing hay, pasture, and small grains would have

limits of about $ 100/acre. (These dollar values are for Federal Reclamation Project areas and

would be different for private development having different institutional constraints).

In general, as climate favours higher farm income, greater expenditure can be made for

land forming, farm distribution systems, leaching salt and exchangeable sodium, profile

modification practices, and farm surface and subsurface drainage. When considered in terms of

land class determining factors such as uneven microrelief; soil texture, structure, and depth; ESP

and soluble salt levels; permeability of substrata; and depth to groundwater barrier, then more

severe deficiencies involving such factors can be tolerated in climates favoring high farm incomes

than in those favoring lower incomes.

Within the broad regional climatic patterns important localized influences occur. The

average climatic conditions on individual farms or small tracts may be different from those of the

region mainly because of slope and exposure. In selecting irrigable lands, this factor can be of

considerable importance. In areas to be devoted to fruit production, land should be differentiated

with respect to frost hazard. This is typical practice in selecting irrigable lands on projects in the

Pacific Northwest where the cropping pattern is dominated by fruit. Here frost hazard lands are

intricately associated with lands, rarely experiencing frost. In regions where experience is lacking

to guide such differentiation, detailed studies of the microclimate may be necessary. Mason (1958)

describes such a study in which lands are identified as frost-free, frost-rare, intermediate, frost-

liable, and frost-subject, all occurring within an area of 10 square miles.

Topography

The principal topographic attributes determining suitability of land for irrigation are degree

of slope, relief, and position. These factors influence land development needs and costs, design of

on-farm water conveyance systems, erosion hazards, crop adaptability, drainage requirements,

water use practices, and selection of management systems. The topographic attributes are

correlated with soil and drainage conditions in selecting the irrigable lands. The quality of land for

irrigation is then derived by appraising the correlated effect of topography on productivity, cost of

land development, and production costs. Size and shape of the areas are usually considered as part

of the topographic factor because of their common relationship to land development and production

costs.

1. Slope : The effect of slope on the suitability of land for irrigation is primarily influenced by the

crops to be grown, the duration and intensity of rainfall, erodibilty of the soil, farm income

potential, and the method of irrigation to be used. In selecting irrigable lands, an initial decision

needs to be made as to whether or not the slope conditions should be modified by landforming.

That is, whether the slope factor is to be treated as permanent or changeable. If the dominant slope

is to remain, then the land is appraised for irrigation suitability primarily on the basis of

productivity and labor inputs.

In this case, an increase in per cent or complexity of slope generally results in a decrease of

the quality of the land for irrigation. The decrease will vary with the irrigation method, being

particularly pronounced with gravity methods and somewhat subdued under the sprinkler method.

As slope increases, shifts in cropping patterns from row to close-growing crops are frequently

made. in general, the row crops produce higher net incomes than the close-growing crops. Hence,

there is a decline in net productivity. Also, the labor costs involved in water management on

sloping lands are usually higher than that of nearly level lands. Surface drainage problems

frequently arise on lands lacking gradient and these can usually be corrected by land forming and

installing surface drainage systems. In areas producing fruit or vegetables, relief and position need

to be considered with respect to frost hazard.

Harris and Hansen (1958) find that the relationship between agricultural land value and

slope is governed by the relative importance of surface drainage, erosion, and water intake. The

relationship is shown in Fig. Values for a particular area will, according to Harriss and Hansen, lie

somewhere within the shaded area depending upon the relative importance of the factor cited.

As the complexity of slope increases, there is a successive reduction in the amount of

gradient that should be permitted within an irrigable land class. Complexity of slope affects the size

and shape of fields, type and number of water control structures needed, size and length of water

conveyance systems, and requirements for relift pumps to reach high areas. In some situations

involving complex slopes the irrigation problems involved can be most efficiently handled by

adopting sprinkler irrigation.

2. Land development : In selecting lands for irrigation the more important land development

factors considered are : clearing vegetation such as trees or brush; removing stones, cobble, or

outcrops of bedrock; leveling land; terracing land; floating and planning; deep plowing or

chiseling; subsoiling and hardpan shattering and perforation; constructing laterals, ditches, borders,

dikes, basins, drops, checks, weirs, flumes, and small storage ponds; installing wells, pumps, main

farm laterals, pipelines, fixed and portable sprinkler systems, nozzles and rotating devices, farm

surface drains, ditch and lateral lining, grassed waterways, inlet and outlet structures, open and

closed subsurface drain laterals, and inverted wells; and by leaching. The requirements for land

development vary from tract to tract. The method of irrigation, value of crops, climatic conditions,

volume of water available for irrigation, slope of land, surface irregularities, soil properties, and

drainability of the substrata are the principal variables considered in determining costs of land

development. The cost will also vary in accordance with preferences of individual farmers. In the

selection process, assumptions are therefore made regarding the amount of land development to be

performed on a project.

Drainage

Prediction of the drainage requirement is the crucial element in selecting land for

irrigation. The adequacy of this prediction and its engineering implementation are among the prime

physical determinants of the success of irrigation enterprises.

Under the concept of conservational use, irrigable land must be drainable land. Systems

devised for selecting irrigable lands should therefore encompass drainability evaluations. This

includes investigating the substrata as well as the soil. Some lands are endowed with adequate

natural drainage to sustain irrigation. Unfortunately, naturally drainable areas occupy but a minor

portion of the landscape. Requirements for engineering works to remove excess water and salts will

therefore arise in most irrigated areas.

In selecting lands for irrigation both surface and subsurface drainage are considered.

Surface drainage is the removal of excess water from the surface of the land. The excess may arise

from precipitation, irrigation, losses from conveyance and storage facilities, or seepage from

groundwater at a higher elevation. Subsurface drainage is the removal of excess water from within

the soil by downward or lateral flow through the soil and substrata ... it involves control of

groundwater levels and the corresponding salt levels in the soil. The source of the water may arise

from deep percolation from precipitation or irrigation; leakage from canals, drains, or surface water

bodies at higher elevations; or upward leakage from artesian aquifers. Surface and subsurface

drainage problems are frequently co-incident and inseparable.

Recognition of Drainage Problems : Existing drainage problems may be identified by careful

field observations. The following generally indicate adverse drainage conditions : (i) water

standing in topographic depressions for prolonged periods; (ii) occurrence of salt-affected soils

with barren surfaces; (iii) soils containing high concentrations of soluble salts in surface layers or

having a distinct surface crust; (iv) shallow water table; (v) mottling or presence of gleyed

horizons; (vi) crop symptoms of unfavorable drainage such as stunted growth or late maturity,

disease, and shallow root development; and (vii) presence of phreatophytic vegetation.

Recognizing a potential drainage problem requires field observations and measurements

coupled with careful engineering analyses. Drainage investigations are designed to identify (i)

possible sources of excess water, (ii) adequacy of outlets for conducting excess water out of the

project area, (iii) capacity of the soil and substrata to conduct water, (iv) volume of excess water to

be removed, and (v) design required to achieve most efficient drainage. The investigations are

initiated by collecting, reviewing, and analyzing relevant data regarding geology and landforms,

soils, topography, well logs, water levels and fluctuations, precipitation and surface flow, and

similar factors.

In general a potential drainage problem may exist when (i) soil and substrata have

hydraulic conductivities of < 1 inch/hr; (ii) soil or unconsolidated substrata consist of textures of

fine sandy loam or finer and contain exchangeable sodium usually > 15%; (iii) soils contain excess

soluble salts for the climate under which they developed; (iv) impervious strata of shale or

sandstone occur within depths of 10 ft or less and have an unevenly weathered or eroded surface

obstructing both lateral and vertical water movement; (v)vertical or nearly vertical barriers occur

such as faults, dikes, and intrusions accompanied by topographic and substrata conditions that

intercept groundwater movement, constrict flow, and favour high water table conditions; (vi)

obstructions to surface drainage occur such as road and railroad embankments; (vii) lands lie

adjacent to a large proposed unlined canal, particularly the lower lying lands enclosed by a U-

shaped canal alignment; (viii) lands border natural drainage channels such as stream bottoms and

low terraces; (ix) lands lie adjacent to lakes or reservoirs whose water elevation may rise

sufficiently to unfavorably influence the groundwater levels; (x) tracts are subjected to sidehill

runoff or seepage from higher lands; and (xi) irrigation water, groundwater, or both contain an

ionic composition that may induce imperviousness in the soil or substrata through chemical

reactions. Recognizing such factors assists in determining the type and detail of drainage

investigations needed in a particular area to select irrigable lands and design drainage facilities.

Soil physical and chemical characteristics

1. Effective Soil Depth : This is a soil mentle resting over parent material which could be either

undifferentiated alluvium, hard pan (murrum layer) weathered (unconsolidated) or unweathered

(consolidated) rock below where roots are unable to pass and even a permanently moistured

saturated zone in which plant roots cannot survive.

The various soil depth classes recognised for this purpose in connection with soil survey

are following :

a) Very Shallow - less than 7.5 cm.

b) Shallow - 7.5 to 22.5 cm.

c) Medium deep - 22.5 to 45.0 cm.

d) Deep - 45 to 90 cm.

e) Very deep - more than 90 cm.

The area, therefore, can be classified into different soil depth classes and average area

under each soil depth class be recorded.

2. Soil Texture : It represents the distribution of particle size in different soils of the command.

Three important particles sizes are being considered for assessing soil texture :

i) Sand (2.0 to 0.02 mm dia as per International method)

ii) Sild (0.02 to 0.002 mm dia as per International method)

iii) Clay (0.002 mm and below).

The number of texture classes of surface and below by auger hole method upto 1 metre

should be grouped as under after profile examinations in the area :

1. Coarse textured soils e.g., sandy, and loamy sand.

2. Medium textured soils e.g. loam, silt loam and sandy loam.

3. Fine textured soils e.g. sandy clay-loam, clay-loam, clay-loam, clayey, fatty clay, silty-clay

etc.

3. Soil Structure : The natural arrangement and orientation of soil particles forming a soil

aggregates of different shapes and sizes and termed as soil structure. The various soil structures

recognised are :

a) Structureless : (a) Single grain (e.g. sand) and (b) Massive (single grain particles clining

together with small amount of clay and organic matter but are without definite lines of

cleavage).

b) With structure : (a) Granular or crumb (rounded granules of soil aggregated in either

uniform or irregular size and shape) and (b) Platty Soil aggregates having horizontal

dimensions greater than vertical as in mica (Biotite, muscovite).

c) Blocky : Soil aggregates conglomerating in (non-angular or angular) a shape having same

horizontal and vertical dimensions.

d) Prismatic and Collumner : Soil aggregates having vertically elongated shape as in prism.

If elongated portions are rounded it is columner (common in sodic soils).

4. Structure Destroyed (puddle soil mess) : The hydro-physical properties of soils are

considerably affected by this parameter.

5. Available Water Capacity : Available water capacity is the amount of water that a soil can

store that is available for use by plants. For designing efficient water delivery or distribution

system for fixing frequency or interval of irrigation and depth etc. the available water capacity of

the different established soil classes has to be worked out. This term when represented on per cent

basis is called available water per cent or A.W.P. It is a difference of moisture at field capacity

(F.C.) minus moisture at permanent wilting point (PWP) for different soil classes. The AWP,

Irrigation frequency and depth of water in a soil is expressed as cm/m soil depth which can be

calculated by formula :

i) AWP = FC-PWP

P x D x AWPii) Irrigation Interval, i = -------------------

ETC

P x D x AWPiii) Application depth, d = -------------------

Water or irrigation Eadepth

Where i = irrigation in days

P = means allowable moisture depletion for particular soil

D = rootzone depth

Ea = application efficiency

ETC = crop evapotranspiration.

6. Infiltration Rate : Infitration is the process of entry of water into the soil. Infiltration rate is the

maximum rate (mm/h) at which water penetrates into the soil at a given moment, the surface being

in contact with water at atmospheric pressure. The basic (Ib) and cumulative infiltration must be

determined at various locations in all the classes and average value for each class should be

recorded. Cumulative infiltration is total quantity of water enters the soil in a given time

Y = a tb + D

Y - cumulative infiltration in cm in time

t - elapsed time (minutes)

a and b - constants for various soils

7. Stream Size : For clayey soils, the stream size should be kept at 10 lits/sec for medium type and

medium textured 15 lits/sec. for medium to medium coarse land, it may be kept 30 lit per second

and for coarse texture upto 60 lits/sec (2 cusecs).

8. Permeability : This term is now designated as hydraulic conductivity and can be determined in

laboratory and in situation fields by auger hole test method in presence of water table or by pit

method in absence of water table. The test be conducted for all soil classes and averaged for each

class.

Permeability may be defined as the characteristic of a porous medium of its readiness to

transmit a liquid. The equation expressing the flow considers the fluidity of liquid and the

permeability factor called intrinsic permeability. Darcy's law according to the definition of

permeability may be written as,

A = K'

Where,

Q = volume of flow, cm3/s

K' = intrinsic permeability, cm/s2

= density of liquid

g = acceleration due to gravity, 9.81 cm/s2

= viscosity of liquid

H = loss of hydraulic head, cm

L = Length of tube, cm

A = cross -sectional area, cm2

From the expression, we find that the hydraulic conductivity K is,

K = = K'f

Where,

f = = fluidity of liquid

Again K' =

The intrinsic permeability has the physical dimension of L2T-1. Only the size and shape of

soil particles and pores influence it. Intrinsic permeability is the same as the hydraulic conductivity

except that it is independent of the fluid properties such as specific weight and viscosity, while the

hydraulic conductivity is dependent on fluid properties and changes with quality of water. For

most studies of water flow in irrigation and drainage, the influence of specific weight and viscosity

is relatively small.

Permeability is dependent on the pore size distribution in the soil. the larger the proportion

of macro-pores, the greater is the permeability. Permeability usually decreases with depth as

subsoil layers are more compact and have a smaller number of macro-pores compared to the

surface soil layer. The organic matter content, soil aggregates, texture, structure, colloidal matters,

plough pan, sodium concentrations of water, tillage and crop management practices influence

greatly the permeability of soil. Permeability decreases as the soil becomes drier following

saturation.

Smith and Browning (1946) described six classes of permeability based on hydraulic

conductivity of soil.

Table Permeability classes based on hydraulic conductivity of soil

Permeability classes Hydraulic conductivity (cm hr-1)

Extremely slow <0.0025

Very slow 0.0025-0.025

Slow 0.025-0.25

Moderate 0.25-2.5

Rapid 2.5-25.0

Very rapid >25.0

Source : Smith and Browning (1946)

9. Soil Salinity and Sodicity : The amount of salts, pH and exchangeable sodium per cent would

determine the type of problem in the area and accordingly different soil classes affected may be

regrouped for design of drainage system and leaching of salts for their suitability to irrigation and

irrigation planning. The reclamation measures be done by gypsum/lime as per requirement and

green manuring with dhaincha and salt resistant crops/paddy with special agronomical practices.

10. Water Quality : The quality of under-ground and irrigation water must be tested prior to

planning or clearance of wetting of any irrigation project. Also a periodic test of water quality be

made to accommodate any change in future planning and maintained soil fertility and sustain land

use in any type of soil in a command. For water quality rating sec Table 1.10.

Water quality rating as proposed by workers in India (Pai and Mukkeri, 1979)

Nature of soil Crops to be grown Upper permissible safe limits to Ec of

irrigation water micromphos/cm

Deep black soils and alluvial

soils clay >30%, fairly to well

drained

Semi-tolerant crops

Tolerant crops

1500

2000

Soils with clay 20-30% well

internal drainage and good

surface drainage

Semi-tolerant crops

Tolerant crops

2000

4000

Soils with 10-20% clay well

internal drainage and good

surface drainage

Semi-tolerant crops

Tolerant crops

4000

6000

Soils with clay <10% excellent

internal and surface drainage

Tolerant crops 8000

11. Classification of Different Soil Classes into Irritability Classes

Five to six irrigable classes are made for command. These are :

Class I = None to slight soil limitation (slope 1%, water table < 3 m).

Class II = Moderate soil limitations (soil, drainage, salt, topography and erosion) slope 1-

3%, water table 3-5 m

Class III = Severe above listed limitations for sustained (slope 3-5%), water table (1.5-3.0

m) use under irrigation.

Class IV = Very severe soil limitations for sustained use under irrigation (slope 5 to 10%),

water table 1.5 m and greater).

Class V (slope 7 to 15%) = Unsuitable for irrigation under prevailing conditions but may

be brought under irrigation by reclamation and management at high cost.

Class VI = Totally unsuitable for irrigation due to poor soil cover, and rock out crops –

Hence not considered for irrigability rating.

Land Development Needs

The irrigation is economical when applied to only I and II irrigability classes where slopes

are not steeper. For gravity or flow irrigation system the ideal slopes is 0.3% to 0.4% in flow

direction. However, by manipulating stream size and application method the soils up to 1.0% may

be irrigated. The land leveling programme should be taken only on deep to very deep soils and

should be avoided where soil depth is less than 0.50 meter. However, since 1'' soil layer of top

takes 300 to 1000 years to develop under natural conditions and 25 to 30 years under highly and

intensive soil management practices the cuttings should be planned to bare minimum and emphasis

should be on terrace making and contour cultivation, contour stripping cropping and other soil

conservation measures for other class of soils as per details of soil survey.

References

Darra B L and Raghuvanshi C S (1999) Irrigation management. Atlantic Publishers, New Delhi. p 8-13

Majumdar D K (2000) Irrigation water management (Principle and practices). New Delhi. p 83-84

Hangon R M, Haise H R, Edminster T W (1967) Irrigation of Agricultural Lands. American Society of Agronomy, Publisher Madison, Wisconsin, USA. p 125-89

Reddi G H S and Reddi T Y (2005) Efficient Use of Irrigation Water. Kalyani Publishers. p 324-25

Mason B (1958) An example of climatic control of land utilization. Climatology and microclimatology. Canberra Symp. Proc, UNESCO 11: 188-94

Harris K and Hansen V W (1958) Relative productive value of land. Utah Stat Univ. Agr. Appl. Sci. Eng. Exp. Sta. p 29

Assignment

On

LAND SUITABILITY FOR IRRIGATION AND

LAND IRRIGABILITY CLASSIFICATION

Course No. : Agron-602Evapo-transpiration

Submitted to :

Dr. S.S. Mahal

Submitted by :

Jagjot Singh Gill L-2009-A-3-D