Detailed soil survey is necessary to assess their degree of suitability for irrigation and to know what precautions should be taken for an ideal irrigation system. Land irrigability assessment by Kolay (2008) is briefly presented.
Irrigation Soil Survey:
Irrigation soil survey is undertaken at a scale of 1:25,000 or 1:20,000 in less developed countries. Aerial photographs and cadastral maps of the area are to be taken for detailed study. In-depth study of these provides clues to the nature of soil, land slope, soil depth etc.
Darker shades in aerial photograph indicate soils of heavier texture. Lighter shades indicate past erosion and soil texture. Such information aids in accurate determination of mapping units in the proposed irrigation soil survey on aerial photographs. Detailed soil survey may be free survey or grid survey.
Free Survey:
Detailed soil survey commences from a permanent object such as bench mark, building etc. The surveyor walks down the slope to observe soil surface features such as land slope, soil colour, texture and structure, erosion, vegetation etc. When he observes a change in any one of the features, he follows the line of change. This is the boundary between soil A and soil B (Fig 15.1).
The surveyor resumes walking down the slope to demarcate soil B from C, soil C from D and so on. Then he digs the soil profile in each of the soil units A, B, C—— . These soil units are provincially designated as the soil series possessing specific combination of surface features and profile characteristics. The soil series nomenclature comprises name of the place and surface soil texture.
Grid Survey:
In grid survey, soil observations are noted down in a grid pattern at short regular intervals. It is of two types: general purpose and special purpose.
General purpose grid survey:
In this, each of the soil units A, B, C…. is characterised by specific combination of surface features and soil characteristics. One combination of surface features and soil characteristics continue up to certain distance when the soil surveyor continues to examine the soil in grid pattern. This means that the soil mapping continues to occur.
Then the surveyor finds another combination of surface features and soil properties. This means that the soil mapping unit A has ended to occur while the soil unit B has begun to occur. He would find the combination of surface features and soil characteristics characterising the mapping unit B to remain unchanged up to certain area.
Then the combination of surface features and soil characteristics characterising the mapping unit B would change. This indicates that the soil mapping unit B has ceased to occur and the soil mapping unit C has begun to occur and so on. When he finishes the soil examination, he draws the boundary between soils mapping units A, B, C … (Fig 15.2).
Special purpose grid survey:
In this, only one soil property (say clay per cent) is examined in a grid pattern.
Boundaries between soils of clay percentages are successively drawn up as has been illustrated in Fig 15.3:
Thorough knowledge of the land characteristics is necessary to assess leveling and drainage requirements for alignment of irrigation channels and drains. Bench terraces are to be made in inclined land on red, laterite soil regions. Then, water falls by gravity to next lower bench and flood it and thereafter falls to next lower bench by gravity. Basins are to be made for fruit trees. Adequate precautions are to be taken to develop land for different methods of irrigation without causing erosion or waterlogging.
Soil Suitability Classes:
Soils are grouped into different irrigability classes on the basis of their degree of suitability for irrigated farming without land degradation. Their suitability to irrigated crop production depends on effective soil depth, surface soil texture, permeability, water retentive capacity, severity of salinity or alkalinity, drainage characterstics etc. Soils have been divided into A, B, C and D irrigability classes, with conditions deteriorating from A to E.
Class A soils:
These soils have no limitations for sustained crop production. Their effective depth exceeds 90 cm and texture varies from sandy loam to clay loam. Available moisture retentive capacity is at least 12 cm per 90 cm soil depth. They are non-saline with EC less than 4 micro mhos cm-1 at 25°C and that of 1:2 soil: water suspension of less than 1 micro mhos cm-1 at 25°C and non-alkaline as indicated by ESP of less than 10.
The soils contain less than 15 per cent gravel and kankar and less than 5 per cent cobbles and stones. Permeability of sandy loam soils varies from 5.0 to 60.0 mm h-1 and that clay loam from 5.0 mm h-1. Lower subsoil of class A is moderately permeable or they possess at least 15.0 cm thick permeable layer sand, gravel etc. within 3.0 m from their ground surface.
Class B soils:
These are of little inferior quality than class A soils because they are moderately limited to be permanently irrigated for crops on sustained basis. They are 45 to 90 cm deep, loamy sand or clayey in texture. Water holding capacity varies from 9, 0 to 12, 0 cm per 90 cm depth. They contain 5 to 15 per cent cobbles and stones of dia exceeding 75 mm and 15 to 35 per cent gravel and kankar of dia 25-75 mm.
Soils may be slightly saline as indicated by EC of saturation extract of 4 to 8 micro mhos cm-1 at 25°C and that of 1:2 soil: water suspension of 1.5 micro mhos cm1 at 25°C. Permeability of loamy sands varies from 50 to 130 mm h-1 and that of clay soils from 1.3 to 5.0 mm h-1. Saline patches may cover 15 to 20 per cent of their surface.
They may be slightly alkaline with an ESP of 10 to 15. Their lower subsoil is moderately permeable or they possess 15 cm thick permeable layer of sand, gravel etc. within 3.0 m from ground surface. Rock out crops may occur at an interval of 20 m apart.
Class C soils:
These soils are of little inferior quality than class B soil because they are severely limited for sustained crop production under irrigation. They are shallower than class B soils, 22.5 to 45 cm deep, sandy or clayey in texture. Water holding capacity varies from 6.0 to 9.0 cm per 90 cm depth. They may be moderately saline as indicated by EC of saturation extract 8 to 12 micro mhos cm-1 at 25°C and that of 1:2 soil: water suspension of 1.5 to 2.5 micro mhos cm-1 at 25°C.
They are fairly stony and gravelly soils as they possess 15 to 35 per cent cobbles and stones of dia exceeding 75 mm and 35 to 55 per cent gravel and kankar of dia 25-75 mm. About 20 to 30 per cent of their surface may show saline efflorescence. They may be moderately alkaline as indicated by their ESP of 15 to 20.
Permeability of sandy soils and clayey soils varies from 130 to 250 cm h-1 and from 0.3 to 1.3 mm h-1 respectively. Subsoil is not moderately permeable to water. No other permeable layer of at least 15 cm thick occurs within 3.0 m from ground surface. Rock out crops may occur at an interval of 15 m apart.
Class D soils:
They are temporarily considered to be non-irrigable due to severe limitations for sustained utilisation under irrigated conditions. They are shallow soils with depth varying from7.5 to 22.5 cm and of texture sand or clayey having available water holding capacity varying from 2.0 to 6.0 cm per 90 cm soil depth.
They may be strongly saline with EC of saturation extract 12 to 16 micro mhos cm-1 at 25°C and that of 1:2 soil: water suspension of 2.5 to 3.0 micro mhos cm-1 at 25°C. Saline patches may cover 30 to 50 per cent of their surface. They may be strongly alkaline as indicated by their ESP of 20 to 30.
Permeability of sandy soils may be around 250 mm h-1 and that of clayey soils less than 0.3 mm h-1. They contain 35 to 65 per cent cobbles and stones of dia exceeding 75 mm and 55 to 70 per cent gravel and kankar of dia 25 to 75 mm. Subsoil is not moderately permeable to water. No other permeable layer of at least 15 cm thickness occurs within 3.0 m from ground surface. Rock out crops may be 5 m apart.
Class E soils:
These soils are not suitable for irrigated agriculture. They are very shallow (< 7.5 cm deep) with variable soil texture and their water holding capacity is less than 2 cm per 90 cm soil depth. They may be very strongly saline/alkaline as indicated by their EC more than 16 micro mhos cm-1 at 25°C, that of 1:2 soil: water suspension of more than 3.0 micro mhos cm-1 at 25°C and ESP exceeding 30.
Salt efflorescence may occur on more than 50 per cent of their surface. They contain more than 65 per cent cobbles and stones of dia more than 75 mm and more than 70 per cent gravel and kankar with dia varying from 25 to 75 mm. Rock out crops may occur at less than 5 m apart.
Land Irrigability Classes:
Land irrigability classification is based on the:
1. Suitability of soil for irrigation
2. Availability of irrigation water
3. Quality of irrigation water
4. Permeability of the subsoil and feasibility for installing drainage system
5. Cost of drainage system installation
6. Potential crop yields and cost of production
7. Cost of development of land for irrigation
8. Factors influencing cost: benefit ratio.
Class I land:
Highly suitable for sustained irrigated agriculture. As the topography and drainage factors are ideal, a variety of crops can be grown with optimum yield and profitability. These are level and deep soils for optimum growth and development of crops. Depth of water table is more than 5 m. Subsurface drainage is not required. Returns from the investment on the land are very high.
Class II land:
They have moderate limitations of soil, topography or drainage for sustained crop production under irrigation. They belongs to A and B irrigability classes. Slope varies from 1.0 to 3.0 per cent. Texture, depth, permeability and other properties are a little less than ideal for arable crop production under irrigation.
Less than 60 m of shallow drains are enough to drain away excess water. There is no necessity for subsurface drainage. Depth of water table varies from 3 to 5 m from ground surface. Soil salinity may be moderate when the soil water is in equilibrium with irrigation water. They are costly for grading to irrigated crop production.
Class III land:
These soils are suitable but approaching marginality for sustained crop production under irrigation. Limitations are in soils topography (3 to 5%) and drainage, enough to be indicated by variation in soil irrigability class from A to C. Depth of water table varies from 1.5 to 3.0 m. Therefore, these soils have severe limitations for sustained arable crop production under irrigation. Drains must be up to 90 cm depth to drain out excess water. Crop production may be risky without appropriate subsurface drainage.
Class IV land:
These moderately steep soils are marginal for sustained use under irrigation to grow arable crops. Soil may not be deep enough to grow a variety of arable crops or may possess impermeable layer or may be too sandy or clayey. Soil water may be too saline or alkaline to support optimum growth and development of different crops for economic yields.
These soils include soil irrigability classes from A to D. Drains are to be dug at 90 cm soil depth. No natural drainage. These are excessively limited for normal crop production. It may or may not be possible to rectify the deficiencies. In the former case, heavy expenditure is involved to bring them under irrigated conditions. In the later case, some fruit trees may be grown under irrigation.
Class V land:
Very severely limited for irrigated crop production under the existing situation but may have potential for irrigated crop production with improvement.
Class VI land:
These lands are not suitable for sustained use under irrigation.
Mapping Units for Land Irrigability Survey:
Land irrigability classification needs informative appraisals by an integrated analysis involving economic, hydrological, engineering, land and soil characteristics. Interaction of productivity and cost of production of crops and land development cost needs estimation. Numerical 1, 2, 3, 4 and 5 denote land irrigability classes which are used for these appraisals relating them. Letters A, B and C indicate low, medium and high water requirements respectively for estimating farm water requirement. Letters X, Y and Z denote good, restricted and poor drainage conditions.
They are used to assess drainage conditions; g, u and f denote slope, undulation and flooding respectively; s, t ad d denotes soil, topography and drainage deficiencies respectively. The United States Bureau of Reclamation has used all these symbols to make mapping unit for land irrigability survey.
An example is:
Where,
2 = Land irrigability class
s, d and t = Soil, drainage and topography deficiencies respectively
C = Cultivated land.
Remaining lower symbols refer to productivity, land development costs, water requirement and drainage conditions and u and f refers to the necessity of land levelling and flooding hazard respectively.