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2. Evaluating and classifying land for irrigated agriculture

2.1 Basic principles
2.2 Terminology
2.3 Main procedures in land evaluation and classification
2.4 Land productivity index and economic measures of suitability

2.1 Basic principles

Certain concepts and principles are fundamental to successful land evaluation for irrigated agriculture. The basic principles advocated by the Framework for Land Evaluation (FAO 1976a), on which the methods described in this bulletin are based, are complementary to the principles of the US Bureau of Reclamation (see Chapter 10) which are specifically for irrigation.

i. The FAO Framework indicates that it is necessary to evaluate land and not just soils. The suitability of soils for irrigated crops is useful information but it is inadequate for making decisions about land use development. Therefore all relevant land characteristics including soils, climate, topography, water resources, vegetation etc. and also socio-economic conditions and infrastructure need to be considered.

ii. The main objective of land evaluation for irrigated agriculture is to predict future conditions after development has taken place. It is necessary to forecast the benefits to farmers and the national economy and whether these will be sustained without damage to the environment. Essentially a classification of potential suitability is required which takes account of future interactions between soils, water, crops and economic, social and political conditions.

iii. Some factors that affect land suitability are permanent and others are changeable at a cost. The costs of necessary improvements may be determined, so that economic and environmental consequences of development can be predicted. Typical examples of permanent features are temperature, soil texture, depth to bedrock and macro-topography. Changeable characteristics which may be altered deliberately or inadvertently, typically may include vegetation, salinity, depth to groundwater, microrelief, and some social and economic conditions (e.g. land tenure, accessibility).

iv. Land suitability must therefore be assessed and classified with respect to specified kinds of land use i.e. cropping, irrigation and management systems. It is obvious that the requirements of crops and irrigation and management methods differ, so the suitability of any land unit may be classed differently for various uses. It can be useless or misleading to indicate suitability for irrigated agriculture in general if the land developer needs to know about its potential for a specific irrigated crop or irrigation method.

v. Land evaluation requires a comparison of the outputs obtained with the inputs needed to generate these outputs, on different kinds of land. In other words, land suitability evaluation is essentially an economic concept, although formal economic analysis may not be necessary for simple surveys. Assessment of physical factors alone does not permit prediction of the results of irrigation; they must be translated into economic terms. It is most important to achieve a land classification that reflects differences in the long-term productivity and profitability of the land under irrigation rather than one that focuses only on physical differences without regard to their economic implications.

vi. The evaluation must take account of the local physical, political, economic and social conditions. The success of irrigation when it is introduced may depend as much on factors such as pricing policies for crops, labour supply, markets, accessibility, land tenure, etc. as on climate and soils. To avoid any misunderstanding all the factors which are relevant in the local situation should be explicitly stated rather than assumed. However, not all conditions need to be considered: only those that can usefully be taken into account in classifying land.

vii. The land suitability must be for sustained use, that is, permanently productive under the anticipated irrigation regime. Either there should be no land degradation anticipated or the cost of prevention or remedial action to control erosion, waterlogging, salinization etc. should be included in the comparison of inputs and outputs.

viii. The evaluation, where more than one apparently viable alternative exists, should compare more than one kind of use. Comparison may be, for example, between the present use and the proposed uses, or between different crops and irrigation methods. The reliability of the evaluation is enhanced by comparing inputs and outputs for several alternatives to ensure that the land use selected is not only suitable but the best of suitable alternatives.

ix. It is evident that an interdisciplinary approach is required, because no one discipline can cover all aspects of land suitability evaluation. Land evaluation can be carried out using general economic considerations to establish a context for selecting appropriate crops and management, and to establish the criteria for boundaries between suitable and unsuitable land. To make a quantitative evaluation at project or farm level, however, requires formal analysis in financial and economic terms.

x. Finally, as emphasized in Chapter 1, land evaluation is an iterative process leading to successive refinements and the need for surveys and investigations that are appropriate in scale and intensity during the different stages from reconnaissance to detailed project planning, and thereafter in successive phases of project implementation.

2.2 Terminology

Full definitions of the FAO Framework terminology are given in the Glossary at the end of this bulletin and in the appropriate Chapters. The most important terms are explained here prior to outlining the evaluation procedures.

i. LAND: An area of land within boundaries, with specified land qualities or land characteristics is known as a land unit. Land units can be mapped and serially numbered (e.g. land unit 1, land unit 2, land unit 3, etc.). The size of an individual land unit can be varied to suit the intensity of the investigation and should be the smallest area of land that it is necessary to evaluate and classify.

ii. LAND USE: A major kind of land use is a subdivision of rural land use. Examples are extensive agriculture, intensive agriculture, forestry, recreation, etc. A land utilization type (LUT), is a subdivision of a major kind of land use describing the cropping, irrigation and management (i.e. farming system), the social and economic context, and other relevant details given in Table 11.

Simplified examples of land utilization types (LUTs)

LUT A: Sprinkler irrigated maize on State farms;
LUT B: Double cropped rice on surface irrigated small farms;
LUT C: Spate irrigated sorghum on tenanted small farms.

Guidelines for the description of LUTs are given in Chapter 4.

iii. LAND SUITABILITY: The FAO Framework land suitability categories are Orders (Suitable or Not Suitable), Classes, and Subclasses, as shown in Table 1. The land suitability classes are Highly Suitable (S1), Moderately Suitable (S2), Marginally Suitable (S3), Marginally Not Suitable (N1) and Permanently Not Suitable (N2). A greater or smaller number of Classes can be used as required. A lower case letter is used to designate Subclass, indicating the reason for downgrading the land from S1 (no Subclasses) to a lower class (symbols given in Table 17).

Example: Land Unit 1 is Highly Suitable (S1) for single cropped rice (LUT D) but is Marginally Not Suitable (N1z) for double cropped rice (LUT B), in a classification for potential suitability.

The FAO Framework distinguishes between classifications for the present suitability of land for a specific use (under existing conditions) and the potential suitability after specified major land improvements (e.g. the rehabilitation of degraded land, irrigation, drainage, etc.).

Two further subdivisions of potential suitability are introduced in this bulletin. These are comparable, but not identical, to the US Bureau of Reclamation's 'arable' land and 'irrigable' land (see Chapter 10). They are defined as follows:

Provisionally-irrigable land: This is land that is classified provisionally, on the assumption that water can be supplied to it, but in the absence of full knowledge about the water supply or the project and land development costs. (Net farm income is a useful measure of the suitability of 'provisionally-irrigable' land, see Section 2.4).

Irrigable land: This is land that is Suitable for irrigation under a 'provisionally-irrigable' classification, that can receive water, and that has been classified according to an economic evaluation of its suitability for specified LUTs, taking into account the water supply, the incremental area-specific development costs, common project costs, and benefits. (Net irrigation incremental benefit or NIIB, may be used to measure the suitability of 'irrigable' land).

One of the merits of the 'provisionally-irrigable' classification is that it contributes to the formulation of the project plan early during the study by eliminating land that is permanently not suitable for reasons other than the water supply. This classification also provides a good basis for updating an evaluation in the event of large price shifts or other major changes at a future date. However, only an 'irrigable' classification meets the needs of a feasibility grade study.

iv. CLASS-DETERMINING FACTORS: These are variables that affect the performance of a land utilization type (LUT) on a land unit and which serve as a basis for classifying the suitability of land for a given use. Many factors affect the performance of a LUT on a given land unit; in suitability classification some are 'class-determining' and others are not. Some factors affect a crop, its irrigation and management, rather uniformily across all land units in the study area, or cause unimportant variations. In contrast, 'class-determining' factors lead to major differences in physical land productivity or benefits and costs on different land units for a given LUT. In land evaluation, only the most important factors (i.e. those that are prospective class-determining ones) need be assessed and these can usually be progressively short-listed or aggregated into estimates of yields, benefits or costs to evaluate the land suitability classes.

Individual factors, that may or may not be selected as 'class-determining' in any given evaluation, can be grouped according to how they affect:

a. Crop yields or production (i.e. agronomic factors)
b. Management
c. Land development or land improvement
d. Conservation and the environment
e. Social and economic conditions

These five broad groupings are subdivided to give a list of 32 factors that may be 'class-determining', in Table 12, Chapter 4. Each factor, and their interactions are discussed in detail in Part Two of this bulletin.

Many factors may be represented in terms of the requirements or limitations of the cropping, management and irrigation systems, and are influenced, for better or worse, by land characteristics. Some land use requirements and limitations, such as fertilizer requirements or drainage requirements are represented as inputs or land improvements, rather than as land characteristics. Thus, land characteristics and land qualities are attributes of the land. Land use requirements and limitations (more strictly, requirements and limitations of the LUT) are attributes of the land use. Many social and economic factors and some economic considerations (e.g. farmers' attitudes) are neither attributes of the land use nor the land, but none-the-less may be 'class-determining'.

v. LAND USE REQUIREMENTS AND LIMITATIONS: These are factors that may or may not be 'class-determining' and that are required for, or limit, the performance of a LUT on a land unit (Subsection 4.2.2, Table 12). Land characteristics, inputs and land improvements interact to satisfy or influence the requirement or limitation.

Examples: Water requirements of a crop, land levelling requirement of an irrigation method, erosion limitation (see list in Table 12).

vi. LAND CHARACTERISTICS: These are any measurable features of land that can be used to characterize a land unit.

Examples: Mean annual rainfall, slope class, soil texture, etc.

vii. LAND QUALITIES: These are descriptors of land in relation to land use. For example, water availability or water deficiency imply a relationship between water supply and water requirement but as an attribute of the land. Land qualities represent complex hierarchical interactions ranging from water availability, nutrient availability, to crop yielding ability, drainability, erodibility, etc. In general, land qualities are the interactions affecting the performance of a LUT.

Note that land qualities are land conditions that affect the LUT, whereas corresponding land use requirements and limitations are the conditions, inputs and land improvements required for, or limiting the cropping, irrigation or management system.

viii. SPECIFICATIONS OF THE LUT: The conditions required for the satisfactory performance of a cropping, irrigation and management system should be specified prior to land survey. These specifications comprise critical ranges, bounded by critical limits, and specify for individual class-determining factors, the land use requirements and limitations at different levels of suitability.

ix. CRITICAL LIMITS: The critical limits of a class-determining factor mark boundaries between s1, s2, s3, n1 and n2 levels of suitability for individual factors, or a single group of factors, in the evaluation of the suitability of a land unit for a LUT. They can be set using the guidelines given in Part Two of this bulletin.

x. FACTOR RATINGS: Factor ratings, namely, s1, s2, s3, n1 and n2, indicate, in terms of a single factor, or a single interaction of a group of factors, whether the land is highly suited, moderately suited, marginally suited, marginally not suited, or permanently not suited, respectively, for a given LUT. (Note that lower case letters are used to avoid confusion with land suitability classes mentioned in iii.)

xi. MATCHING: This term is used in two senses. In its broadest sense 'matching' is the adjustment of the land by inputs and improvements to match the requirements and limitations of the LUT. Conversely, it is the adjustment of the LUT to match more closely the conditions of the land.

In a more restricted sense, matching is the comparison between the land conditions (land qualities or land characteristics), and the requirements or limitations of the LUT, to give factor ratings. Thus, factor by factor, each class-determining factor is given a factor rating, s1, s2, s3, n1 and n2.

Example: Assume critical limits corresponding to s3 for the factor 'Rooting' are a soil depth range limited by 50-100 cm under LUT A. Land unit 1 has a soil depth of, say, 75 cm. This falls within these critical limits, therefore, the factor rating for land unit 1 with LUT A = s3 (see Chapter 6, Example 1).

xii. FACTOR SIGNIFICANCE: This indicates whether an individual factor, or interaction, is Very Important (VI), Moderately Important (MI), Less Important (LI), or Not Important (NI) in deciding the land suitability class from factor ratings (see Section 6.3).

xiii. MEASURES OF SUITABILITY: The land suitability classes can be defined in terms of various physical, financial or economic indicators. A land productivity index based on relative yields is described as an example of a physical measure of land class in Section 2.4. Net farm income, and net incremental irrigation benefit are also defined in Section 2.4 and may be used as economic measures of land suitability class.

2.3 Main procedures in land evaluation and classification

2.3.1 Need for preliminary studies
2.3.2 Identification of relevant land utilization types
2.3.3 Inventory of land resources
2.3.4 Selection of class-determining factors
2.3.5 Classification of 'provisionally-irrigable' land and 'irrigable' land

The main procedures for evaluating and classifying land should comprise:

i. The study of relevant existing information and, wherever practicable, field appraisals of land conditions and experiences in a fully developed area having physical, climatic and socio-economic conditions similar to the area under investigation.

ii. The selection of cropping, irrigation and management alternatives and the description of prospective land utilization types (LUTs) for evaluation (Chapter 4).

iii. The selection of types of data required for the evaluation and the preparation of a land resource inventory (Chapter 5).

iv. The selection of class-determining factors having significance from a physical and economic standpoint, and the specification of critical limits to designate factor ratings and land suitability categories (see Step-by-Step Guide, Chapter 3, also Chapter 6, and Part Two).

v. The classification and mapping of 'provisionally-irrigable' land (see Sections 2.2 and 2.3.5, also Chapters 6 and 7).

vi. Modification of the 'provisionally-irrigable' classification as additional pertinent physical, engineering, hydrologic and economic information is obtained assisted where necessary by updated class-determining factors and critical limits.

vii. The classification and mapping of the 'irrigable' land delineating the location of the specific lands found to be suitable for irrigation development under a project plan.

Some of the above aspects are now discussed.

2.3.1 Need for preliminary studies

Wherever possible, preliminary studies should be undertaken to reduce the guesswork in land evaluation investigations. The most reliable guide to crop performance and future management problems is often experience in a locality with similar physical, climatic and socio-economic conditions to those of the area to be developed. If there is little or no previous experience of the proposed cropping or irrigation practices in the locality, two approaches are possible: first, comparative studies, recognizing relevant similarities and differences on a worldwide basis (Higgins and Kassam 1981), and second, a programme of agronomic work on representative land units, together with soil, topographic and drainage studies including laboratory tests and analyses. Experimental work started early in an irrigation development programme often proves of value during the later operation and management of the irrigation project, as well as during the later stages of land evaluation.

2.3.2 Identification of relevant land utilization types

In reconnaissance and project preparation studies there may be many land use alternatives. Major kinds of land (e.g. extensive agriculture, intensive agriculture) may be later subdivided to identify LUTs, first at a general level and later, during semi-detailed survey, in more detail. A checklist to assist in the description of LUTs is given in Chapter 4, Table 10. In some evaluations the choice of LUTs, that is of cropping, irrigation and management systems, is a major product of the study. In other evaluations the LUTs may be obvious from the outset (e.g. irrigated rice, irrigated sugarcane, irrigated tree crops), or only irrigation methods may have to be decided (e.g. surface, sprinkler or drip).

2.3.3 Inventory of land resources

Checklists are given in Chapter 5 of data necessary; for a general characterization of the project area including inventories from surveys of topography, soils, climate, water resources, drainage, vegetation and fauna, present land use, and socio-economic conditions. Guidelines on how to carry out the inventory are provided in other publications (see references in the appropriate sections).

Three sets of information are obtained from surveys of land resources:

i. definitions and descriptions of land units;
ii. maps showing the distribution of these land units;
iii. values of land characteristics of the land units.

The land units may be soil series or phases on specified slope categories in detailed surveys or land systems and land facets in less detailed surveys. They are described in terms of climate, relief, soils, vegetation and present land use. The results of water resources studies at national, basin, project or village, farm or field level (see Table 2) may include data on hydrology, hydrogeology, and irrigation water supply or requirement. These data will be progressively refined and developed during the course of the evaluation.

2.3.4 Selection of class-determining factors

The agronomic, management, land development, conservation and socio-economic factors that may affect the production or cost of crop production of LUTs on the land units, may be selected from the list given in Table 12, Chapter 4. In the early stages of evaluation, it is usual to consider a large number of factors, but during the study it will be apparent that many factors affecting the cropping, irrigation and management systems, do so relatively uniformly or without important variations across all the land units under study. Other factors will be short-listed because they may be 'class-determining' and their critical limits for factor ratings in the matching of a land unit in terms of its suitability for a LUT will need to be decided (see Chapters 3, 6 and Part Two).

2.3.5 Classification of 'provisionally-irrigable' land and 'irrigable' land

These two types of classification have been defined in Section 2.2 (iii). Separate classifications, first of 'provisionally-irrigable' land and later of 'irrigable' land may be required at successive stages of the evaluation. In the early stages of irrigation investigations, the amount of water available for irrigation and the exact locations to which water can be economically transported are often uncertain. The suitability of the land must therefore be classified on condition that water can be supplied to it. Only later, when the studies of water supply systems and economics are completed will it be certain whether the land can be irrigated or not. In this publication, land for which the supply is not yet assured and for which land development costs and benefits are unclear, is classified as 'provisionally-irrigable', and land that could be supplied with water under a project plan is 'irrigable'. The appropriate measures of suitability for each classification are explained in Section 2.4 and Chapter 7.

The above distinction between 'provisionally-irrigable' and 'irrigable' are similar to the USBR's use of 'arable' and 'irrigable' land (Chapter 10 and Glossary). However, it should be noted that the USBR does not normally use the water supply as a class-determining factor (except for its quality). The FAO Framework includes water supply (water quantity, quality, and seasonality) among the resources of the land, and the land evaluator may or may not choose to classify land suitability according to whether the volume of water is sufficient, the period of year during which it is available, and the cost of conveying water to the different tracts of land.

One of the main advantages of the 'provisionally-irrigable' classification is that it contributes, early in the study, to the formulation of the project plan, and to the separation of land that is suitable for irrigation, from land that is not. The delineation of an 'irrigable' area from land already classified as Suitable under a 'provisionally-irrigable' classification leads to reductions in land area under the plan of development. Typical adjustments include:

i. elimination of uneconomic increments of land such as those that are too costly to serve, drain or provide with distribution works;

ii. adjustment of land areas to the available water supply;

iii. elimination of tracts of land located above water surface delivery elevations, or elevations which cannot be feasibly irrigated by lift;

iv. exclusion of isolated segments, odd-shaped tracts, and severed areas that cannot be efficiently fitted into a farm unit;

v. deletion of proposed public rights-of-way;

vi. elimination of areas unable to meet minimal criteria for economic returns under the plan;

vii. elimination of land for socio-economic reasons including land tenure, water rights, etc.

In establishing land suitability classes, the physical and economic standard defining the lowest quality land that can be considered as Suitable under a 'provisionally-irrigable' classification should be specified as early as possible in the investigations. This boundary (or cut-off) between Suitable and Not Suitable is progressively revised eliminating marginal lands as new data on water supply and project investment costs become available until the 'irrigable' lands are defined.

2.4 Land productivity index and economic measures of suitability

Land suitability classes (i.e. S1, S2, S3, N1, N2) can be defined by various physical or economic indices or measures of suitability. Three convenient measures of suitability which lend themselves to progressive application as data become more readily available are suggested below: i) land productivity index, ii) net farm income, and iii) net incremental irrigation benefit.

i. Land Productivity Index: In this bulletin, this is defined as the physical productivity of land for a specific land use, relative to that of the best land. Relative yield can be a convenient land productivity index. This is the yield per hectare relative to that of the best land as a percentage or fraction. Thus the top yields of Class S1 land for a given LUT may be taken as 100% or 1.0, the top of s2 as a fraction of S1 (e.g. 80% or 0.8), S3 as 0.6, etc. as appropriate. Other standards such as absolute yields or relative production can be used as alternative measures of physical productivity. Productivity may be for a present or potential suitability classification. It would normally be necessary to use a physical land productivity index in reconnaissance studies and as a necessary preliminary to economic evaluation.

ii. Net farm income: This is a convenient measure of land suitability class for 'provisionally-irrigable' land. Net farm income is defined as the value obtained by subtracting both the variable and fixed costs from the gross value of production. At the 'provisionally-irrigable' stage of the investigations the common project costs are generally not known and are not taken into account in estimating net farm income. Net farm income may be calculated for 'without project' and 'with project' situations. At the 'provisionally-irrigable' stage it will often suffice to base the classification on the 'with project' net farm income. However, this lacks the refinement necessary for detailed project planning and analysis and 'provisionally-irrigable' land may include marginally suitable land that can be eliminated by further project economic analysis and land classification mapping during the final classification of 'irrigable' land. To ensure maximum consistency with the final classification it is convenient to express net farm income in 'economic' rather than 'financial' terms (see Chapter 7).

iii. Net Incremental Irrigation Benefit (NIIB): This is the standard required for measuring the suitability of 'irrigable' land in the final classification. NIIB is a measure of the potential increase in productivity of a unit area of land when developed under a project plan, expressed in economic terms, such as an Annual Equivalent Value. It is derived by estimating the difference in net benefit accruing to a unit area of land under 'with' and 'without' project situations taking into account:

a. farm investment and operating costs, and returns ordinarily accruing from the agricultural use of the land;

b. all project investment, operating and maintenance costs (including a share of common project costs and area-specific land development costs, whether or not these are paid for by the farmer).

Further explanatory details are given in Chapter 7.

Land evaluation using the above three measures of suitability becomes progressively refined and more detailed as the investigations move from reconnaissance through to the proposing of a specific plan of development. However, it is important to understand that the classes into which given land unit-LUT combinations fall, may differ abruptly according to the index used. The land productivity index does not take into account prices or costs of production, and net farm income does not take account of the increase or increment in moving from a 'without' to a 'with' project situation. Generally there will be a progressive elimination of the marginally suitable lands as the investigations intensify until the 'irrigable' lands are delineated. Examples of the implications of the above are given as follows.

Consider, for example, two LUTs A & B based respectively on a high and low value crop, e.g. vegetables and grain. In terms of a land productivity index a land unit may, say, be classed S1 for both crops. The S1 for LUT A may indicate that the vegetables would yield 20 t/ha on this land unit. The S1 for LUT B may indicate that the grain would yield 3 t/ha. (It is also conceivable that this land unit with a third LUT C, e.g. irrigated rice, might fall into class N1; illustrating incidentally, that the land suitability class must relate to a specified land use.)

If monetary values are introduced in the above example, there is a common basis for comparing the LUTs on a land unit, one with another. The high value vegetables (LUT A) might, say, remain classified as S1 using net farm income. The low value grain crop (LUT B) might fall into a lower class, say, N1 on the basis of net farm income, even though the grain crop is S1 in physical terms on this particular land unit. Such consequences must be anticipated where net farm income is used as a standard instead of physical productivity.

In classifying 'irrigable' land, the existing agriculture, the project costs (common project costs and area-specific land development costs) must be taken into account to arrive at NIIB as defined in paragraph iii. If the land unit already produces, say, vegetables without irrigation, it may have an existing high net farm income per hectare for the 'without' project situation. Thus, although LUT A based on irrigated vegetables may be S1 on both the above two criteria (a land productivity index and net farm income), the increment in net farm income from investment in irrigation may be small and the additional project costs may not be worthwhile. The land may be downgraded using NIIB, for example, to S3, or even to N1. The land unit for LUT A (irrigated vegetables) may be highly suitable in physical terms or in terms of net farm income, but the proposed development is not viable because of the limited or negligible incremental net benefit.

The land productivity and economic measures of suitability described above are usually expressed as production or value per hectare. However, it is also possible to evaluate the suitability of land on the basis of a value per volume of water. This may be appropriate where the water supply systems are proposed as additional in a rehabilitation project. Rather than expressing NIIB as a value per hectare it could be expressed as a value per cubic meter of water delivered to each hectare. (In many places water is priced volumetrically.) In using the value per volume of water method of attributing benefits, the land can be classified as if it were receiving a full supply of irrigation water, with the land classification expressing values equivalent to a full water supply project.

Further details of the economic evaluation of land suitability are given in Chapter 7. Economic considerations need to be introduced from the very outset of the land suitability classification with increasing degrees of refinement in successive iterations as suggested above.

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