1. Introduction

1.1 General
1.2 From project identification to project implementation
1.3 Levels of intensity of investigations
1.4 Planning a land evaluation investigation

1.1 General

A fuller use of land and water resources by the development of irrigation facilities could lead to substantial increases in food production in many parts of the world. The process whereby the suitability of land for specific uses such as irrigated agriculture is assessed is called land evaluation.

Land evaluation provides information and recommendations for deciding 'Which crops to grow where' and related questions. Land evaluation is the selection of suitable land, and suitable cropping, irrigation and management alternatives that are physically and financially practicable and economically viable. The main product of land evaluation investigations is a land classification that indicates the suitability of various kinds of land for specific land uses, usually depicted on maps with accompanying reports.

The evaluation and suitability classification system described in this bulletin is based on 'A Framework for Land Evaluation' (FAO 1976a). The structure of the FAO Framework classification is given in Table 1 with details in Appendix 1.

Table 1 STRUCTURE OF THE SUITABILITY CLASSIFICATION

ORDER	CATEGORIES CLASS	SUBCLASS
S Suitable	S1
	S2	S2t
		S2d
		S2td
		etc.
	S3
N Not suitable	N1	N1y
		N1z
		etc
	N2

Legend:
S1 Highly Suitable
S2 Moderately Suitable
S3 Marginally Suitable
N1 Marginally Not Suitable
N2 Permanently Not Suitable

Lower case letters in a Subclass indicate the nature of a requirement of limitation (e.g. t and d for topography and drainage). See list of Subclass symbols in Table 17. Land suitability units (subdivisions of Subclasses) may also be used to indicate minor differences in management.

1.2 From project identification to project implementation

In the early stages of land resources investigations, land evaluation studies indicate in a preliminary way, the suitability of land for alternative crops and irrigation methods and the land improvements that may be worthwhile. With further field studies, projects are identified and a plan of irrigation development is worked out. Individual projects are ranked in order of priority. The priority projects are planned in more detail and each project plan is progressively refined. The proposed crops, methods of irrigation, inputs and land improvements are progressively adjusted until a satisfactory project plan is produced.

Various criteria are used to decide whether a project plan is satisfactory. Apart from social and political objectives, which in practice are often paramount, a satisfactory plan is one that leaves the farmers, the community and the national economy better off. In other words, it results in the largest practicable increment in net benefits in an economic comparison of 'without project' and 'with project' situations. Such a plan will generally utilize limited resources of land, water or inputs for the most productive use. A satisfactory plan is one which is practicable and likely to work out under actual field conditions, not necessarily the most economically attractive on paper.

Land evaluation reports, maps and data continue to be useful after the planning stage during design and implementation, and for monitoring the project.' The detailed design of engineering works may depend on information collected earlier during the evaluation study. During the implementation and later management of the irrigation project, the land evaluation study provides a basis for monitoring changes in physical, social and economic conditions. In response to such changes, the recommendations may need modification and updating from time to time.

Currently, the rehabilitation of existing irrigation projects is an important aspect of land evaluation work. This highlights the need for thorough evaluation of land and water resources in the preparation of irrigation projects from the start; obviating the need for later rehabilitation.

1.3 Levels of intensity of investigations

The study of land and water resources and the production of irrigation development proposals may be conducted at national level, at the level of a river basin or hydrogeological basin, at project development level, or at village, farm or field level (Horning 1979). The types of studies undertaken at these different levels are indicated in Table 2.

Table 2 LEVELS OF INTENSITY OF INVESTIGATIONS

LEVELS	TYPES OF STUDY	TYPES OF SURVEY
National Basin	Project identification	Reconnaissance
Project	Pre-feasibility	Semi-detailed
	Feasibility	Detailed
Village, farm or field	Detailed design	Very detailed

Project identification at a national and basin level leads to a need for prefeasibility and feasibility studies. These are followed by detailed design studies of water supply systems and field layouts. These various studies are served by different scales of survey.

At the national level, investigations are required to provide a Master Plan for land and water resources development including an assessment of the priorities accorded to respective regions and areas within the country. At the level of individual river basins or hydrogeological basins, investigations provide the basis for water development, water control for different uses and for land use planning (e.g. catchment projection, flood zoning, potential areas for irrigation, reclamation of delta and swamp and tidal zones, etc.). At the irrigation development project level, a plan is- formulated for investment in irrigation, drainage and flood protection. At the village, farm or field level, investigations provide information for farm water management and improvements or rehabilitation.

Reconnaissance surveys on a small scale i.e. 1:100 000 to 1:250 000 (Table 3) are useful for broad resource inventory (see Chapter 5), the identification of promising areas for development, and to provide a basis for more detailed study. Mapping units are usually compound and provide only estimates of the proportions of the conditions for the various land suitability categories. The 'land system' method of survey (Christian and Stewart 1968) is often used and it may suffice to broadly distinguish land which is promising for specific kinds of irrigated agriculture from land which is not. Economic studies at this, stage broadly indicate levels of production and income.

Semi-detailed surveys in pre-feasibility and feasibility studies are typically at scales from 1:25 000 to 1:50 000. Soil mapping units consist of a mixture of homogenous units (soil series) and compound units (e.g. soil associations). With sufficiently intense sampling, such surveys can be used for planning some developments up to the design stage.

Table 3 SURVEY INTENSITY, MAPPING SCALE AND KIND OF MAPS

KIND OF SURVEY	RANGE OF SCALES
Very High Intensity (very detailed)	Larger than 1:10 000	Soil maps showing special features or phases of soil series and occasionally soil complexes; detailed topographic maps with spot heights; cadastral maps; groundwater maps; present crops and vegetation etc.
High Intensity (detailed)	1:10 000 to 1:25 000	Soil maps showing phases of soil series and soil complexes; detailed topographic maps, groundwater maps, present land use, etc.
Medium Intensity (semi-detailed)	1:25 000 to 1:100 000	Soil maps showing series or associations of series; land system maps, physiographic units, topographic contour maps, present land use maps, etc.
Low Intensity (reconnaissance)	1:100 000 to 1:250 000	Soil maps with associations and phases of Great Groups or Subgroups; land system maps, physiographic or contour maps, present land use, climatic zones, etc.
Exploratory	1:250 000 to 1:1 000 000	Land units of various kinds.
Syntheses	Smaller than 1:1 000 000	Climatic maps, soil taxonomic maps, vegetation and land use, physiographic and geomorphological maps, agro-ecological zones, etc.

Detailed surveys may be required separately for soils and topography. Soil surveys, typically at scales of 1:10 000 to 1:25 000, with soils series and phases as the main soil mapping units, are used for project planning and implementation and for some surveys at village or catchment level, including layout of farms and irrigation systems. If topography is an important consideration in delineating land to be brought under command by gravity irrigation, a more intensive survey (e.g. at 1:5 000) may be required for land levelling and engineering applications.

Very detailed surveys, at scales of 1:5 000 or larger, are necessary where small contour intervals must be mapped in order to determine slope classes, or align irrigation and drainage channels.

1.4 Planning a land evaluation investigation

Land evaluation investigations may be carried out by a government department or private company with or without external help. Large irrigation projects often involve a client, a funding agency and a consultant organization. Prior to field work, initial discussions will take place to decide the objectives of the evaluation, and the data and assumptions on which it is to be based. The extent and boundaries of the area to be evaluated, and the kinds of land use or irrigation systems may either be prescribed in the terms of reference, or may be part of the evaluation. Appropriate physical or economic measures of suitability must be decided (see Section 2.4). The intensity and scales of the required surveys, and the phasing of activities should be agreed prior to the start of field work. The administrative, logistical and financial implications of the work being undertaken should also be agreed.

During initial discussions the requirements for reports and maps at various stages during the study should be decided. The regular production of progress reports and maps is a feature of all efficiently organized irrigation development investigations. These are essential as a basis for making major policy decisions at crucial stages of the study. It is also customary to produce interim reports in order to facilitate discussions and amendments before producing the final reports and maps.

This introductory chapter has indicated how the physical and economic aspects of land evaluation for irrigated agriculture become more detailed and refined as the land evaluation progresses from reconnaissance and project identification, to the detailed planning of irrigation projects. Chapter 2 describes the principles, terminology and outlines the FAO land evaluation procedures as applied to irrigated agriculture.