This bulletin is written for all concerned with land evaluation or irrigation planning, and is divided into two parts. Part One explains the recommended procedures of land evaluation and land suitability classification for irrigated agriculture and is based on the FAO Framework for Land Evaluation (1976). Part Two provides technical information on individual factors commonly of importance for the specification of critical limits in land suitability classification. The bulletin can be used as a general reference, for example, as a source of checklists, or more specifically, as a detailed guide to procedures and technical information.
The structure of the FAO Framework classification of land suitability is given in Table 1 of Part One and is described in detail in Appendix 1. Further details are given in FAO Soils Bulletin No. 32.
Chapter 1 of Part One outlines the role of land evaluation in the development of land and water resources for reconnaissance surveys through to the detailed design of irrigation projects. Chapter 2 explains the basic principles and terminology, and provides a summary of the procedures to evaluate and classify land for a specific use. The concepts of 'provisionally-irrigable' and 'irrigable' land at successive levels of detail, and the use of productivity and economic indices to evaluate land suitability classes are introduced in this chapter.
Chapter 3 provides a step-by-step guide to the procedures, accompanied by data record sheets (Formats) to assist land evaluators in the field. The individual steps are described in detail in subsequent chapters.
Chapter 4 explains how to select and describe relevant land utilization types (LUTs) or farming systems for evaluation. It then lists the factors that may affect a crop, its irrigation and management (i.e. land use requirements and limitations) and the land qualities or land characteristics, inputs and land improvements that should be evaluated.
Chapter 5 describes procedures for making a land resources inventory. The principal categories of data required are dealt with under eight headings: general characteristics of the project area, topography, soils, climate, water resources, drainage, vegetation and fauna, social and economic data.
Chapter 6 describes how land suitability for a specific use is classified by 'matching', and the assignment of land suitability classes to the land units.
Chapter 7 describes the economic evaluation of land suitability for irrigated agriculture. It explains economic criteria for separating land which is 'Suitable' from land which is 'Not Suitable' first In 'provisionally - irrigable' and later in 'irrigable' classifications. Procedures are set out for translating estimates of physical productivity into economic terms and for estimating ranges of permissible development costs. The chapter explains how to verify both the economic and financial viability of a project from the national and farmers' viewpoints.
Chapter 8 discusses the presentation of the results of land evaluation in land suitability maps and reports and Chapter 9 outlines the role of land suitability classification in the appraisal of irrigation development projects.
Chapter 10 gives a brief account of the USBR system of classifying land for irrigation with some comments on similarities and differences from that described in the foregoing chapters.
Part Two provides technical guidance on factors that are commonly used to determine land suitability class and on the setting of critical limits to help in the choice of land suitability classes. The discussion focuses on the individual factors and their interactions that may affect crop yields or production, management, land development costs, conservation and the environment, and socio-economic conditions.
The reader should use these Guidelines selectively, as not all the factors listed will be relevant in a given evaluation. The procedure provides for a sifting out of considerations deserving special emphasis in order to avoid needless investigations and unnecessary expense. The evaluation procedure is an essential preliminary to project planning and should be conducted in a manner that will minimize costs, but at a level that is consistent with achieving practicable recommendations.