Land Evaluation for Irrigated Agriculture
The main goal of these guidelines is to provide a methodology for implementing in irrigated agricultural systems the key principles of the Framework for Land Evaluation (FLE): (1) matching the requirements of the land use against the qualities of the land, (2) comparing alternative sustainable uses, and (3) multidisciplinary approach. Similar guidelines have been prepared by FAO for rainfed agriculture, extensive grazing and forestry.
This bulletin is written for all concerned with land evaluation for irrigation planning, and is divided into two parts. Part One explains the recommended procedures of land evaluation and land suitability classification for irrigated agriculture. Part Two provides guidelines for developing the specifications and critical limits of class-determining factors, which include agronomic factors, management, land development and improvements, conservation and the environment, and socio-economic factors. 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 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) for evaluation, using a checklist of headings. It then lists the factors that may affect a crop, its irrigation and management (in the form of land use requirements or 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 the headings: general characteristics of the project area, topography, soils, climate, water resources, drainage, land use, vegetation and fauna, environmental health, social and economic data. Useful checklists are included for soils, climatic and socio-economic data to be compiled.
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. The matching process consists itself of different steps: (i) matching the class-determining factors individually, (ii) evaluating the interactions between individual factors, and (iii) evaluating the importance of each individual factor in its impact on land productivity or the benefit/cost balance.
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 for studies at reconnaissance, pre-feasibility and feasibility level.
Chapter 9 outlines the role of land suitability classification in the appraisal of irrigation development projects for the purposes of either feasibility appriasal or project implementation and monitoring.
Chapter 10 gives a brief account of the US Bureau of Reclamation (USBR) system of land classification for irrigated agriculture in terms of principles, terminology, financial and economic considerations, land classes and subclasses, map symbology, and additional classification specifications.
Part Two provides further technical guidance on developing the specifications and critical limits of factors that are commonly used to determine land suitability class. The discussion focuses on 32 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 is advised to 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.