Guidelines for predicting crop water requirements

Abstract

This publication is intended to provide guidance in determining crop water requirements and their application in planning, design and operation of irrigation projects. Part 1.1 presents suggested methods to derive crop water requirements. The use of four well-known methods for determining such requirements is defined to obtain reference crop evapotranspiration (ETo), which denotes the level of evapotranspiration for different climatic conditions. These methods are the Bla.ney- Criddle, t he Radiation, the Penman and Pan Evaporation methods, each requiring a different set of climatic data. To derive the evapotranspiration for a specific crop, relationships between crop evapotran.spiration (ETcrop) and reference crop evapotranspiration (ETo) are given in Part I . 2 for different crops , stages of growth, length of growing season and prevailing climatic conditions. The effect of local conditions on crop water requirements is given in Part 1.3; this includes local variat ion in climate, advection, soil water availability and agronomic and irrigation methods and practices . Calculation procedures are presented together with examples. A detailed discussion on selection and calibration of the preSented methodologies together with the data sources is given in Appendix II. A computer programme on applying the different methods is given in Appendix III. Part 11 discusses the application of crop water requirements data in irrigationproject planning, design and operation.. Part II. 1 deals v.rith deriving the field water balance, which in turn forms the basis for predicting season.al and peak irrigation supplies for general planning purposes. Attention is given to irrigation efficiency and water requirements for cultural practices and leaching of salts. In Part 11.2 methods are presented to arrive at field and scheme supply schedules with emphasis towards the field water balance and field irrigation management. Criteria are given for op erating the canal system using different methods of water delivery, and for subsequent design parameters of the system. Suggestions are made in Part 11.3 on refinement of field and project supply schedules once the project is in operation. The presented guidelines are based on measured data and experience obtained covering a wide range of conditions. Local practical, technical, social a.nd economic considerations will, however, affect the planning criteria selected. Therefore cauti on and a critical attitude should still be taken when applyin.g the presented methodology.