FAO IRRIGATION AND DRAINAGE PAPER 24

1 FAO IRRIGATION AND DRAINAGE PAPER 24. revi sed 1977. at uirements FOOD AND AGRICULTURE ORGANIZATION. OF THE UNITED NATIONS ROME. FAO IRRIGATION AND DRAINAGE PAPER (2L., revised 1977. clu Mims for [preck'dng i:e[r [aqukramernm by j. d renbos water management specialist land and water development division fao and W. O. PrErfg" fao coreamN:anr1. isTigadzqi isrdivevzKy oaNcinTje, davis, california, , in consultation a. thcautthEieE: gsbar-dc<, ciarivagnez dastane 7,D(AF: . c. van den be.,v2. Lii:artna ashINDscl ( ,-11.)))]]]

2 M. Va-16. and fao field s FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS. Rome 1977. SUMMARY. This publication is intended to provide guidance in determining crop water requirements and their application in planning, design and operation of IRRIGATION projects. Part 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 evapo- transpiration for different climatic conditions.

3 These methods are the Criddle, the 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 (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 ; this includes local variation in climate, advection, soil water availability and agronomic and IRRIGATION methods and practices.

4 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 irriga- tionproject planning, design and Part II. 1 deals deriving the field water balance, which in turn forms the basis for predicting and peak IRRIGATION supplies for general planning purposes.

5 Attention is given to IRRIGATION efficiency and water requirements for cultural practices and leaching of salts. In Part 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 operating the canal system using different methods of water delivery, and for subsequent design parameters of the system. Suggestions are made in Part on refinement of field and project supply schedules once the project is in operation.

6 The presented guidelines are based on measured data and experience obtained covering a wide range of conditions. Local practical, technical, social econo- mic considerations will, however, affect the planning criteria selected. Therefore caution and a critical attitude should still be taken when the presented methodology. TABLE OF CONTENTS. Summary Table of Contents List of Tables List of Figures Conversion Factors Climatological Nometiclature PART I - CALCULATION OF CROP WATER REQUIREMENTS. CALCULATION OF REFERENCE CROP EVAPOTRANSPIRATION 3.

7 Blaney-Criddle Method 3. Radiation Method 8. Penman Method 15. Pan Evaporation Method 30. SELECTION OF CROP COEFFICIENT 35. FACTORS AFFECTING CROP EVAPOTRANSPIRATION 55. Climate: variation with time, distance, size of IRRIGATION development, advection, altitude 55. Soil Water: level of . soil water, groundwater, salinity, water and crop yields 59. Method of IRRIGATION : surface, sprinkler, drip or trickle, sub- surface IRRIGATION 62. Cultural Practices: fertilizers, plant population, tillage, mulching, windbreaks, anti-transpirants 65.

8 PART II - APPLICATION OF CROP WATER REQUIREMENT DATA IN IDENTI- FICATION , DESIGN AND OPERATION OF IRRIGATION PROIECTS 67. PROJECT IDENTIFICATION AND PRELIMINARY PLANNING 68. Introduction 68. Seasonal and Peak Project Supply Requirements 69. Crop water requirements 71. Net IRRIGATION requirements: effective rainfall, groundwater contribution, stored .soil water 72. IRRIGATION requirements: leaching requirements, IRRIGATION efficiency 76. Summary of calculation of seasonal and peak project supply requirements 81.

9 PROJECT DESIGN 83. introduction 83. Field and Project Supply Schedules 83. Field IRRIGATION schedules: depth of IRRIGATION application, IRRIGATION application interval, calculation of field IRRIGATION schedules 86. Field IRRIGATION supply schedules: surface, sprinkler drip 90. Design and operation of supply system: continuous, rotational, supply on demand 96. Summary calculation of project design and operation 100. PROJECT OPERATION 102. Refinement of Field Supply Schedules: adaptive research, data collection, project monitoring 102.

10 Application of Field IRRIGATION Data 104. Appendix I Persons and Institutes Consulted 107. Appendix II Background and Development of Methods to Predict Reference Crop Evapotranspiration 108. Appendix III Computer Programme for Estimation of Reference Crop Evapotranspiration 120. Appendix IV Glossary 133. Appendix V References 137. Appendix VI Constants for the Radiation Equation Rs = (a + b njol)Ra 144. LIST OF TABLES. Page I. Mean daily percentage (p) of annual daytime hours for different latitudes 6. Extra terrestrial radiation (Ra) expressed in equivalent evaporation in mm/day 12.