WORLD METEOROLOGICAL ORGANIZATION LECTURE NOTES FOR TRAINING AGRICULTURAL METEOROLOGICAL PERSONNEL Prepared by Jon WIERINGA Jakob LOMAS SECOND EDITION WMO-No. 551 Secretariat of the World Meteorological Organization Geneva - Switzerland 2001 © 200 l , World Meteordogical Organization ISBN 978-92-63- 10551-6 NOTE The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological Organization concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitotion of its frontiers or boundaries. TABLE OF CONTENTS FOREWORD . vii PREFACE.......................................................... ix CHAPTER 1: AGRICULTURAL METEOROLOGY - ITS SCOPE AND AIMS . 1 1.1 Aims . 1 1.2 Range of subject matter . 1 1.2.1 Soil and water . 1 1.2.2 Plants and crop microclimate . 2 1.2.3 Farm animals (farm livestock) . 2 1.2.4 Diseases and pests of crops and animals . _2 1.2.5 Farm buildings, equipment and operations . 3 · 1.2.6 Artificial modification of meteorological regimes . 3 1.2.7 Climate change . 3 1.3 Use and provision of agrometeorological information . 4 Review questions on Chapter 1 . 6 CHAPTER 2: RADIATION AND THE SURFACE ENERGY BALANCE . 7 2.1 Solar energy ('short-wave' energy) from Sun and sky . 7 2.1.1 Direct and diffuse components of solar short-wave radiation . 7 2.1.2 Estimation of global radiation on a horizontal surface . 9 2.1.3 Emission and reflection of radiation . 10 2.1.4 Energy in the visible spectrum - light . 11 2.2 The energy balance and its components . 12 2.2.1 The long-wave budget . 12 2.2.2 Surface radiation temperatures . 14 2.2.3 Total radiation budget and complete surface energy balance . 14 2.3 Special aspects of radiation and temperature in agriculture . 16 Review questions on Chapter 2 . 18 CHAPTER 3: THE SOIL AND ITS HEAT BALANCE.......................... 21 3.1 What is 'soil'? . 21 3.2 Transmission of heat in the soil . 21 3.3 Soil freezing, and the role of snow cover . 23 3.4 Diurnal and annual variations of soil temperature and moisture. 25 3.5 A model of soil temperature diurnal course at different depths . 27 Review questions on Chapter 3 . 28 CHAPTER 4: WATER AND THE HYDROLOGICAL CYCLE IN AGRICULTURE . 31 4.1 Water and vegetation . 31 4.2 Moisture characteristics of soils . 33 4.3 Determination of water loss from land surfaces . 35 4.3.1 Fundamentals of the evaporation process . 35 4.3.2 Existing methods to determine evaporation . 37 4.3.3 Energy balance estimation of evaporation . 38 4.3.4 Aerodynamic estimation of evaporation . 40 4.4 'Combination' methods of Penman and others . 42 4.4.1 Development of the original Penman equation . 42 4.4.2 Evaporation formulae of Priestley-Taylor and Penman-Monteith . 44 4.5 Special forms of precipitation . 46 4.5.l Dew . 46 4.5.2 Snow . 46 4.6 Soil moisture budgets - irrigation need . 47 Review questions on Chapter 4 . 49 iii WMO LECTURE NOTES - AGRICULTURAL METEOROLOGY CHAPTER 5: SMALL-SCALE CLIMATE, REPRESENTATIVITY, AND THEIR DEPENDENCE ON TOPOGRAPHY . 53 5.1 Micro-, topo- and meso-climatology . 53 5.2 Observation representativity, exposure and sampling . 55 5.3 Wind behaviour in common inhomogeneous terrain . 57 5.3.1 Wind around barriers of varying porosity . 57 5.3.2 Wind reduction by shelterbelt arrays . 60 5.3.3 Wind representativity at toposcale . 63 5.4 Toposcale representativity of meteorological observations . 66 5.5 Topoclimatologi(al effects a'rising from landscape variations . 70 5.5.1 Effects of slopes on incoming solar radiation . 70 5.5.2 Soil temperatures on slopes . 71 5.5.3 Effects of slopes and hills. on airflow . 72 5.5.4 Local mesoscale circulations . 73 Review questions on Chapter 5 . 74 Chapter 6: AGROMETEOROLOGICAL MANAGEMENT AT MICROSCALE AND TOPOSCALE ... 79 6.1 Introduction . 79 6.2 Soil cultivation and·treatment . 79 .6.2.1 Effects of surface-colouron soil temperature . -80 ·6.2.2 Mulching ._. ... ... ... •8.1 6.2.3 Surface geometry effects on temperatures . ·83 ,6.3 ·, Crop management and layout . 84 ~6.3.l Spacing of.crop rows . .84 6.3.2 Shading,-cover crops and weeding . 86 6.4 r-Wind shelter . -88 6.4.1 Effect of wind-shelter on microclimate . 88 6.4.2 Windbreaks.against damage and erosion . ·89 6.5 Jrrigation and drainage . 91 6;6 Frost, and protectioffagainst frost damage . -93 6.6.1 Passive methods of protection against frost . 93 6.6.2 Active methods of .protection against frost . 95 6.6.3 Short-term frost forecasting . 97 6. 7 Artificial climate in _glasshouses and stables_. 98 ·Review questions on Chapter 6 . 102 ·.CHAPTER 7: WEATHER·HJ\ZARDS.ADVERSELY AFFECTING AGRICULTURAL O.UTPUT ... .......... ... : . .. ... .107 .7. -1 Drought . .... ..... .. .. .. ... 107 7.2 Artificial stimulation-of.precipitation . ... 110 ·7: 3 ·ttail ............• ..,. 112 7.3.1 Distribution- of hail in space and time . 112 7.3.2 Active suppression of hail . 114 7.4 Fire in vegetation . 115 7.5 Atmospheric.transports . 116 7.5.1 Elementary aspects of transport over mesoscale distances . 116 7.5.2 Point sources . 117 _ 7.5.3 Line sources, e.g. sea-salt tran~port . 119 ·Review questions on Chapter 7 . ... : .. ....... .. .. 120 CHAPTER"S: OPERATIONAL AGROMETEOROLOGY . 123 8.1 Alternative forms of agrometeorological decision-supporting activity . 123 8.2 Operational modelling for tactica_l agrometeorology . 126 8.3 Protectidn of crops against pes~s and diseases . 128 8.4 Agroclimatological surveys ....· . ...... ·. 130 iv TABLE OF CONTENTS 8.4.1 Agroclimatology of the Sahel - an example of presentation . 131 8.4.2 Irrigation need - a climatological case study . 132 8.4.3 Agroclimatological analysis of a rainfed semi-arid situation . 134 8.5 Computer weather modelling for agriculture.