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Ref ET Manual REF-ET: REFERENCE EVAPOTRANSPIRATION CALCULATION SOFTWARE for FAO and ASCE Standardized Equations Version 2.0 for Windows for Windows 95, 98 and NT Copyright 1999, 2000 University of Idaho and Dr. Richard G. Allen TABLE OF CONTENTS LIST OF TABLES ............................................................................................................................... iv LIST OF FIGURES ............................................................................................................................. iv Reference Evapotranspiration Calculator ............................................................................................1 Reference ET Methods Calculated by REF-ET for Windows Version 2..............................................2 Computation Time Steps .....................................................................................................................2 REF-ET Installation and Computer System Requirements .................................................................3 Creating a Shortcut and Screen Icon for REF-ET ...............................................................................6 Disclaimer ............................................................................................................................................6 Distribution and Support Limitations ....................................................................................................6 Example Data Files..............................................................................................................................7 Algorithms Used in the REF-ET Program............................................................................................8 Reference ET Options .........................................................................................................................9 Equation Requirements .......................................................................................................................9 Actual Vapor Pressure of the Air...................................................................................................9 Parameter Equations Used in Various Penman Equations.........................................................10 Variations on the ASCE Penman-Monteith Equation in REF-ET.......................................................10 Intermediate Computation Files.........................................................................................................13 Definition File Editor...........................................................................................................................13 Program Operation ............................................................................................................................13 Opening the data file ...................................................................................................................14 Creating an ASCII data file from a spreadsheet ...................................................................15 The definition file of REF-ET .......................................................................................................16 Specifying where to Start and where to Stop reading a parameter.............................................18 Other Data File Reading Techniques..........................................................................................19 Data Parameter Definitions .........................................................................................................20 Second Definition File Editor Screen...........................................................................................22 Selecting the Reference Type, Output Options and ET Equations .............................................23 Selection of Reference Equations and Reference Type .............................................................25 Estimation of Missing Data..........................................................................................................25 Soil Heat Flux and Net Radiation Measurements .......................................................................27 Other Calculations.......................................................................................................................27 Estimation of the Timestep..........................................................................................................27 Saving the Definition File Data ....................................................................................................28 Processing of the Weather Data File ..........................................................................................29 Final Results ......................................................................................................................................30 RET-ET for Windows ver. 2.0, 2000 Page i Graphical Comparisons of Reference ET Estimates......................................................................... 31 ASCE.DAT Example Data File .......................................................................................................... 31 Examples of Intermediate File 2 and Final Results File.............................................................. 32 Example of Final Results ............................................................................................................ 35 Errata to Various Publications ........................................................................................................... 37 Errata to ASCE Manual 70 that should be noted if using it as a source of equations................. 37 Errata to FAO-56 that should be noted if using it as a source of equations................................ 37 References ........................................................................................................................................ 38 Bibliography ....................................................................................................................................... 40 Appendix 1 ASCE and FAO Penman-Monteith Equations and Kimberly Penman equation........... 41 Supporting Calculations for the ASCE and FAO Penman-Monteith Methods ............................ 42 Psychrometric and atmospheric parameters .............................................................................. 43 Net radiation (Rn )....................................................................................................................... 48 Soil Heat Flux Density (G)........................................................................................................... 49 Clear-sky solar radiation (Rso )................................................................................................... 50 The Standardized ASCE Penman-Monteith Equation(s) ............................................................ 51 The FAO-56 Penman-Monteith Equation.................................................................................... 53 Precautions when Comparing the FAO-56 and Standardized ASCE Penman-Monteith Equations to Measured Values............................................................................................. 54 The Penman Equation ................................................................................................................ 54 The Kimberly Penman Method ............................................................................................. 54 The CIMIS Penman Method ................................................................................................. 56 Other Penman Methods ....................................................................................................... 56 Hourly vs. 24-hour Computation Time Steps .............................................................................. 57 The 1985 Hargreaves Method .................................................................................................... 58 Appendix 2 Additional Supporting Equations.................................................................................... 60 Dew Point Temperature (Tdew).................................................................................................. 60 Short Wave Radiation on a Clear-Sky Day (Rso) ....................................................................... 60 FAO56c method ................................................................................................................... 61 In some situations, the estimation for Rso can be improved by considering the effects of water vapor on short wave absorption, so that:.................................................................... 61 Extraterrestrial radiation for 24-hour periods (Ra ) ..................................................................... 62 Extraterrestrial radiation for hourly or shorter periods (Ra )........................................................ 64 Solar radiation (Rs ) .................................................................................................................... 65 Daylight hours (N) ....................................................................................................................... 66 Wind profile relationship.............................................................................................................. 66 Daylight Savings Time ................................................................................................................ 67 Appendix 3 Estimating Missing Climatic Data
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