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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />2481 <br /> <br />ET estimates when calibrated to a given area and crop. The form usee <br /> <br />in this study is the FAO modified Penman method as presented in l/: <br /> <br />ETa = c [W x Rn T (l-W) x F(u) x (ea-ed)] (17) <br /> <br />in which: <br /> <br />ETa is grass reference crop ET in mm/day, <br /> <br />w is a temperature related weighting factor, <br /> <br />Rn is net radiation in equivalent evaporation in <br />mm/day, <br /> <br />F(u) is a wind function in which, <br />F(u) = 0.27 (1 + U/IOO) <br /> <br />Where U is 24 hr. wind run in km/day at 2 m height, <br /> <br />(ea-ed) is the difference between the saturation <br />vapor pressure at mean air temperature and the mean <br />actual vapor pressure of the air in mb, and <br /> <br />c is an adjustment factor to compensate for the <br />effect of day and night weather conditions. <br /> <br />Tables and formulas are needed for calculation of most terms in <br /> <br />equation (17). Additional information on the calculation of the <br /> <br />individual terms in each equation is given in the section on sample <br /> <br />project calculations. To obtain the c factor, it is necessary to <br /> <br />interpolate between relative humidity, solar radiation, wind speed, <br /> <br />and day/night wind ratios. This difficult interpolation can be <br /> <br />facilitated by use of the equations in Appendix A-l.2. <br /> <br />1.3.6 General Comparison of Methods <br /> <br />Equations vary greatly in the amount of data required, ease of <br /> <br />computation, and accuracy of the predicted results. Table A-l.3 <br /> <br />compares the different methods discussed above, and Figure A-l.2 <br /> <br />1- 18 <br />