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=+? <br />(3) <br />RsoABdC cos[()/() 2365 <br />where A, B = are empirical functions of the latitude (degree) and elevation (m); d = calendar day (1-365); <br />and C = the phase constant for the longest day = 172. In the spreadsheet calculator, this parameter is <br />calculated as a function of the extra-terrestrial radiation Rso = cRa where Ra = extra-terrestrial radiation <br />(40.24 MJ/m2/d); and <br />2 <br />=?+?? <br />c exp(./(sin(.sin(/.)(.). 000206085236513903042 Pdll ) <br />(4) <br />where P = total atmospheric pressure (kPa); and l = latitude (degree). There is no apparent significant <br />difference between the two equations when estimating the cloudless solar radiation. <br />4. The aerodynamic term in CRDSS-CU is approximated by eq. 6.17b ((Jensen , 1992: p. 93) and is <br />expressed as follows: <br />* o <br />=+? <br />AEROKe (/( )).()/ 0622 er P <br />(5) <br />1 a <br />where the quantity; <br />=? <br />(6) <br />0622 ./. KPT 1710685 <br />1 <br />The spreadsheet calculator uses the expression: <br />* o <br />=++? <br />AEROPPTe (/( )(./)(./.())(()/ 0622348610127386400 er )) <br />(7) <br />a <br />for the aerodynamic term which is adopted from eq. 2.16 of Allen et al. (1994). There is no apparent <br />significant difference between these two approximations. <br />Table 1 shows the calculated reference alfalfa evapotranspiration (ETr) and reference grass <br />evapotranspiration (ETo) as computed using CRDSS-CU and the spreadsheet calculator developed by <br />Marvin Jensen. The results are similar as reflected by the computed ratio being very close to unity. <br />Figure 1 and 2 shows graphically the comparison of the results between CRDSS-CU and the spreadsheet <br />calculator. <br />2 <br />12/3/96 2.09-09- IDS/CSU <br />