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CRDSS <br />TASK MEMORANDUM 2.09-09 <br />Consumptive Use Model <br />Verification of the Penman-Monteith Reference Method <br />1.0 ISSUE <br />The calculation of the reference evapotranspiration by the Penman-Monteith method in CRDSS-CU is <br />verified by comparing its results to those of a spreadsheet calculator developed by Dr. Marvin Jensen. An <br />excellent comparison between the results was found indicating the Penman-Monteith method in CRDSS- <br />CU was implemented correctly. <br />2.0 DISCUSSION <br />The Penman-Monteith method in CRDSS-CU computes the reference evapotranspiration using an alfalfa <br />(ETr) and a grass (Eto) reference crop. It uses the procedure and equations that are documented in the <br />ASCE Manual 70 (Jensen, 1992). Its performance is compared to a spreadsheet calculator that includes <br />Penman-Monteith as one of its supported methods. The procedure used by CRDSS-CU and the <br />spreadsheet calculator are almost identical except for the following equations: <br />1. Albedo in CRDSS-CU is computed automatically as a function of the month and day using the <br />equation <br />(1) <br />02900630001753225 ..sin((.)(/.)) mn <br />where = albedo; m = month (1-12); and n = day (1-31). <br />In the spreadsheet calculator, the correct value should be provided by the user. To eliminate this <br />difference, the spreadsheet calculator was modified to compute albedo using the equation adopted by <br />CRDSS-CU. <br />2. The soil heat flux is considered in CRDSS-CU. It is computed as an empirical function of the mean air <br />temperature using the equation <br />=? + + <br />GT 0377 .(()/) T T T 3 <br />(2) <br />? ? ? <br />i i 123 i i <br />2 <br />where G = soil heat flux (MJ/m /d); and T = mean air temperature (degree C); and i = index for the <br />current day. <br />In the spreadsheet calculator, this parameter is not considered. To eliminate this discrepancy, the <br />spreadsheet calculator was modified to account for the heat flux to the ground using the equation adopted <br />by CRDSS-CU. <br />3. The cloudless solar radiation in CRDSS-CU is estimated using eq. 6.66 (Jensen , 1992: p. 135) <br />1 <br />12/3/96 2.09-09- IDS/CSU <br />