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CRDSS <br />TASK MEMORANDUM 1.14-15 <br />Consumptive Use Model <br />Penman-Monteith Equation <br />1.0 ISSUE <br />This task memoradum describes the Penman-Monteith (PM) reference crop equation and its use in the <br />consumptive use (CU) model. <br />2.0 DISCUSSION/ANALYSIS <br />Introduction <br />The PM method is a daily reference crop method that uses complete meteorological data (including <br />precipitation, temperature, wind run, and solar radiation) in addition to crop, and soil characteristics. The <br />concept of a daily reference crop evapotranspiration (ET) is widely used in estimating crop ET, and is <br />considered to be more accurate where the type and frequency of climate data is adequate Jensen et al <br />(1990). <br />Reference crop ET (Etr for alfalfa and ETo for grass) is the rate at which water, if available, would be <br />removed by evaporation and transpiration from soil and plant surfaces expressed as the depth of water <br />used by a standard reference crop per unit time. <br />The Penman-Monteith Equation <br />In general, a combination method such as the PM equation uses meteorological and crop data input. In <br />this approach, the energy balance, heat, and mass transfer components are used in estimating ET. The <br />first form of a combination equation was derived by Penman (1948). <br />If the PM equation uses the concept of canopy resistance to approximate the effects of vegetation leaf <br />area, the height on surface roughness, and the bulk stomata resistance, it appears to improve the <br />applicability to a wide variety of climates and locations. <br />The general mathematical equation used in the PM method involves two steps. First, ETr or ETo are <br />calculated for a well-watered standard crop base using appropriate meteorological data and the PM <br />equation. <br />5360 <br />=?+?? <br />ETrRnG ().)( 402 uee ) <br />ad <br />T <br /> (eq.1) <br />++? <br />() 1407 u <br />1 <br />A 275 01.09.95 1.14-15 Al-azzawe <br />