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5360 <br />?+?? <br />()(.)( RnG 402 uee ) <br />ad <br />T <br />= <br />ETo <br /> (eq.2) <br />++? <br />() 133 u <br />-2 -1 <br />ETrandETo <br />where are the vapor flux densities for the base reference (MJm d ) of alfalfa and grass, <br />-2 -1 <br />respectively; Rn is the net radiation flux density to the plant canopy (MJm d ); G is the soil heat flux <br />-2 -1 o -1 <br />density (MJm d ); is the slope of the saturation vapor pressure curve [Kpa (C ) ]; is the <br />o -1 <br />psychrometric constant [Kpa (C ) ]; e and e are the saturation vapor pressures at the current and dew- <br />a d <br />-3 <br />point air temperature, respectively (Kpa); T is the density of air (Kgm ); is the latent heat of <br />-1 -1 <br />vaporization (MJKg ); and u is the wind speed (ms ). <br />The PM equation is based on a standard alfalfa and grass reference crop equation as presented in Jensen <br />et al (1990). The PM reference crop method uses daily climate data, allows for daily irrigation events, <br />and a daily soil moisture budget. The ET will be calculated at complete weather station locations. <br />Reference crop equations estimate ET rates for a reference crop and then apply the rate to other crops <br />using crop coefficients. <br />Applying Crop Coefficients <br />One of the methods implemented in the CU model, is the PM reference crop equation. The crop ET <br />estimated using this method can calculate the crop ET in reference to standard crop ET, like alfalfa and <br />grass. The common procedure for computing ET for specific crops involves two steps: (1) estimating <br />reference ET for a well-watered standard crop base like alfalfa or grass using the appropriate <br />meteorological data and PM equation, and (2) applying an appropriate empirical crop coefficient factor <br />(Kc) and soil coefficient (Ka) mathematically shown as: <br />Et = Kc * Etr or Kto (eq.3) <br /> Where : <br /> Et = Specific crops evapotranspiration, unit (L/T). <br /> Kc = Empirical crop coefficient. <br /> Etr and Kto = Reference crop evapotranspiration alfalfa and grass base respectively. <br />In the following section, attention will be directed towards the methods used in computing the Kc values <br />required by the CU model. <br />Methods in Use for Developing Crop Coefficients <br />The crop coefficients (Kc) are empirical ratios of specific crop ET to the ET of reference crops (usually <br />alfalfa and grass). The Kc is derived from control experiments and its value depends on the crop?s <br />growth stage, crop canopy characteristics, and surface soil moisture conditions. <br />Generally, there are two methods used for computing the Kc values for specific crops. Each method has <br />its advantages and disadvantages. <br />2 <br />A 275 01.09.95 1.14-15 Al-azzawe <br />