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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 />I <br />I <br />I <br />I <br />I <br /> I <br /> I <br /> ~- <br /> <br />2471 <br /> <br />season, growth and development characteristics, and cuttings or <br /> <br />harvest intervals. Values have been determined for many crops and <br /> <br />are given in tabular form, graphical form, or mathematical <br /> <br />equations. More recently, basal crop coefficients have been <br /> <br />developed which represent conditions when the soil surface is dry so <br />that the evaporation component of ET is a minimum. Basal <br /> <br />coefficients must be adjusted for surface soil wetness based upon the <br /> <br />frequency of rainfall or irrigation. <br /> <br />Alfalfa crop coefficients developed for an arid region with an <br />intermountain climate such as the project area are used in this <br />report for all methods except the three FAOmodified methods and the <br />SCS Blaney-Criddle method l/. The coefficients are shown in Table <br />A-l.2. A mean alfalfa crop coefficient of 0.95 which reflects the <br />cyclic effect of harvest on crop ~ater use is recommended in an arid <br /> <br /> <br />climate with light to moderate wind.~/ This coefficient is used in <br /> <br /> <br />this report for the FAO methods with grass based reference crop. <br /> <br />Caution must be used in applying crop coefficients as they are not <br />interchangeable to all methods. The user must be sure that the crop <br /> <br />coefficients he intends to use are compatible with his reference crop <br /> <br />methodology. Most crop coefficients in use today are based upon an <br /> <br /> <br />alfalfa based reference; exceptions are the FAO modified methods <br /> <br /> <br />(dicussed later in this section) which use a grass based reference <br /> <br /> <br />and the SCS method which computes crop ET directly using a locally <br /> <br /> <br />calibrated adjustment factor. <br /> <br />1- 8 <br />