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The Basal Crop Coefficient Method (Kcb) <br />In this method, the computation of wet soil surface evaporation is separated from the transpiration of <br />crops; therefore, individual functions describing the effects of wet soil and crop evaporation are needed. <br />Mathematically the Kcb is: <br />Kc = Kcb * Ka + KS (eq.4) <br />Where : <br />Kc = Crop coefficient values, <br />Kcb = Empirical basal crop coefficient curve that is experimentally driven for each crop, <br />Ka = Dimensionless coefficient to account for available soil water and soil moisture stresses, and <br />KS = Soil coefficient that accounts for the affect of increased evaporation for wet soil surfaces after rain <br />or irrigation events. <br />The Mean Crop Coefficient Method (Kcm) <br />In this method, the crop coefficient curve accounts for the effect of both wet soil surface evaporation and <br />crop transpiration together, mathematically shown as: <br />Kc = Kcm * Ka (eq.5) <br />Where : <br />Kcm = Mean crop coefficient curve experimentally determined for each crop, and <br />Ka = Dimensionless coefficient to account for available soil water and soil moisture stresses. <br />For both methods, the crop coefficient can be derived from an alfalfa or a grass base. This leads to four <br />types of Kc curves for each crop (i.e. basal Kcb for alfalfa and grass and mean Kcm for alfalfa and <br />grass). Since the Kc value required experimental data in the specific area, which is time consuming and <br />unreliable, the CRDSS CU team decided to use Kc values given in the reference Jensen et al. (1990). <br />These data were modified for the crops in the Gunnison River basin using local crop growth data , such <br />as planting date, planting to full covers days, and the end date of the growing season in the Gunnison <br />River basin region. These modifications adjust the given data in the reference to local conditions that <br />make the data more reliable for this specific region. <br />Crop Coefficients Values in the Gunnison River Basin <br />As previously mentioned, the crop coefficients were adjusted for local crop growth conditions for the <br />Western Slope region of Colorado and should be used regionally. It is important to re-adjust the crop <br />coefficients if these data need to be used in a new region. Also, the CU model uses 20 different crop <br />types. Not all of these crop types have a reference crop coefficient. For example, cotton crops have a <br />mean crop coefficient only, so the CU model will use this type of Kc in calculating the cotton reference <br />crop ET. <br />3 <br />A 275 01.09.95 1.14-15 Al-azzawe <br />