Crop yield response to water - Cra

ox 5 (CONTINUED)F 2 VALUES (Northern Hemisphere)Month F 2January 0.70February 0.75March 0.80April 0.90May 1.05June 1.25July 1.25August 1.20September 1.10October 1.20November 1.10December 0.70B CALCULATING EVAPOTRANSPIRATION with the K c MethodIf it is not possible to calculate Tr separately, what is needed is an overall crop coefficient (K c )that embodies all the components of ET c . The FAO I&D No. 56 provides the K c values for treecrops and vines, derived from the original values published in FAO I&D No. 24. The differentcrop sections in this publication provide sets of K c values for the different tree crops and vines,which are an update of those published earlier in FAO I&D No. 56.As shown in Figure 10, the crop coefficient (K c ) for deciduous orchards on bare soil, increasessharply in spring in response to bud break and leafout until it reaches a maximum in earlysummer. The peak K c value is maintained until harvest, after which it declines temporarily forsome tree crops. In others, the maximum K c stays more or less the same until the beginningof senescence, when it starts to decline until leaf fall is complete. For evergreen trees, theremay be variations in K c within the season, caused by changes in leaf area, and by responseseither to environmental changes or to internal tree signals. It should be emphasized that thestandard K c values include an average E component but do not include the Tr cc from covercrops, unless specifically stated.Determining the ET of partial tree canopiesWhen information exists to compute ET c as the sum of its components (Equation 4), the sameprocedure may be applied to non-mature, developing orchards where trees have not yetachieved their mature size. It suffices to measure the canopy parameters that characterizethe radiation intercepted by the young trees and to compute the Tr, using a method suchas the one in Box 5. The standard K c procedure described above specifically applies to264crop yield response to water