Kc for Nonpristine and Unknown Conditions

For crops or vegetation where the Kc is not known, but where estimates of the fraction of ground surface covered by vegetation can be made, Kcb mid can be approximated as [76]

where Cf is the effective fraction of ground covered by vegetaton (0.01-1), Kcmin is the minimum Kc for dry, bare soil (0.15-0.20), and (Kcb)h is the maximum value for Kcb for vegetation having complete ground cover. (Kcb) h is estimated for areas of vegetation > 500 m2 as

(Kcb)h = min[(1.0f 0.1h), 1.2] f 0.04(U2 - 2) - 0.004(RHmin - 45)(h/3)0 3. (5.85)

For small, isolated stand sizes, Kcbh may need to be increased beyond the value given by the equation above.

The value for Kcbh may be reduced for vegetation that has a high degree of stomatal control, such as some types of brush and trees. The exponent in Eq. (5.84) represents the effects of microscale advection (transfer) of sensible heat from dry soil surfaces between plants toward plant leaves, thereby increasing ET per unit leaf area and the effects of increased roughness as the value for Ceff decreases.

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