The basal crop coefficient, Kcb [see Eq. (5.53)] is defined as the ratio of ETc/ET0 when the soil surface is dry but transpiration is occurring at a potential rate (e.g., water is not limiting transpiration) [2, 71]. Therefore, Kcb represents primarily the transpiration component of ET. Its use provides for separate and special adjustment for wet soil evaporation immediately following specific rain or irrigation events. This results in more accurate estimates of ETc when it must be computed on a daily basis.
Recommended values for Kcb are listed by Allen et al. [91, 95] but they can be obtained from Kc in Table 5.1 using the procedures indicated below.
Kcb during the initial period, denoted as Kcbini, is estimated for annual crops by assuming that a constant value can be used to represent evaporation from a mostly dry and bare surface soil layer that may occur during the initial period (see [2, 71, 92, 100]).
For annual crops having a nearly bare soil surface at the time of planting, it is recommended that
For perennial crops having a nearly bare soil surface at the beginning of the growing period before the initiation of new leaves, Kcbini can be estimated as 0.15 to 0.20.
The Kcbini for perennial crops with ground cover can be estimated by subtracting 0.10-0.20 from the Kcini values in Table 5.1:
Kcb ini = Kc ini - 0.1 or Kcb ini = Kc ini - 0.2, (5.72)
the second being recommended when crops are routinely irrigated frequently.
Most agricultural crop canopies nearly completely cover the soil surface during the midseason. As a result, soil wetness beneath the canopy has less effect on total ETc. Therefore, Kcbmid is often nearly the same value as Kcmid listed in Table 5.1 and can be estimated by
for C > ~0.8, that is, for crops that have nearly complete ground cover, such as cereals; and
for C < ~0.8, where C is the average fraction of soil surface covered (or shaded) by vegetation (0-0.99).
The basal Kc at the end of the growing season, Kcbend, has a lower value than the Kcend values recommended in Table 5.1. In general, Kcbend values can be estimated from Kcend in this table:
The Kcbmid and Kcbend need to be adjusted for climate using Eq. (5.67). The Kcb for the crop development period and for the late-season period are obtained by linear interpolation as indicated for the Kc curve (see "The Kc Curve," above). This is illustrated in Fig. 5.7.
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