Agriculture Reference
In-Depth Information
Fig. 6.12. Values of the crop coefficient for evapotranspiration, K c (see Eqns 6.8 and
6.9) for well-fertilized bulb onion crops growing in a large field without water stress.
Values of K c change with the stage of crop development and vary with the frequency
of soil surface wetness, particularly during early growth and with climatic condition
(see Eqn 6.10 and Table 6.3) (diagram based on Allen et al ., 1998, Fig. 24, and data
for onions given by these authors in their Chapters 5 and 6).
from full cover to the start of maturity; and (iv) 'late-season', from the start of
maturity to senescence or harvest.
Figure 6.12 shows how K C for a bulb onion crop changes through these
growth stages and how K C values range depending on the frequency of soil
wetting, the humidity and wind speed. For directly sown onions and leeks the
initial stage is comparatively long because of their slow germination, small
emergence weight and low RGR compared with many crops. It is usually
necessary to keep the soil moist before emergence, so values of K C typical of wet
soil (i.e. close to 1.0) will occur. The duration of the initial and crop
development phases can be predicted using models based on day-degrees or
effective day-degrees (Eqns 4.7 and 4.18). The duration of the mid-season and
late-season phases will also depend on climate, tending to be shorter the
warmer the conditions (see Fig. 4.4a).
In Eqn 6.9 K CB represents the ratio of crop evaporation to the reference
grass evaporation when the soil surface layer is dry but where there is adequate
water in the root zone for maximum transpiration rate. During the mid- and
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