Geoscience Reference
In-Depth Information
where the factor μ (Pa) is a measure for transpiration eficiency, which is more or less
independent of the climatic conditions during crop growth. The factor 1000 depends
on the units of T a (m) and DM a (kg m -2 ). Note that the dependence of Eq. ( 6.35 ) on
VPD is roughly consistent with the dependence of the leaf scale water use eficiency
on VPD (Eq. ( 6.31 )). An extensive literature review of Tanner and Sinclair (1981)
conirms that Eq. ( 6.35 ) is very useful to relate water use and plant yield. Important
for application of Eq. ( 6.35 ) is that the saturation deicit is calculated for the daytime,
when the stomata are open, and not for the entire day. Ehlers and Goss ( 2003 ) provide
μ values for a number of crops, which are listed in Table 6.4 . As discussed earlier, C 4
crops are more effective in ixation of CO 2 within the leaf than C 3 crops. Therefore,
at a certain light intensity, the CO 2 uptake rate and the photosynthesis of C 4 plants are
much higher ( Figure 6.18 ; compare with the sketch in Figure 6.13b ). This causes also
the higher water use eficiency of C 4 crops ( Table 6.4 ).
Using the crop-speciic factor μ and the average daylight saturation deicit during
the growing period, we may calculate with Eq. ( 6.35 ) the amount of transpiration for
an expected yield (provided no other stresses occur) or predict the yield for a given
amount of water extracted by the roots. This is illustrated in Table 6.5 for locations in
Germany and Colorado. When the climate becomes more arid, the amount of water
for crop transpiration increases. In the case of Göttingen and Akron the transpiration
amount more than doubles. That is only the water used for transpiration. Additional
water will be required for evaporation and possibly for drainage. Conversely, from
a ixed quantity of water, stored in the soil proile and replenished by precipitation
or irrigation, only a comparatively small amount of dry matter can be attained in the
dry climate with high saturation deicit of the air. Whereas in Göttingen 15 t ha -1 of
wheat biomass can be produced from 300 mm of water, only 7 t ha -1 will be obtained
in Akron ( Table 6.5 ).
Question 6.7: Which water productivity (kg m -3 ) and amount of transpiration (mm)
do you expect for a wheat crop grown near Wageningen? The average saturation deicit
amounts 1200 Pa, and a yield (dry matter above ground) of 16 t ha -1 is expected. Which
yield do you predict if 300 mm of soil water is extracted by the roots?
At a speciic location, we may apply Eq. ( 6.35 ) to potential and actual conditions,
yielding:
M
DM
DT
T
a
=
a
(6.36)
p
p
Therefore a irst approximation of the relative yield of dry matter, grain or other
marketable products is the relative transpiration. Equation ( 6.36 ) is often used by
hydrologists. However, at certain stages of crop development, such as pollination,
marketable yield may be extraordinarily affected (Kirkham, 2005 ). Figure 6.19
shows a generalized relation between yield and adequacy of water at different stages
Search WWH ::




Custom Search