Integrated Applications - Transport in the Atmosphere-Vegetation-Soil Continuum - page 286

J F M A M J J A S O N D

E ref 7.9 15.1 31.4 54.5 82.9 86.7 91.5 80.2 48.2 27.1 11.0 6.2

P 67.0 47.5 65.4 44.5 61.5 71.7 70.0 58.2 72.0 77.1 81.2 76.8

Crop factors for use with the Makkink reference evapotranspiration can be found in
Appendix E .
a) Compute the (climatological mean) optimal evapotranspiration for grass for the
period April-September (determine the monthly mean crop factor for each month
from the three decadal values).
b) Compute the (climatological mean) optimal evapotranspiration for potatoes for the
period April-September.
c) Can each of the crops (grass and potatoes) grow on the precipitation that falls in each
of the months during the growing season in an average year (i.e., is the precipitation
suficient to sustain optimal evapotranspiration)?
d) Suppose that 120 mm of water is stored in the root zone of each of crops on
March 31. Is the precipitation in each of the months suficient to let the crop grow
unstressed?

8.2 Evapotranspiration Measurement: Lysimeters
In previous chapters a number of methods to measure evapotranspiration have been
dealt with. The eddy-covariance method, as well as the proile method (using simi-
larity relationships) have been dealt with in Chapter 3 . In Chapter 7 the Bowen ratio
method has been described. Here we discuss a measurement technique that does not
consider the turbulent lux in the atmosphere, but rather determines the evapotranspi-
ration as a residual of the water balance of a well-deined soil column.
The water balance of a soil column accounts for all incoming and outgoing luxes
of a soil proile and has been discussed in Section 4.2 . The actual evapotranspiration
E can be calculated when all the other luxes and the change of soil water storage are
known. This means that all errors in the other luxes and soil water change will be
relected in the estimate of E .
In the soil water balance method it can be dificult to quantify the drainage or deep
percolation lux D . In the case of deep groundwater levels this term is equal to the
percolation lux. In the case of shallow groundwater levels, D includes both percola-
tion and capillary rise. Despite the effort of many researchers, until today no practi-
cal device could be developed to measure soil water luxes. For a proper evaluation
of D therefore lysimeters should be used. A lysimeter is an isolated undisturbed soil
column, typically 0.5-2.0 m in diameter, with or without a crop, in which apart from
the evapotranspiration all terms of the water balance can be assessed. The lysimeter
permits the measurement of drainage or makes it zero.
Figure 8.4 shows a nonweighable lysimeter, in which a bottom porous plate is
used to apply a soil water pressure head corresponding with that in the ield at the