Integrated Models in Hydrology and Meteorology - Transport in the Atmosphere-Vegetation-Soil Continuum - page 321

In the actual production situation, the productivity achieved will be the result of a
combination of growth-deining, -limiting and -reducing factors (Van Ittersum et al.,
2003 ).
In the 1980s a wide range of scientists in Wageningen became involved in the
development and application of crop models. A comprehensive overview of the devel-
opment and application of 'Wageningen' crop models is given in Van Ittersum et al.
( 2003 ). We will discuss the WOFOST (WOrld FOod STudies model; Supit et al.,
1994 ), which has been incorporated in the SWAP model, and aims to calculate the
crop production level as determined by climate and crop factors and limited by water
and oxygen shortage or salt excess.
Figure 9.10 shows a low diagram of the WOFOST model components. Plant dry
matter production results from the photosynthesis process, in which CO 2 from the air
is converted into carbohydrates (CH 2 O) n according to the overall reaction:

CO + + HO solar energy →+ CH OO (9.23)

2 2 2 2

This process is known as CO 2 assimilation. For each kg of CO 2 absorbed, 30/44 kg
of CH 2 O is formed, where the numerical values represent the molecular weights of
CH 2 O and CO 2 .
Canopy photosynthesis is calculated from the absorbed amount of photosynthet-
ically active radiation (PAR; wavelength 400-700 nm) and the photosynthesis-light
response of single leaves. Use of average illumination intensities of the leaves in
the calculations would overestimate assimilation because of the convex assimilation-
light response. Temporal and spatial variation in illumination intensity over the leaves
therefore has to be taken into account. First, the instantaneous radiation lux at the
top of the canopy at a certain time of day is derived from measured daily global radi-
ation. A distinction is made between diffuse skylight and direct sunlight because of
the large difference in illumination intensity between shaded leaves receiving only
diffuse radiation, and sunlit leaves, receiving both direct and diffuse radiation. The
assimilation rate in a leaf layer is calculated for sunlit and shaded leaves separately.
Daily crop assimilation is obtained by integrating these assimilation rates over the
leaf layers and over the day (Goudriaan, 1986 ).
The potential photosynthesis might be reduced by water or oxygen shortage or salt
excess. The reduction is assumed to be proportional to the reduction in transpiration,
as discussed in Chapter 6 .
Part of the daily production of assimilates is used to provide energy for the mainte-
nance of the existing biomass ( maintenance respiration ). Maintenance respiration is
related to the standing biomass and to the metabolism intensity. Higher temperatures
accelerate the turnover rates in plant tissues and hence the cost of maintenance. An
increase in temperature of 10 ° C increases maintenance respiration by a factor of
about 2 (Penning de Vries and Van Laar, 1982 ).