Crop Modelling: Method

These calculations have been done with the WOFOST crop growth model (Van Diepen et al., 1989; Rabbinge and Van Diepen, 2000) for two possible levels of atmospheric CO2. The effects of management adaptation on crop yields under future conditions have also been calculated.

Incorporating CO2 effects on C4-crops

For C4 plants such as maize and other tall tropical grasses such as millet, sorghum and sugar cane, the photosynthetic response to CO2 is only very steep for atmospheric CO2 concentrations well below the current level. In the present and also the future range of atmospheric CO2 concentrations (e.g., 300 to 1000 μmol/mol), the rate of CO2 assimilation practically does not change at increasing CO2, even under high light intensities (Goudriaan and Unsworth, 1990). But, concurrently, the transpiration rate of the maize crop strongly decreases. Literature reviews by Cure (1985) and Cure and Acock (1986) indicated a decrease in stomatal conductance by 40% and a decrease in transpiration by 28% for maize at doubled atmospheric CO2 and high light conditions. The reduction of the transpiration by maize at doubled CO2 was calculated in a study with a stratified micrometeorological model (Goudriaan, 1977; Chen, 1984). If the stomatal conductance at doubled CO2 was reduced by 45%, this resulted in a reduction of the transpiration by 26% if both the effects of increasing leaf temperature and decreasing air humidity in the canopy were taken into account (Goudriaan and Unsworth, 1990). As this micrometeorological feedback is not included in the method applied in the WOFOST model for calculating the rate of transpiration (i.e. the Penman approach), calculated transpiration rates were reduced by an overall factor. This reduction factor increased with increasing CO2 and was set at 26% for a doubled CO2 concentration (i.e., increase by 355 μmol/mol).

Incorporating CO2 effects on C3-crops

Direct effects of increasing atmospheric CO2 concentration on the CO2 assimilation and growth of the wheat crop is incorporated in the WOFOST model as follows:

• Maximum and initial angle of the CO2 assimilation - light response curve of single leaves increase with  increasing CO2 concentration;
• Limited decrease in transpiration rate.

The changes in WOFOST model parameters that reflect these changes in plant behaviour at increasing CO2 concentrations, are summarized in the table below. These parameter adaptations are based on studies by Allen et al. (1990), Goudriaan et al. (1984), Goudriaan et al. (1985), Goudriaan (1990), Goudriaan & Unsworth (1990), Goudriaan & de Ruiter (1983) and Idso (1990), and on literature surveys on crop responses to CO2 doubling by Cure (1985), Cure & Acock (1986), and Kimball (1983). Doubling CO2 resulted in yield increases by 40 to 60%  (see Old parameter values in Table), being mainly based on pot experiments. Field experiments under doubled CO2 done more recently, appeared to give lower CO2 responses due to more plant interaction (e.g. shadowing in canopy), being about 25 to 40% yield increase for doubled CO2 (De Temmermans et al.; 2002, Wolf & Van Oijen, 2002; Wolf & Van Oijen, 2003; Wolf et al., 2002). Hence, the change in the model parameters that determine the CO2 assimilation (EFF, AMAX) due to CO2 doubling have been multiplied with 60% (see New parameter values in Table).

Changes  in initial angle (EFF) and in maximum (AMAX) of the CO2 assimilation - light response curve and in the reduction factor for potential transpiration (RTRA) for  adaptation of the WOFOST model to doubling of the actual atmospheric CO2 concentration (i.e. increase by 355 μmol/mol) on C3-crops. The relative figures are given in brackets.

Changes in WOFOST model parameters for KNMI scenarios

The CO2 concentrations used as inputs for the WOFOST growth simulations are combined with  the KNMI  climate scenarios for 2050 (see table below; for more information http://www.knmi.nl/climatescenarios/knmi06/index.php), and are derived from the SRES emission scenarios in the IPCC assessment report from 2001 (Scientific basis, Appendix II, Table II.2.1 with CO2 abundances; http://www.grida.no/publications/other/ipcc_tar/).  We used the CO2 concentrations from the ISAM model (reference) for 2050 for first the high emission scenario A1FI and second, the low emission scenario B2, being respectively 567 and 478  μmol CO2/mol, and for the current situation around year 2000, we use 369 μmol CO2/mol. The CO2 concentration from A1FI scenario might correspond best with the W and W+ scenarios of KNMI and the CO2 concentration from B2 scenario with the B and B+ scenarios (Table 2), but initially we will do simulation runs for all (4* 2) combinations.

Source: Van den Hurk et al., 2006; daily weather data for the four scenarios have been supplied by Janette Bessembinder (KNMI); these weather data are based on observed daily weather data for Lelystad around year 2000 with transformation of mainly minimum and maximum temperature and rainfall to produce the four scenarios for 2050.

We assume that the relationship between  the CO2 increase and the growth processes is roughly linear, and that it is alright to change  the WOFOST model parameters for C3-crops (first table) for the A1FI and the B2 scenarios as follows:

• change in EFF= +11% × (567-369)/355=  +6%    and +11% × (478-369)/355= +3%
  
• change in AMAX= +60% × (567-369)/355= +33% and +60% × (478-369)/355= +18%
  
• change in RTRA= -10% × (567-369)/355= -6%     and -10% × (478-369)/355= -3%
  

For C4-crops (i.e. maize) the model parameters EFF and AMAX are not affected by the increase in atmospheric CO2 but the change in RTRA = -26% × (567-369)/355= -15%     and -26% × (478-369)/355= -8%.   

Source

Wolf, J., M. Mandryk, A. Kanellopoulos, P. van Oort, B. Schaap, P. Reidsma and M. van Ittersum, 2010. Methodologies for analyzing future farming systems in Flevoland as applied within the AgriAdapt project. AgriAdapt project report no. 1, Wageningen UR.