Impact of Climate on Radiation-Use Efficiency

Radiation-use efficiency is sensitive to variations in atmospheric $CO_2$ concentrations and equations have been incorporated into SWAT+ to modify the default radiation-use efficiency values in the plant database for climate change studies. The relationship used to adjust the radiation-use efficiency for effects of elevated $CO_2$ is (Stockle et al., 1992):

$RUE=\frac{100*CO_2}{CO_2+exp(r_1-r_2*CO_2)}$ 5:2.1.4

where $RUE$ is the radiation-use efficiency of the plant (kg/ha⋅(MJ/m$^2$)$^{-1}$ or 10$^{-1}$ g/MJ), $CO_2$ is the concentration of carbon dioxide in the atmosphere (ppmv), and $r_1$ and $r_2$ are shape coefficients.

The shape coefficients are calculated by solving equation 5:2.1.4 using two known points ($RUE_{amb}$, $CO_{2amb}$) and ($RUE_{hi}$, $CO_{2hi}$):

$r1=1n[\frac{CO_{2amb}}{(0.01*RUE_{amb})}-CO_{2amb}]+r_2*CO_{2amb}$ 5:2.1.5

$r_2=\frac{(1n[\frac{CO_{2amb}}{(0.01*RUE_{amb})}-CO_{2amb}]-1n[\frac{CO_{2hi}}{(0.01*RUE_{hi})}-CO_{2hi}])}{CO_{2hi}-CO_{2amb}}$ 5:2.1.6

where $r1$ is the first shape coefficient, $r2$ is the second shape coefficient, $CO_{2amb}$ is the ambient atmospheric $CO_2$ concentration (ppmv), $RUE_{amb}$ is the radiation-use efficiency of the plant at ambient atmospheric $CO_2$ concentration (kg/ha⋅(MJ/m$^2$)$^{-1}$ or 10$^{-1}$ g/MJ), $CO_{2hi}$ is an elevated atmospheric $CO_2$ concentration (ppmv), $RUE_{hi}$ is the radiation-use efficiency of the plant at the elevated atmospheric $CO_2$ concentration, $CO_{2hi}$, (kg/ha⋅(MJ/m$^2$)$^{-1}$ or 10$^{-1}$ g/MJ). Equation 5:2.1.4 was developed when the ambient atmospheric $CO_2$ concentration was 330 ppmv and is valid for carbon dioxide concentrations in the range 330-660 ppmv. Even though the ambient atmospheric concentration of carbon dioxide is now higher than 330 ppmv, this value is still used in the calculation. If the $CO_2$ concentration used in the simulation is less than 330 ppmv, the model defines RUE = $RUE_{amb}$.

Stockle and Kiniry (1990) have shown that a plant’s radiation-use efficiency is affected by vapor pressure deficit. For a plant, a threshold vapor pressure deficit is defined at which the plant’s radiation-use efficiency begins to drop in response to the vapor pressure deficit. The adjusted radiation-use efficiency is calculated:

$RUE=RUE_{vpd=1}-\Delta rue_{dcl}*(vpd-vpd_{thr})$ if $vpd>vpd_{thr}$ 5:2.1.7

$RUE=RUE_{vpd=1}$ if $vpd \le vpd_{thr}$ 5:2.1.8

where $RUE$ is the radiation-use efficiency adjusted for vapor pressure deficit (kg/ha⋅(MJ/m$^2$)$^{-1}$ or 10$^{-1}$ g/MJ), $RUE_{vpd=1}$ is the radiation-use efficiency for the plant at a vapor pressure deficit of 1 kPa (kg/ha⋅(MJ/m$^2$)$^{-1}$ or 10$^{-1}$ g/MJ), $\Delta rue_{dcl}$ is the rate of decline in radiation-use efficiency per unit increase in vapor pressure deficit (kg/ha⋅(MJ/m$^2$)$^{-1}$⋅kPa$^{-1}$ or (10$^{-1}$ g/MJ)⋅kPa$^{-1}$), $vpd$ is the vapor pressure deficit (kPa), and $vpd_{thr}$ is the threshold vapor pressure deficit above which a plant will exhibit reduced radiation-use efficiency (kPa). The radiation-use efficiency value reported for the plant in the plant growth database, $RUE_{amb}$, or adjusted for elevated carbon dioxide levels (equation 5:2.1.4) is the value used for $RUE_{vpd=1}$. The threshold vapor pressure deficit for reduced radiation-use efficiency is assumed to be 1.0 kPa for all plants ($vpd_{thr}=1.0$).

The radiation-use efficiency is never allowed to fall below 27% of $RUE_{amb}$. This minimum value was based on field observations (Kiniry, personal communication, 2001).