  A TL-LUE model modified with a radiation scalar (RTL-LUE) is developed in
this paper. The same maximum LUE is used for both sunlit and shaded leaves, and
the difference in LUE between sunlit and shaded leaf groups is determined by
the same radiation scalar. The RTL-LUE model was calibrated and validated at
global 169 FLUXNET eddy covariance (EC) sites. Results indicate that although
GPP simulations from the TL-LUE model match well with the EC GPP, the RTL-LUE
model can further improve the simulation, for half-hour, 8-day, and yearly time
scales. The TL-LUE model tends to overestimate GPP under conditions of high
incoming photosynthetically active radiation (PAR), because the
radiation-independent LUE values for both sunlit and shaded leaves are only
suitable for low-medium (e.g. average) incoming PAR conditions. The errors in
the RTL-LUE model show lower sensitivity to PAR, and its GPP simulations can
better track the diurnal and seasonal variations of EC GPP by alleviating the
overestimation at noon and growing seasons associated with the TL-LUE model.
This study demonstrates the necessity of considering a radiation scalar in GPP
simulation in LUE models even if the first-order effect of radiation is already
considered through differentiating sunlit and shaded leaves. The simple RTL-LUE
developed in this study would be a useful alternative to complex process-based
models for global carbon cycle research.
