A comparison of models to estimate in-canopy photosynthetically active radiation and their influence on canopy stomatal resistance

The models for photosynthetically active radiation (PAR) used in a multi-la yer canopy stomatal resistance (CSR) model developed by Baldocchi et al. (A tmospheric Environment 21 (1987) 91-101) and in a two-big-leaf CSR model de veloped by Hicks et al. (Water, Air and Soil Pollution 36 (1987) 311) are i nvestigated in this study. The PAR received by shaded leaves in Baldocchi e t al. (1987) is found to be larger than that predicted by a canopy radiativ e-transfer model developed by Norman (in: Barfield, Gerber, (Eds.), Modific ation of the Aerial Environment of Crops. ASAE Monograph No. 2. American So ciety for Agricultural. Engineering, St. Joseph, Ml, 1979, p. 249) by as mu ch as 50% even though the Baldocchi et al. (1987) model is indirectly based on Norman's model. This larger value of PAR results in turn in a smaller C SR by as much as 30% for canopies with larger leaf area indexes. A new form ula to predict vertical profiles for PAR received by shaded leaves inside a canopy is suggested in the present study based on Norman (1979) and agrees well with the original model of Norman (1979). The simple treatment used i n Hicks et al. (1987) for canopy-average PAR received by shaded leaves is f ound to diverge for canopies with leaf area indexes not close to two A new empirical formula for canopy-average PAR is then suggested for use in a two -big-leaf model, and it is shown that under most conditions the modified tw o-big-leaf CSR model can predict reasonable values when compared with the m ore complex multi-layer CSR model. Both the modified multi-layer CSR model and the modified two-big-leaf CSR model are also shown to predict reasonabl e dry deposition velocities for O-3 when compared to several sets of measur ements. (C) 2001 Elsevier Science Ltd. All rights reserved.