On the scaling up leaf stomatal resistance to canopy resistance using photosynthetic photon flux density

In addition to the limited availability of canopy resistance (rc) data for a variety of vegetation surfaces at different development stages, and at a range of soil water and climatic conditions, one challenge in practical application of the Penman–Monteith (PM) model is scaling up leaf level stomatal resistance (rL) to rc to represent an integrated resistance from plant communities to quantify field-scale evaporative losses. We present an integrated approach to scale up rL to rc. We measured rL for subsurface drip-irrigated (non-stressed) corn (Zea mays L.) plants and integrated a number of microclimatic and in-canopy radiation transfer parameters to scale up rL to rc. With the espousal of microclimatic and crop factors such as leaf area index for sunlit and shaded leaves, solar zenith angle, direct and diffuse solar radiation, we scaled up rL as a function of measured photosynthetic photon flux density (PPFD). We also estimated rc by solving the PM model on an hourly time-step using the Bowen ratio energy balance system-measured latent heat. There was an asymptotic relationship between the rL and PPFD. The rL showed a sudden (almost instantaneous) response to changing PPFD. The minimum value of rL was measured in the morning as 74 s m−1 (PPFD = 400 μmol m−2 s−1), whereas the maximum resistance occurred in the late afternoon as 910 s m−1 (PPFD = 111 μmol m−2 s−1). Beyond a certain amount of PPFD (approximately 800 μmol m−2 s−1), rL became less responsive to PPFD. Despite the changes in microclimatic conditions, the resistance remained relatively constant during the midday hours. A relatively constant pattern of rL was most likely during the period when the supply of water from plant roots kept pace with the transpiration rate. At lower PPFD (0–250 μmol m−2 s−1) and higher rL range (>150 s m−1), rL was very sensitive to PPFD, as a small change in PPFD caused a large change in rL. PPFD alone explained 85% of the variability in rc. The average root mean square difference between the measured and estimated rc was 11.1 s m−1 (r2 = 0.93). Results are encouraging, as the integrated PPFD vs. rc approach to scale up rL to rc for non-stressed plants does not account for the effect of other factors such as vapor pressure deficit, carbon dioxide concentration, wind speed, and soil evaporation.