Development and evaluation of simplified models for simulating canopy photosynthesis and transpiration.
Comparisons were made of three micrometeorological models which simulated daily rates of photosynthesis and transpiration in a maize crop. A complete soil-plant-atmosphere model (SPAM) simulated wind speed, CO2 concentration, atmospheric water vapour content, air and leaf temperatures, and radiation distribution in canopies. The model predicted that transpiration rates were highly dependent on air temperature and mildly dependent on water vapour content, but only on wind speed when air temperature was high. Predicted CO2 assimilation rates were strongly dependent on air temperature. A simplified model assumed that there were no vertical gradients of air temperature, water vapour and CO2 above and within the canopy. This model produced predictions of CO2 assimilation within 12% of SPAM, but predictions of transpiration agreed only under conditions of low transpiration; the difference could be as great as 40%. A third model assumed that the maize canopy could be viewed as a 'big leaf'. This leads to the Penman-Monteith equation for expressing transpiration. Daily transpiration rates agreed within 10% of SPAM, except for conditions producing the lowest transpiration rates. Estimates of CO2 exchange agreed within 5% in all but three cases.