A canopy conductance and photosynthesis model for use in a GCM land surface scheme

Land surface schemes are vital components of general circulation models (GCMs) which provide the fluxes of heat, water and momentum at the land-atmosphere interface. The fluxes simulated by these schemes are especially dependent on the way in which the canopy (or 'bulk stomatal') conductance for plant transpiration is modelled. Considerable research has been carried out into the dependences of canopy conductance on the local environment, and empirical relationships for such dependences have been obtained by fitting the data collected in field and laboratory experiments. However, observed leaf level relationships between stomatal conductance and net photosynthesis suggest an alternative approach. Given an appropriate algorithm for scaling these values up to canopy level, such relationships allow canopy conductance values to be derived from (comparatively) well validated models of leaf photosynthesis. This approach is likely to become especially attractive as land surface schemes are extended to simulate CO2 fluxes, since the shared environmental dependences of canopy conductance and photosynthesis reduce the number of model parameters which require independent specification. This paper is concerned with the evaluation of canopy level relationships between photosynthesis and conductance using data from the first international satellite land surface climatology project (ISLSCP) held experiment (FIFE). Simultaneous measurements of CO2 and water vapour fluxes, taken over a predominantly C-4 grassland, are used to independently test the conductance and photosynthesis models. Based on these tests, a simple coupled model of canopy conductance and photosynthesis is developed which produces a good match to the experimental data. (C) 1998 Elsevier Science B.V. All rights reserved.