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

Land surface schemes are vital components of general circulation models (GC Ms) which provide the fluxes of heat, water and momentum at the land-atmosp here interface. The fluxes simulated by these schemes are especially depend ent on the way in which the canopy (or 'bulk stomatal') conductance for pla nt transpiration is modelled. Considerable research has been carried out in to the dependences of canopy conductance on the local environment, and empi rical 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 sug gest an alternative approach. Given an appropriate algorithm for scaling th ese values up to canopy level, such relationships allow canopy conductance values to be derived from (comparatively) well validated models of leaf pho tosynthesis. This approach is likely to become especially attractive as lan d surface schemes are extended to simulate CO2 fluxes, since the shared env ironmental dependences of canopy conductance and photosynthesis reduce the number of model parameters which require independent specification. This pa per is concerned with the evaluation of canopy level relationships between photosynthesis and conductance using data from the first international sate llite land surface climatology project (ISLSCP) held experiment (FIFE). Sim ultaneous measurements of CO2 and water vapour fluxes, taken over a predomi nantly C-4 grassland, are used to independently test the conductance and ph otosynthesis 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.