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 CO 2 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) ﬁeld experiment (Fife). Simultaneous measurements of CO 2 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.  1998 Elsevier Science B.V.