A COMPARISON OF DAILY REPRESENTATIONS OF CANOPY CONDUCTANCE BASED ON 2 CONDITIONAL TIME-AVERAGING METHODS AND THE DEPENDENCE OF DAILY CONDUCTANCE ON ENVIRONMENTAL-FACTORS

In hydrological models which incorporate vegetated surfaces, non-stead y state responses in stem sap flow to diurnal evaporative demand can l ead to unreasonable values of computed canopy conductance, which corru pt diurnal courses and daily averages. Conductance computations based on daily averaged constituent variables are a potential method for cir cumventing this problem. However, comparisons between these two averag ing methods are lacking. In this study, both methods for computing dai ly canopy conductance were compared in a pine forest. A simplification of the Penman-Monteith equation under conditions of high aerodynamic coupling was used to calculate instantaneous canopy conductance. Large variation between the two methods was observed due to biases introduc ed under conditions of low sap flow or vapor pressure deficit. Two con ditional averaging schemes were developed to exclude data which were s trongly affected by such conditions, and as a result of the conditiona l averaging, a tighter relationship between these two averaging scheme s was found. We calculated daily representations of canopy conductance for an entire growing season in a 15-year-old Pinus taeda stand. Desp ite clear declines in conductance between rain events in direct respon se to soil water depletion, and large seasonal dynamics in canopy leaf area canopy conductance remained generally uniform until low late sea son temperatures. ((C) Inra/Elsevier, Paris.).