A comparison of daily representations of canopy conductance based on two conditional time-averaging methods and the dependence of daily conductance on environmental factors
In hydrological models which incorporate vegetated surfaces, non-steady state responses in stem sap flow to diurnal evaporative demand can lead to unreasonable values of computed canopy conductance, which corrupt diurnal courses and daily averages. Conductance computations based on daily averaged constituent variables are a potential method for circumventing this problem. However, comparisons between these two averaging methods are lacking. In this study, both methods for computing daily 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 introduced under conditions of low sap flow or vapor pressure deficit. Two conditional averaging schemes were developed to exclude data which were strongly affected by such conditions, and as a result of the conditional averaging, a tighter relationship between these two averaging schemes was found. We calculated daily representations of canopy conductance for an entire growing season in a 15-year-old Pinus taeda stand. Despite clear declines in conductance between rain events in direct response to soil water depletion, and large seasonal dynamics in canopy leaf area, canopy conductance remained generally uniform until low late season temperatures.
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