tailieunhanh - Báo cáo khoa học: "A comparison of daily representations of canopy conductance based on two conditional timeaveraging methods and the dependence of daily conductance on environmental factors"

Tuyển tập các báo cáo nghiên cứu về lâm nghiệp được đăng trên tạp chí lâm nghiệp quốc tế đề tài: "A comparison of daily representations of canopy conductance based on two conditional timeaveraging methods and the dependence of daily conductance on environmental factors. | Ann. Sei. For. 1998 55 217-235 Inra Elsevier Paris 217 Original article A comparison of daily representations of canopy conductance based on two conditional timeaveraging methods and the dependence of daily conductance on environmental factors Nathan Phillips Ram Oren Nicholas School of the Environment Duke University Durham NC 27708-0328 USA Received 15 January 1997 accepted 23 September 1997 Abstract - 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. Inra Elsevier Paris. Pinus taeda canopy conductance sap flux I soil water balance Resume - Comparaison des estimations de conductance de couvert journaliere basées sur deux .

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