A canopy model (RM-CWU) for determining transpiration of subalpine forests. I. Model development

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Abstract
A model has been developed for calculating consumptive water use of subalpine forests in the Rocky Mountains. This model, RM-CWU, is based upon the use of a submodel of stomatal behavior in which stomatal activity is predicted from photosynthetic photon flux density (PPFD) and the absolute humidity difference from leaf to air (DAH). Instantaneous transpirational flux density is calculated for layers of leaf area for each species, and transpiration is summed for the entire canopy. PPFD profiles are derived using extinction coefficients estimated for each stand, for a range of sun angles, along with estimates of PPFD above the canopy corrected for slope, aspect, obstruction of the horizon, time, and date. DAH is estimated from records collected at a centrally located weather station, using a submodel for calculating canopy temperature at higher elevations. In preliminary applications, conductances calculated with RM-CWU varied with physiographic features of the site, date and time, position in the canopy, and weather conditions, through effects on PPFD and DAH. Consumptive water use, calculated for entire stands using leaf areas predicted from basal area measurements, also varied with stand composition.

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