The water and energy dynamics of an old-growth seasonal temperate rainforest

Graduation date: 2002In the Pacific Northwest (PNW), concern about the impacts of climate and land\ud cover change on water resources, flood-generating processes, and ecosystem dynamics\ud emphasize the need for a mechanistic understanding of the interactions between forest\ud canopies and hydrological processes. A detailed measurement and modeling program\ud during the 1999 and 2000 hydrologic years characterized hydrological conditions and\ud processes in a 500-600 year old Douglas fir-western hemlock seasonal temperate\ud rainforest. The measurement program included sub-canopy arrays of radiometers,\ud tipping bucket rain gauges, and soil temperature and moisture probes, to supplement a\ud vertical temperature and humidity profile within the forest canopy. Analysis of the\ud precipitation interception characteristics of the canopy indicated that the mean direct\ud throughfall proportion was 0.36, and the mean saturation storage was 3.3 mm.\ud Evaporation from small storms insufficient to saturate the canopy comprised 19% of\ud the net interception loss, and canopy drying and evaporation during rainfall accounted\ud for 47% and 33% of the net loss, respectively. Results of the measurement program\ud were used to modify the Simultaneous Heat and Water (SHAW) model for forested\ud systems. Changes to the model include improved representation of interception\ud dynamics, stomatal conductance, and within-canopy energy transfer processes. The\ud model effectively simulated canopy air and vapor density profiles, snowcover\ud processes, throughfall, soil water content profiles, shallow soil temperatures, and\ud transpiration fluxes for both a calibration period and for an uncalibrated year. Soil\ud warming at bare locations was delayed until most of the snowcover ablated due to the\ud large heat sink associated with the residual snow patches. During the summer,\ud simulated evapotranspiration decreased from a maximum monthly mean of 2.17 mm\ud day⁻¹ in July to 1.34 mm day⁻¹ in September, as a result of declining soil moisture and\ud net radiation. Our results indicate that a relatively simple parameterization of the\ud SHAW model for the vegetation canopy can accurately simulate seasonal hydrologic\ud fluxes in this environment. Application and validation of the model in other forest\ud systems will establish similarities and differences in the interactions of vegetation and\ud hydrology, and assess the sensitivity of other systems to natural and anthropogenic\ud perturbations