Modeling program

Water-balance simulations of the WRCCRF were made using the SHAW model. The SHAW model simulates a vertical, one-dimensional system composed of a vegetation canopy, snowcover (if present), litter, and soil profile. A conceptual diagram of the model structure is shown in Figure 11.3. The model integrates the detailed physics of interrelated mass and energy transfer through the multilayer system into one simultaneous solution. Hourly predictions include evaporation, transpiration, snow depth, runoff, and profiles of soil water content and temperature.

Upper boundary conditions are defined by meteorological variables (solar radiation, air temperature, humidity, windspeed, and precipitation) measured above the canopy. Lower boundary conditions are defined by soil temperature and soil water content, potential or flux at a specified depth. A layered system is established though the model domain, with each layer represented by a node. After computing fluxes at the upper boundary, the heat, liquid water, and vapor fluxes between layers are simulated. Vegetation height, biomass, leaf area index, rooting depth, and leaf dimension are specified by the user. Details of the numerical implementation of the SHAW model are presented in Flerchinger (2003) Flerchinger et al. (1998, 1996b), and Flerchinger and Saxton (1989).

For this investigation, the SHAW model was modified to simulate saturated flow in soils and to more accurately simulate ET from forest vegetation. Modifications to

Thermal A^P®.^ radiation - Wlnd veloclty

Solar radiation

Vapor pressure

Precipitation

Thermal A^P®.^ radiation - Wlnd veloclty

Vapor pressure

Solar radiation

Canopy

Snowpack Litter Soil frost

Underlying soil

Canopy

Snowpack Litter Soil frost

Underlying soil

Soil temperature (Tg) Soil water content (0) Figure 11.3 Conceptual diagram of the Simultaneous Heat and Water (SHAW) Model. Reproduced with permission from Link et al., Simulation of water and energy fluxes in an old growth seasonal temperate rainforest using the Simultaneous Heat and Water (SHAW) model; published by American Meteorological Society, 2003

the model included improved parameterizations for intercepted rainfall storage, canopy conductance to water vapor transport, and transmission of radiation within the canopy. Details of the modifications and implementation of the model for the WRCCRF canopy are provided in Link et al. (2004a).

Simulations were completed for the 1999 and 2000 water years. In summary, the canopy was simulated as a single species with an LAI of 8.6, rooting depth of 1.2 m, and maximum stomatal conductance of 4.2 mm s-1, based on measurements at the site. The soil was simulated as two relatively high hydraulic conductivity layers from 0 to 0.50 and 0.50 to 1.00 m, overlying a low conductivity lower layer from 1.00 to 2.00 m, based on soil properties measured on 18 cores collected at the site. The canopy, litter, and soil layers were represented as 10, 6, and 24 nodes respectively in the model domain.

Figure 11.4 Precipitation intensity, snow depth, soil moisture, water table elevation, and streamflow at WRCCRF, hy1999-2000. Snow depth was recorded in a canopy gap and represents a maximum depth at the site. The range and mean of individual soil water content measurements are shown with symbols (•, o) and the approximate site average soil water content is shown by the record from a continuously logged proxy sensor

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1998 1999 2000

Figure 11.4 Precipitation intensity, snow depth, soil moisture, water table elevation, and streamflow at WRCCRF, hy1999-2000. Snow depth was recorded in a canopy gap and represents a maximum depth at the site. The range and mean of individual soil water content measurements are shown with symbols (•, o) and the approximate site average soil water content is shown by the record from a continuously logged proxy sensor

11.3 RESULTS AND DISCUSSION

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