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United States Department of Agriculture

Agricultural Research Service

SHAW Model
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Overview
The SHAW model simulates heat, water and solute transfer in a one-dimensional profile extending from the top of a plant canopy or the snow, residue or soil surface to a specified depth within the soil. The system is represented by integrating detailed physics of vegetative cover, snow, residue and soil into one simultaneous solution. The model is sufficiently flexible to represent a broad range of conditions and the system may or may not include a vegetative canopy, snow, or a residue layer. Interrelated heat, water and solute fluxes are computed throughout the system and include the effects of soil freezing and thawing. Daily or hourly predictions include evaporation, transpiration, percolation, soil frost depth, snow depth, runoff and soil profiles of temperature, water, ice and solutes.

Unique features of the model include: simultaneous solution of heat, water and solute fluxes; detailed provisions for soil freezing and thawing; and a sophisticated approach to simulating transpiration and water vapor transfer through a multi-species plant canopy. The model has been used to accurately predict the effects of management, climate, slope and vegetation on energy and water transfer at the soil-atmosphere interface and temperature and water conditions near the soil surface and within the soil profile. References for the SHAW model are available in pdf format.

Soil Freezing and Thawing
The SHAW model is one of the most detailed models available for snowmelt and soil freezing and thawing. The model has been shown to accurately simulate frost depth for a wide range of soil, climatic and surface conditions. It is capable of simulating complex wintertime phenomena of freezing effects on moisture and solute migration, solute effects on frost formation, and frozen soil related runoff. The interrelated transfer of heat, liquid water, water vapor, and solutes between layers within the soil profile is solved using an iterative procedure. Transfer within the soil profile is solved concurrently with the surface energy and mass balance, which includes solar and long-wave radiation exchange, evaporation, and sensible and latent heat transfer.

Evapotranspiration
The model is capable of simulating the effects of a multi-species plant canopy (including standing dead plant material) on heat and water transfer. Variation in plant size, rooting depth, and leaf area index of each plant species is defined by the user. Provisions for a plant canopy in the SHAW model were made using detailed physics of heat and water transfer through the soil-plant-atmosphere continuum. The plant canopy may be divided into several layers (up to 10) and transfer of water vapor and energy are solved for each layer within the canopy. Heat and water flux within the canopy include solar and long-wave radiation, turbulent transfer of heat and water vapor, and transpiration from plant leaves. Transpiration from plants is linked mechanistically to soil water by flow through the roots and leaves. Within the plant, water flow is controlled mainly by changes in stomatal resistance, which is computed as a function of leaf water potential.

Snowmelt
Energy and mass transfer calculations for snow within the SHAW model are patterned after the point energy and mass balance model developed by Anderson (Anderson, E.A. 1976. A point energy and mass balance of a snow cover. NOAA Tech. Rep. NWS 19, U.S. Department of Commerce, National Oceanic and Atmosphere Administration, National Weather Service, Silver Spring, MD. 150 p.). The energy balance of the snow includes solar and long-wave radiation exchange, sensible and latent heat transfer at the surface, and vapor transfer within the snowpack. Absorbed solar radiation, corrected for local slope, is based on measured incoming short-wave radiation, with albedo estimated from grain size, which in turn is estimated from snow density. Liquid water is routed through the snowpack using attenuation and lag coefficients, and the influence of metamorphic changes of compaction, settling and grain size on density and albedo are considered.

Input Requirements
Input to the SHAW model includes: initial snow depth and density; initial soil temperature and water content profiles; daily or hourly weather conditions (temperature, wind speed, humidity, precipitation and solar radiation); general site information; and parameters describing the vegetative cover, snow, residue and soil. General site information includes slope, aspect, latitude, and surface roughness parameters. Residue or litter properties include residue loading, thickness of the residue layer, percent cover and albedo. Input soil parameters are bulk density, saturated conductivity, coefficients for the matric potential-water content relation, and albedo-water content relation.

User Interface
A user-interface called ModShell (i.e. Model Shell) has been developed for the SHAW model. ModShell contains menus designed for ease of data entry. ModShell will assist in creating the required input files for the SHAW model and run SHAW. It provides information about input parameters and performs range and error checking for input data.

Downloads
SHAW model, documentation and example input files. The current version of SHAW is 2.3.6 and was released in November of 2004 (history, bug notices, and fixes)


Last Modified: 4/16/2008
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