Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/1996
Publication Date: N/A
Interpretive Summary: Radiation energy absorbed by a snowpack (including short-wave radiation from the sun and terrestrial long-wave radiation) provides a significant amount of the energy for melting snow, particulary for deep snowpacks which remain late into the spring and summer. These deep snowpacks provide much of the water supply for industry and agriculture in the western United States. Simulating the radiative balance over snow is particularly challenging because: the reflectivity of the snow changes as the snowpack ages; surface temperature, which affects long-wave radiation exchange, is difficult to simulate; and incoming terrestrial long-wave radiation can be somewhat variable. Few snowmelt models are available that include a comprehensive energy and water balance for cold-season conditions, which is required to adequately compute surface temperature and emitted long-wave radiation. The Simultaneous Heat and Water (SHAW) Model is a detailed, physical process model of a vertical, one-dimensional canopy-snow-residue- ystem which integrates the physics of heat and water transfer through a plant canopy, snow, residue and soil into one simultaneous solution. The SHAW model was applied to a site near St. Paul, Minnesota U.S.A. where a comprehensive data set for the complete radiative energy balance was collected to test the representation of the radiative balance within the model and to evaluate parameter values used in the model. Application of the model led to changes in the model to yield better agreement with measured radiation balances. This work has led to a better understanding of the radiation balance over a snowpack and improved capability to simulate rate and timing of snowmelt.
Technical Abstract: Snow and ice present interesting challenges to hydrologists. Simulating the radiative balance over snow, which is an important part of surface-atmo e interactions, is particularly challenging because of the decay in albedo ver time and the difficulty in estimating surface temperature and incoming long-wave radiation fluxes. Few models are available which include a comprehensive energy and water balance for cold-season conditions. The Simultaneous Heat and Water (SHAW) Model is a detailed, physical process model of a vertical, one-dimensional canopy-snow-residue-s ystem which integrates the detailed physics of heat and water transfer through a plant canopy, snow, residue and soil into one simultaneous solution. Detailed provisions for metamorphosis of the snowpack are included. The SHAW model was applied to data for one winter/spring season (November through May) on a ploughed field in Minnesota without prior calibration to test the performance of the radiation components. Maximum snow depth during this period was 30 cm. For the nearly 100 days of snowcover, the model accounted for 69% of the variation in net solar radiation, 66% of the variation in incoming long-wav iation, 87% of the variation in emitted long-wave radiation, 26% of the variation in net long-wave radiation and 55% of the variation in net radiation balance. Mean absolute error in simulated values ranged from 10 w m-2 fro emitted long-wave radiation to 27 W m-2 for the entire net radiation balance. Mean bias error ranged from 8 W m-2 for emitted long-wa diation to -16 w m-2 for the entire net radiation balance. When the entire 170 days of simulation, which included periods without snow cover, were included in the analysis, the variation in observed values increased