Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 3/31/1996
Publication Date: N/A
Interpretive Summary: Accurate prediction of snowmelt is critical to the forecasting of floods, and it is also needed for predicting the movement of contaminants on and within the soil. The Simultaneous Heat and Water (SHAW) model simulates heat, water, and solute flow in a soil/snow/atmosphere system, but some of its components have not been tested, primarily due to the difficulty in making the necessary measurements. We tested the performance of the model in simulating sensible heat flux, which is a critical process during snowmelt, in a farm field at Rosemount, MN. We measured the parameters necessary to operate the model, and we also measured sensible heat flux by eddy correlation for a 31 day period beginning on February 14, 1994. The model agreed reasonably well with measurements, with some discrepancies. During periods of strong stability, when the sensible heat flux was toward the snow surface, the model tended to overestimate the heat flux. Since predictions of surface temperature were reasonably accurate, the model was under estimation of aerodynamic resistance during these periods. After the snow was gone, but the soil below was still frozen, the model tended to overestimate midday sensible heat fluxes away from the surface. In this case the aerodynamic resistance measurements appeared to be accurate, and the errors in heat flux were due primarily to errors in estimation of surface temperature. These results will be used to further improve model performance.
Technical Abstract: Prediction of spring floods and solute transport requires prediction of snowmelt in areas where snow occurs.We tested the Simultaneous Heat and Water (SHAW) model, which simulates spring thaw within the broader context of a soil/plant/residue/atmosphere system. The model has been in use for several years, and portions of it have been tested, but there has not been an examination of the surface/atmosphere heat exchange component, a key factor during snowmelt. We tested it by comparing its predictions against 31 days of direct measurements of sensible heat flux made by eddy covariance during the snowmelt period in 1994 in Rosemount, MN. The model assumes that sensible heat transfer is proportional to the difference between surface and air temperature and inversely proportional to aerodynamic resistance, which is a function of windspeed and surface roughness. Air temperature is an input to the model, while surface temperature is an output, iteratively determined at each timestep by balancing all energy exchange at the soil surface. The model agreed reasonably well with the measurements, though there were times when they diverged. The model tended to underestimate aerodynamic resistance during periods of strong stability, overpredicting sensible heat flux toward the snowpack when it was melting. After the snow was gone, the model predicted aerodynamic resistance well, but tended to overestimate midday soil surface temperatures, causing overestimation of sensible heat flux away from the surface.