Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 3, 1998
Publication Date: N/A
Interpretive Summary: In regions with significant snowfall, the spring snowmelt is often equivalent to the largest rainstorm of the year. During snowmelt, water can move into and through the soil to replenish groundwater or it can flow over the soil into rivers and lakes. When snow melts on agricultural fields, the meltwater can carry nutrients and pesticides with it. The objective of this study was to improve our understanding of the snowmelt process in agricultural fields to begin developing recommendations to reduce water quality impacts of snowmelt. Sensors were placed in a no-till corn field in Iowa and monitored through the winter of 1994-95. There was continuous snow cover for over 70 days then a rapid melt period. After a few days, additional snowfall occurred that melted several days later. Both snowpacks melted very quickly. In the first instance the soil was still frozen so most of the meltwater ponded in depressions on the surface. .The second melt period coincided with the thawing of the soil so most of the meltwater moved into and through the soil profile. It was concluded that, in order to predict the melt process accurately, careful measurements of the surface temperature and roughness were required.
Technical Abstract: Transport of agricultural chemicals in runoff and recharge waters from snowmelt may represent a significant event in terms of annual contaminant loadings in temperate regions. Improved understanding of the melt dynamics of shallow snowpacks is necessary to fully assess the implications for water quality. The objective of this study was to measure the energy balance components of a corn (Zea mays L.) stubble field during the meltin of its snowcover. Net radiation (Rn), conductive soil (G), sensible (H), and latent (Q) heat fluxes were measured in a field near Ames, IA during the winter of 1994-95. Energy consumed by melting including change in energy storage of the snowpack was determined as the residual of the energy balance. There was snowcover at the field site for 71 days (maximum depth = 222 mm) followed by an open period of 11 days before additional snowfall and a second melt period. The net radiation and snowmelt/energy storage (S) terms dominated the energy balance during both measurement intervals. Peak daily sensible and latent heat fluxes were below 100 W m**-2 on all days except the last day of the second melt period. There was good agreement between predicted and measured values of H and Q during the melting of an aged snow layer but poorer agreement during the melt of a fresh snowpack. Both snowpacks melted rapidly and coincident changes in soil moisture storage were observed. Improved estimates of Q and H, especially for partially open surfaces, will require better characterization of the surface aerodynamic properties and spatially-representative surface temperature measurements.