|Spaans, Egbert - UNIVERSITY OF MINNESOTA|
Submitted to: International Symposium on Physics Chemistry and Ecology of Seasonally Froz
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 12, 1997
Publication Date: N/A
Interpretive Summary: Late winter/early spring is an important time of year hydrologically in the Upper Midwestern United States. The combination of melting snow, spring rain, and frozen soil often creates runoff that collects in depressional areas within farm fields. For accurate flood forecasting and for estimation of agricultural impacts on groundwater quality, it is important to predict how much snowmelt infiltrates in place, how much flows overland to these ephemeral ponds, and how the ponded water eventually moves through the soil. We conducted research to address these questions by developing instrumentation capable of simultaneously measuring pond formation and infiltration and the associated soil moisture and temperature profiles beneath the pond. The results were at first glance paradoxical. The ponds form because the frozen soil beneath the snowpack is frozen and relatively ice-rich, limiting its permeability. However, the ponds that are formed can ninfiltrate rapidly, even while much of the soil profile is still frozen. This may occur after the thawing front has penetrated beyond the ice-rich surface layer. There appears to be little that can be done to control the permeability of these ponds, but it may be possible to prevent their formation or limit their size through cultural practices that maximize air-filled porosity at the soil surface. These results may be useful in designing site-specific tillage and residue management to retain more snowmelt in place on the landscape.
Technical Abstract: The disposition of water that is released as snow melts in agricultural fields is of considerable importance with respect to flooding, solute transport, and recharge of soil moisture depleted during the previous growing season. We have made a broad range of measurements during the snowmelt season for several years at the University of Minnesota Agricultural Experiment Station in Rosemount, MN. In level portions of untilled fields the contribution of snowmelt to moisture recharge is generally small (less than 20 percent of overwinter liquid equivalent precipitation). Evaporation is also not a large sink for water from the snowpack; model estimates suggest that it accounts for less than 20 percent of the overwinter precipitation. Most of the water released during snowmelt find its way to low points within the field, resulting in the formation of large ephemeral ponds, which can form within a day to depths approaching 1 m. Infiltration is often almost as rapid, despite the presence of a frozen layer in the soil below.