SNOWMELT WATER AND ENERGY FLUX UNDER EPHEMERAL PONDS IN THE CANADIAN PRAIRIES

The Northern Prairies region of North America is characterized by undulating topography with numerous hydrologically closed depressions. Snowmelt runoff in the prairies occurs on the frozen ground and flows into depressions to form ponds. There have been many studies on seasonal and semi-permanent prairie wetlands, which occur in relatively larger depressions that usually hold water for a few months or longer. However, little attention has been paid to ephemeral ponds forming in smaller depressions, which are usually cultivated over, but far more abundant than prairie wetlands. These ephemeral ponds are short-lived (< 1 month) but play an important role in the hydrology and ecology of the prairies, by storing runoff water, recharging soil moisture and shallow groundwater, and by providing food and habitat for many organisms. Infiltration of snowmelt water under small depressions (< 1000 m2) was studied in the St. Denis National Wildlife Area in Saskatchewan, Canada. The depressions stored most of the snowmelt runoff generated within their respective watersheds. The water level in a typical depression declined at 0.6 mm/hr while the underlying frozen soil was thawing. This rate was likely limited by the rate of downward movement of the thawing front, which was functioning as a moving impermeable boundary. The rate of thawing was controlled by the external energy input from the ground surface. Standing water in depressions efficiently trapped incoming radiation and conducted it to the thawing front. The recession rate increased to 1.1 mm/hr when the soil under the depression thawed completely. The thawing front under each depression had a highly irregular shape with localized area of preferential thawing, thereby creating preferential flow paths for snowmelt infiltration. This may have a significant implication in the contamination of shallow groundwater since snowmelt runoff frequently contains residue of farm chemicals.