The partitioning of water released from a melting snowcover into surface runoff and soil infiltration is a crucial topic for a number of environmental questions, such as erosion or solute leaching. It is highly dependent on the extent to which the underlying soil is frozen. There is experimental evidence for snowmelt infiltration into frozen soil, although the liquid soil water which is adsorbed by the particles has a very low hydraulic conductivity. Previous research has shown that snowmelt infiltration has to do with the saturation of the soil, i.e., with the amount of air-filled pore space. In our view, these air-filled pores have to be accounted for as a second water flow domain when the infiltration into a frozen soil needs to be quantified. The concept of two flow domains has recently be introduced into an established one- dimensional soil water and head model (SOIL) and first applications showed an improved prediction of the timing of runoff. However, it is difficult to assess a hydraulic conductivity for the water moving through the air-filled pore domain. Another uncertainty is related to the amount of infiltrating water that refreezes while passing the frozen soil.
With the aim to test and improve the new model concept, a small-plot experiment was performed at Uppsala (Central Sweden) during the winter 1995/96. It included soil profile measurements of temperature, liquid and total water content, as well as a thorough climate monitoring. The soil plots (2 x 2 m, 1.4 m deep) were separated with a rubber sheet which allowed to quantify the percolation by monitoring the ground water level. The winter 1995/96 turned out to be the coldest since the fifties at Uppsala with a frost depth down to more than 70 cm.
Results and analysis of this experiment will be one major task of the presentation. In addition, the new model concept will be presented in detail and applied to the small-plot experiment. A special focus will be put on the quantification of water refreezing in the air-filled pore domain.
Manfred Stahli SLU, Department of Soil Sciences Box 7014 S - 750 07 Uppsala, Sweden Tel.: 46 18 67 29 29 Fax.: 46 18 67 27 95 E-mail.: manfred.stahli@mv.slu.se http://bgfserver.mv.slu.se/markvet/bgf/defeng.htm