No abstract

No abstract

No abstract

No abstract

Abstract. Infiltration into frozen soil is a result of the whole climate dynamics of the preceding winter with all its importance for the freezing of the soil. Therefore a predictive infiltration model needs to include a proper description of the main processes of soil water and heat transfer during season-long periods. Such a model may assume two waterconducting flow domains. A lysimeter experiment was set up with the aim of studying these processes in two different sandy soils. Frequent measurements of total and liquid soil water content, soil temperature, and groundwater level were made during two winters with contrasting meteorological conditions. The main problems in the simulation of the two winters were (1) frost-induced upward water redistribution, (2) rate of infiltration in the initially air-filled pores, and (3) heat transfer caused by snowmelt refreezing in the frozen soil. An extensive calibration of the model suggested that some key empirical parameters were not constant for the two soils and the two seasons. Complementary methods for determining the hydraulic conductivity of frozen unsaturated field soils are necessary to further improve the model. IntroductionKnowledge of hydraulic processes in frozen soils are of particular practical importance in Nordic and high-altitude regions. The snowmelt periods at the end of the winter have a major influence on the hydrological environment. When these occur, the amount of meltwater may be very large, causing erosion where it runs off on the surface or solute leaching where it infiltrates. Consequently, the soil and its varying infiltration capacity plays a crucial role. Usually, it is still frozen, at least partly, when snowmelt occurs, which reduces the hydraulic conductivity. However, it is important to stress that frozen soil is not impermeable to infiltration. A number of field studies [e.g., Kane, 1980] where 0to t is the total water content, 0i½ e is the ice content, and 0•f is the water content in the low-flow domain. Stadler [1996] argued that (2) overestimates khf for the case of high saturation because it does not account sufficiently for the water flow-impeding effect of ice. In fact, with a high ice content the water-conducting pores are assumed to be blocked by ice and the tortuosity increases markedly. Following Stadler's ideas a reduction term was added in the hydraulic conductivity function of the high-flow domain (equation (2) where fci is an impedance factor and Q is the thermal quality of the soil, that is, the mass ratio of frozen water to total amount of water. The impedance factor will be a point of discussion later in this paper (section 4). Experimental SetupFour small lysimeters (2 x 2 m, 1.4 m depth) were installed at Ultuna (central Sweden) and identically instrumented (Figure 2): TDR probes and thermocouples were placed at six different soil depths (10, 17, 25, 35, 50, and 70 cm) to monitor profiles of liquid soil water content and temperature. A tube was installed vertically for measuring the total water content (ice plus li...

No abstract

Earlier modelling studies have shown the difficulty of accurately simulating snowmelt infiltration into frozen soil using the hydraulic model approach. Comparison of model outputs and field measurements have inferred the occurrence of rapid flow even during periods when the soil is still partly frozen. A one-dimensional, physically based soil water and heat model (SOIL) has been complemented with a new two-domain approach option to simulate preferential flow through frozen layers. The ice is assumed to be first formed at the largest water filled pore upon freezing. Infiltrating water may be conducted rapidly through previously air-filled pores which are not occupied by ice. A minor fraction of water is slowly transferred within the liquid water domain, which is absorbed by the solid particles. A model validation with field measurements at a location in the middle-east of Sweden indicated that the two-domain approach was suitable for improving the prediction of drainage during snowmelting. In particular, the correlation between simulated and observed onset of drainage in spring was improved. The validation also showed that the effect of the high flow domain was highly sensitive to the degree of saturation in the topsoil during freezing, as well as to the hydraulic properties at the lower frost boundary regulating the upward water flow to the frozen soil and ice formation.

No abstract