Movement of snowmelt freshet occurs in two forms, vertical flow (infiltration, percolation) and horizontal flow (baseflow, overland flow). Liquid water percolate into lower layers in the snowpack when the total volumetric capacity of snowpack exceeds the water retention capacity. During the time of rapid snowmelt, overland runoff occurs when the infiltration rate becomes lower than snowmelt rate. Figure-4 shows the snowmelt freshet movement for different soil condition. Water follows horizontal unsaturated zone into stream (Hirashima et al., 2010). Canadian prairie is characterized as cold semi-arid climate with clay-rich soils and underlying glacial till. Snowmelt cause significant amount of runoff in upland. Prairie topography allow to hold a significant amount of water storage in the snowmelt period, but high evaporation and low precipitation dry up the soil by late fall. Upland snow accumulation, evaporation, antecedent soil moisture condition are important features for prairie snowmelt freshet movement (Shook et al., 2013; Winter & Rosenberry, 1995; Woo & Rowsell, 1993).
|Figure- Movement of snowmelt freshet for different conditions (a) water table percolating meltwater into stream, (b) impermeable water table, and (c) lower portion of water table rise above ground (Dingman, 2002)|
Infiltration into frozen soil is a complex coupled heat and mass transfer process. These processes are dependent on the hydro-physical and thermal properties of the soil, the antecedent soil moisture, temperature regimes, the rate of release of water from the snow cover; and the energy content of the infiltrating water. The macro pores and cracks in the soil are inactive in frozen state, because the presence of ice refreezes and reduces the effective porosity. Saturated frozen soil do not have enough macropores and considered as impermeable. Infiltration in unsaturated frozen soil varies with respect to SWE of top snow cover and soil moisture content (Pomeroy & Brun, 2001). The ability of absorption and transmittance of the soil drives the water into infiltration and percolation. The soil absorption capacity in the Prairies is denoted as unlimited, limited and restricted, where unlimited capacity indicates soil with large, connected, air filled macropores and cracks; limited capacity indicates infiltration zone of 0- 300 mm soil layer; and restricted capacity indicates impervious soil layer at shallow depth. In the Prairies, most of the snowmelt freshet is intercepted by the prairie depression or sloughs. Hayashi et al. (1998) shows that about 25% of water evaporates and rest of the water infiltrates. Infiltration flows laterally into the shallow subsurface. Evaporation is dominant process in the upland of Prairie regions. The net recharge rate of aquifer is very low (about 1-3 mm/year).
During time of initial snowmelt period, evapotranspiration is relatively low and the soil condition is frozen and saturated with water. This phenomenon lead to less infiltration and high overland runoff. This promotes excess water in the watershed and steep hydrograph peak just after the inception of snowmelt (Pomeroy & Brun, 2001).
Prairie snowmelt runoff is vastly dependent on topography and antecedent soil moisture condition. The effect of topography in Prairie snowmelt runoff was overlooked initially (Gray et al., 1985). Later Euliss et al. (2004) shows the linkage of groundwater connectivity of prairie pothole with nearest outlet, which was partially valid for the case of wetlands. Van der Kamp & Hayashi (2008) shows the mechanism of lateral groundwater flow in the prairie watershed. Runoff in a watershed normally begins when the total melt water or excess rainfall exceeds the threshold storage capacity of topography. In the prairies, melt water rarely exceeds the threshold capacity. As a result, water is stored in the local depressional storages and evaporates or infiltrates locally. A significant portion of prairie watershed do not contributes to the outlet. Prairie potholes have a substantial storage capacity. These potholes show a dynamic behavior in storing water. They develop an interlinked hydraulic connectivity when they are filled with water and disconnects when excess water is spilled (Mekonnen et al., 2014; Shook et al., 2013). A surface water survey at St. Denis in 2006 reveals that only 39% of the watershed contributes actively to the outlet. Anthropogenic features also affects the spatial distribution of contributing area. This ‘non-contributing area’ affects the stream flow and develop a dynamic precipitation- runoff relationship which is a function of snow accumulation, antecedent soil moisture condition, melt rate etc. (Shaw et al., 2012).
In the winter, infiltration becomes inactive, because of frozen soil. Gray et al. (1985) classified the infiltration capacity into unlimited, limited and restricted based on soil condition. About 75% of stored water infiltrated into shallow subsurface. Snowmelt runoff is intercepted by the local depressional storages. Storage capacity is also dependent on antecedent soil moisture condition. Prairie pothole intercepts snowmelt freshets and makes a significant portion of the watershed as non-contributing area to the outlet. When the potholes become full, they establish a temporary local connectivity and contributes to the outlet.
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