Numerical modeling of soil frost and groundwater recharge the COUPmodel

Conceptually, the COUP-model consists of a soil profile with a number of layers of (measured or estimated) physical properties. Water and heat flow between the layers is calculated with the well-known Richards equation and Fourier's law assuming homogeneous, nonpreferential fluxes. Hourly or daily values of standard meteorological variables are the driving forces for the model, and parameterized hydrological properties characterize the soil profile. With regard to winter conditions, COUP simulates a snow cover assuming uniform properties, such as thermal conductivity, liquid water retention or density. Melting of the snowpack is based on a complete surface energy balance calculation. At freezing, both soil water and heat fluxes are coupled. If the soil temperature decreases below 0°C, a user-defined freezing characteristic curve defines the partitioning of heat loss into a latent part (transformation of water to ice) and a sensible part (temperature decrease). During melt periods, the infiltration capacity of the frozen soil depends to a large extent on the available air-filled porosity. Surface runoff, lateral subsurface runoff (from saturated soil layers), as well as deep percolation are examples of resulting outputs from the simulation. The model package is freely available at http://www.lwr.kth.se/english/OurSoftWare/.

The model was applied in the area of Grachen to five different elevation zones ranging from 1600 m to 2600 m. In a first step, we used the field experiment at Hannigalp (2100 m) to calibrate and validate the soil/snow thermal and hydraulic parameters in COUP. The second step consisted of extrapolating the meteorological data, assuming a mean air temperature gradient of —0.88°C/100m (average winter gradient between Grachen and Hannigalp), an altitudinal variation in the radiation of 1.1 Wm—2/100m (Marty 2001), and, from adjacent meteorological stations, a precipitation

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Percolation Surface runoff Subsurface runoff Accum. percolation Accum. surface runoff Accum. subsurface runoff

Figure 7.6 Snow and frost depth (a), measured deep percolation, surface runoff and subsurface runoff (daily discharge and total accumulation) for spring 2001 (b) and 2002 (c) at Hannigalp

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Figure 7.6 Snow and frost depth (a), measured deep percolation, surface runoff and subsurface runoff (daily discharge and total accumulation) for spring 2001 (b) and 2002 (c) at Hannigalp increase of 2%/100m. Finally, the simulated snow and frost depths were compared with the water-table rise in spring measured at Grachen.

7.3 SELECTED RESULTS FROM THE TWO-YEAR FIELD EXPERIMENT

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