Routines have been developed that allow the calculation of snow depth and density, water storage in snow and water melting from the snow pack, using daily input of air temperature. The original snow model was developed at the University of Helsinki by Vehviläinen & Lohvansuu (1991) for calculating water equivalent, but modified by Tuomo Karvonen to calculate snow depth, which is important for determining soil freezing and thawing. We have further modified this model and have calibrated it by iterative simulation using a 10-year dataset from Norway, as described by Riley and Bonesmo (2005). The approach has later been validated with independent data.

The chosen soil frost model is based on two approaches, one for freezing and one for thawing. The approach for soil freezing was proposed by Olsen and Haugen (1997), at the Norwegian University of Life Sciences, Ås, assuming uniform thermal properties throughout the profile. Values for the latter properties are taken from the Swedish SOIL model (Jansson, 1991). The model requires input of surface temperature as modified by the snow pack. The approach used for thawing is that in the ECOMAG model developed at the University of Oslo (Molitov et al., 1999). We have validated both freezing and thawing processes for Norwegian conditions.

The snow and frost calculation procedures, including all parameters used, are described in detail in a programming note (Riley, 2004a). This note also describes how these processes interact with water infiltration and associated processes such as leaching. In brief, it is assumed that infiltration ceases when soil freezes. During snowmelt and soil thaw, an amount of water equal to the difference between field capacity and total porosity is stored for later infiltration, whilst the remainder passes to surface runoff. An example of the interaction of frost with runoff appeared in an article in the second project newsletter (Riley, 1994b).

Literature

Jansson, P.E. 1991. Simulation model for soil water and heat conditions. Report 165, Swedish University of Agricultural Sciences, Uppsala, 72 pp.

Karvonen T. (internet publication). Model of snow accumulation and snowmelt. www.water.hut.fi/wr/kurssit/Yhd-12.135/kirja/paa_e.htm

Motovilov, Y.G., L. Gottschalk, K. Engeland, & A. Belokurov,1999. ECOMAG: Regional model of hydrological cycle. Application to the NOPEX region. Dept. Geophysics, University of Oslo, Report no. 1051, 88 pp.

Olsen, P.A.& L.E. Haugen, 1997. Jordas termiske egenskaper. Dept. Soil and Water Science, Agricultural University of Norway, Rapport nr. 8, 14 pp. (in Norwegian).

Riley, H. 2004a. Description of submodels on snow dynamics and frost depth. Programming note of 18th June 2004, 9 pp.

Riley, H. 2004b. New submodel: Snow and Frost Article intended in newsletter in June 2004,

Riley, H. and Bonesmo H. 2005. Modelling of snow and freeze-thaw cycles in the EU-rotate_N decision support system. Planteforsk Grønn Kunnskap (e) vol. 9, no. 112, 8 pp.

Vehviläinen, B. & Lohvansuu, J. 1991. The effects of climate on water discharges and snow cover in Finland. Hydrological Sciences 36 (2): 109-121.