A feasible method for fractional snow cover mapping in boreal zone based on a reflectance model Sari J. Metsa ¨ma ¨ki * , Saku T. Anttila, Huttunen J. Markus, Jenni M. Vepsa ¨la ¨inen Finnish Environment Institute, Geoinformatics and Land Use Division, Mechelininkatu 34a, P.O. box 140, Helinski, FIN 00251, Finland Received 19 July 2004; received in revised form 1 November 2004; accepted 28 November 2004 Abstract A feasible method for mapping the fraction of Snow Covered Area (SCA) in the boreal zone is presented. The method (SCAmod ) is based on a semi-empirical model, where three reflectance contributors (wet snow, snow-free ground and forest canopy), interconnected by an effective canopy transmissivity and SCA, constitute the observed reflectance from the target area. Given the reflectance observation, SCA is solved from the model. The predetermined values for the reflectance contributors can be adjusted to an optional wavelength region, which makes SCAmod adaptable to various optical sensors. The effective forest canopy transmissivity specifies the effect of forests on the local reflectance observation; it is estimated using Earth observation data similar to that employed in the actual SCA estimation. This approach enables operational snow mapping for extensive areas, as auxiliary forest data are not needed. Our study area covers 404 000 km 2 , comprising all drainage basins of Finland (with an average area of 60 km 2 ) and some transboundary drainage basins common with Russia, Norway and Sweden. Applying SCAmod to NOAA/AVHRR cloud-free data acquired during melting periods 2001–2003, we estimated the areal fraction of snow cover for all the 5845 basins. The validation against in situ SCA from the Finnish snow course network indicates that with SCAmod , 15% (absolute SCA-units) accuracy for SCA is gained. Good results were also obtained from the validation against snow cover information provided by the Finnish weather station network, for example, 94% of snow-free and fully snow-covered basins were recognized. A general formula for deriving the statistical accuracy of SCA estimates provided by SCAmod is presented, complemented by the results when the AVHRR data are employed. Snow melting in Finland has been operatively monitored with SCAmod in Finnish Environment Institute (SYKE) since year 2001. The SCA estimates have been assimilated to the Finnish national hydrological modelling and forecasting system since 2003, showing a substantial improvement in forecasts. D 2004 Elsevier Inc. All rights reserved. Keywords: Snow-covered area; Reflectance model; Boreal zone; Hydrological model; Snow mapping 1. Introduction The boreal zone is characterized by a thick seasonal snow pack. As a temporary water storage, snow is a valuable source of energy when a large volume of water is released during the melting process. On the other hand, sudden runoff raises a risk of flooding in certain areas. The high albedo of snow is an important climatological factor that affects the earth’s radiation budget for several months during the year. Thus, the snow melting process is also an essential part of hydrological models used for simulating and forecasting the runoff (Lide ´n & Harlin, 2000; Lindstro ¨m et al., 1997; Vehvila ¨inen, 1994). Evidently, these models would benefit from reliable information on the extent of snow cover (Ramamoorthi, 1987), as typically they include the fraction of snow-covered area (SCA) in a drainage basin as one of the parameters. Remote sensing can provide spatially and temporally well- distributed information for snow hydrology (Grayson, Blo ¨schl et al., 2002; Kite & Pietroniro, 1996; Rango, 0034-4257/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2004.11.013 * Corresponding author. Tel. +358 9 40300664; fax: +358 9 40300690. E-mail address: sari.metsamaki@ymparisto.fi (S.J. Metsa ¨ma ¨ki). Remote Sensing of Environment 95 (2005) 77 – 95 www.elsevier.com/locate/rse