Estimating drainage area-based snow-cover percentages from NOAA AVHRR images
During the melting season in spring, up-to-date information on the areal extent of snow cover is needed; for example for hydrological modelling and hydropower production. A method for estimating drainage area-based snow-cover percentages from NOAA AVHRR images was developed and tested by using data from the melting season of 1999 in Finland. When estimating the snow-cover percentage of a drainage area, the reflectance of the area is compared with the reflectance of the same area in the conditions of full snow cover and with the reflectance of snow-free ground. It is assumed that the mean reflectance of each land-cover class in the drainage area decreases linearly as snow-free ground appears. By estimating the percentages for drainage areas rather than pixels and using local comparisons rather than overall threshold values, some problems related to varying land cover and imaging geometry, as well as small inaccuracies in image registration, can be reduced. The drainage area is also a feasible unit for further use of the results; for example in hydrological models. The results were compared with ground measurements and appeared to be satisfactory. Further research, however, is needed to account for various effects that can change the reflectance of the snow cover and forest during the melting season. Along with the research work, operational use of the developed method began in 2000.
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