Temporal Variation in Snowcover Area During Melt in Prairie and Alpine Environments

Abstract

Information on the temporal variation in snow-covered area of watershed during melt is requisite for accurate predictions of runoff. The amount of the gross watershed area that is snow-covered affects runoff primarily in two ways: a) it influences the melt rate, because patches of bare ground affect the energy balance of the snow field, and b) it governs the contributing area of runoff. This paper examines the area-frequency and perimeter-area characteristics of soil and snow patches that form during ablation of seasonal snowcovers in Prairie and Alpine environments. It uses fractal geometry as a basis for quantifying these properties. Image analyses are applied to aerial photographs taken during snowmelt on two small watersheds: one in the West-central part of the Province of Saskatchewan in the Canadian Prairies, the other in the alpine region of the Austrian Alps. The results of the study suggest that the soil and melting snow patches behave as fractals, that is their perimeter-area and area-frequency characteristics can be described by simple power equations with patch area. The perimeter-area ratio of the soil and snow patches decreases with increasing size of patch, but at a smaller rate than for Euclidean objects. The area-frequency characteristics of snow patches follow a hyperbolic distribution with relatively few large patches and numerous small patches. It is suggested that the soil and snow patches have the same fractal dimension. It is concluded that snow patches are not random and their size distribution is predictable. The variation in the edge length of a snow field per unit basin area during ablation is demonstrated. A maximum value of the ratio is reached when a basin has 45-65 % snowcover. With snow coverage in this range the potential for local advection increasing melt under a specific set of climatic conditions is greatest.