Terrain Characteristics and Snow Cover Variability
in Mountain Areas: An Example from the Spanish Pyrenees
Steve Anderton1, Bernardo Alvera2, and Sue White3
An evaluation of the spatial variability of snow water equivalent (SWE) in a small headwater basin
in the Spanish Central Pyrenees is presented here. This work constitutes the first stage of a project
whose overall objectives are to assess the causes of spatial variability in snow processes in
mountainous environments, and to parameterize this variability into hydrological models in a phys-
ically based manner in order to assess its hydrological effects.
Continuous monitoring of meteorological and flow data has been carried out at the experimental site
since 1984. In addition, intensive field campaigns took place during the 1997 and 1998 melt seasons,
involving approximately weekly sampling of snow depth and density at a large number of points
throughout the basin.
As a preliminary exploration of the causes of the observed spatial variability in SWE within the
basin, the strength of relationships between observed SWE and a number of terrain characteristics
was evaluated. These terrain characteristics, elevation, slope gradient, aspect and curvature, and
deviation of pixel elevation from a neighborhood mean elevation, were derived from a 1-m resolu-
tion basin DEM. A simple terrain-based model of potential direct solar radiation input, taking into
account slope gradient aspect and topographic shading, was also constructed. It was found that char-
acteristics describing slope form (slope curvature, and deviation of pixel elevation from neighbor-
hood mean) exhibited the strongest relationship with the spatial distribution of SWE, suggesting a
topographic control on snow accumulation and redistribution. SWE distribution also displayed a
secondary, but nonetheless significant, correlation with potential direct solar radiation input, demon-
strating a topographic control on energy inputs during the melt season.
Scale effects were also evident, with stronger relationships between observed SWE and areal aver-
ages of the key terrain characteristics described above than for raw pixel values. This would seem
physically reasonable, because snow accumulation tends to smooth out small-scale terrain variabil-
ity. In addition, snow cover, by virtue of its high albedo, also promotes reflection of radiation from
adjacent terrain, thereby reducing small-scale variability in solar radiation flux density.
Water Resource Systems Research Laboratory, Department of Civil Engineering, University of Newcastle
upon Tyne, Claremont Road, Newcastle upon Tyne NE1 7RU, UK.
2 Instituto Pirenaico de Ecologa, Avda. Regimento de Galicia s/n, 22700 Jaca, Spain
3 School of Engineering, University of Durham, South Road, Durham DH1 3LE, UK.