PLANNING AND EXECUTION OF FIELDWORK

Criteria for Choice of Fieldwork Area  The site used for fieldwork had to satisfy two main crite-ria: 1) the need for a diverse range of slopes, elevations and aspects; 2) the need for wet and dry snow to be pre-sent within the site to permit the signatures of both to be characterised. Additionally, the site needed to be rea-sonably accessible, easy to traverse and safe. Prior to the fieldwork neither the extent of the snowpack nor the level of the freezing line were known. Hence a number of potential survey routes were planned for a suite of likely snow cover conditions on the day of the field-work.

Development of the Winter Snowpack  The very mild winter of 1997/98 prevented the devel-opment of an extensive seasonal snowpack in the Spey basin, so that two weeks before the start of the proposed fieldwork there was almost no snow in the catchment. However, an extensive snowfall event did occur between 2-3 March which resulted in the whole of the Spey basin receiving some snow. A thaw over the weekend of 6-8 March caused the removal of snow from lower areas of the basin, but westerly winds induced drifting at higher altitudes.

The Area used for Fieldwork  Snow conditions dictated that the only viable survey traverses were those radiating from the Cairn Gorm Ski Centre. The ski area itself was avoided because back-scatter is likely to be affected by the ski lifts and snow fences. Snow cover observations were taken along tran-sects covering: the Coire an t-Sneachda sub-basin; the vicinity of Cairn Gorm summit, and the Coire Laogh Mór sub-basin.

Fieldwork Observations

Observations were taken over a range of elevations and aspects along the three transects. The observations taken included GPS-derived position, elevation, slope, aspect, snow depth, the extent of snow, protruding vegetation and bare ground, the depth of wet snow horizons and underlying vegetation type. A number of qualitative observations were also made: snow surface hardness, snow/vegetation/soil thermal state (frozen, dry, wet), the presence of saturated soils underlying the snow cover and the presence and characteristics of snow surface windforms. These measurements were used to charac-terise snow cover conditions within 100´100 m survey sites along the three traverses. The size of the sites was considered to be representative of post-processing SAR image resolution.

The snow depth, hardness and wetness characteristics were sampled at 5 points within each site. Sampling involved following a checklist which only allowed the estimates of snow, vegetation and bare ground extents to be placed into 5 categories (1-20%, 21-40%, 41-60%.......). Although this reduced the resolution of the potential extent estimates, the procedure will have re-duced user-based variance considerably. Photographs were also taken in areas below the cloud base.

Timing of Fieldwork  Surveying was planned to coincide with the SAR de-scending pass at 11:16. Poor weather conditions resulted in a late start to the surveying (due to closure of the ac-cess road) and significantly affected navigation once the surveying had begun. As a result, observations were collected between 10:30 and 14:45 with only 38 sites being characterised over this interval.

Last updated: 13th October, 1998 by Roger Dunham