At both sites, we set up a similar experiment. On a delimited soil plot (6 m2), the different components of the water balance were measured, that is, precipitation, snow water equivalent, surface runoff, subsurface runoff and deep percolation.
At the lower edge of the plot, surface and subsurface runoff were collected from the depth intervals 0 to 3 cm and 3 to 28 cm, respectively (Figure 7.3). The collecting gutters were filled with gravel and sand so
as to diminish the capillary barrier between the soil and the collecting device, and to allow water to drain to the two collecting gutters under unsaturated conditions. The gutters were equipped with heating wires to prevent water from freezing inside the apparatus. The discharge was measured manually the first winter (once per day), and automatically with a tipping bucket the second winter. Lateral inflow from the terrain above was prevented by a 50-cm-deep open ditch. However, lateral inflow through the snowpack was not inhibited.
On an adjacent plot, the deep percolation was collected at a depth of 40 cm using an open lysimeter with a surface area of 0.52 m2 (see Figure 7.4). To maintain a hydraulic connection between the lysimeter and the undisturbed soil, the excavated gap was filled with sand and gravel.
Additionally, we measured the liquid soil water content with Time Domain Reflectometry, the soil temperature at depths of 5, 10, 20 and 30 cm using thermocouples, as well as the short wave solar radiation with a pyranometer. Complementary meteorological data (air temperature, precipitation, sky cover, wind speed, relative humidity) were received from the MeteoSwiss station in Grachen.
In both winters, a dye tracer experiment was set up to observe the development of the water flow paths from the surface down to the bedrock (60-80 cm depth) along the route of the snowmelt. In autumn, we selected suitable plots with a length of approximately 5 m and a width of 1 to 1.5 m. At the beginning of the winter, a dye tracer was applied on the selected plots. We used the food dye Brilliant Blue FCF, which has been used in numerous soil physical field studies (e.g. Forrer et al. 2000). Brilliant Blue FDF is non-toxic, well visible in normal field soils and, depending on the pH, either neutral or anionic with a rather high mobility. During the spring snowmelt, we returned to the sites and excavated vertical profiles on the tracer plots, starting at the onset of the snowmelt and finishing shortly after the complete melt of the snow. For each date, we excavated two to four profiles. Each profile was then photographed using a digital camera (Nikon Coolpix 990) at a pixel resolution of less than 1.5 mm.
The digital images (Figure 7.5) were processed with the following method (details in Stahli et al. 2004). The average level of the color channels was standardized to a reference common for all profiles. Then, we applied a supervised classification procedure to separate the pixels stained with Brilliant Blue tracer from the unstained profile pixels. Next, the image analysis operations erosion and dilation eliminated erroneously classified single pixels and filled up small gaps in connected stained
areas. Finally, the depth distribution of the fraction of stained areas was plotted.
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