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Report: Assessing the "Snow Sensitive Zone" at Multiple Scales in the Kettle River Watershed, Southern British Columbia

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A method was developed to identify the snow sensitive zone (SSZ), the area contributing snowmelt during the peak flow period, for eight watersheds in the Kettle River basin. Snow covered area (SCA) was mapped at the onset of the peak flow period for 2010-2020. The SSZ was identified as the median SCA for this period. The SSZ maps can inform planning forest harvest activities to reduce the potential for snowmelt synchronization and increased peak flow.

Author:  Natasha Neumann

Date Published:  Feb 2022

Report ID:  59526

Audience:  Government and Public

The area contributing snow meltwater during the peak flow period is referred to as the "snow sensitive zone" (SSZ) of a watershed. Forest harvest and other forms of vegetation cover disturbance within the SSZ alters the accumulation and melt of snow relative to adjacent forest stands, potentially leading to the synchronised delivery of meltwater from multiple parts of the catchment that had previously been decoupled. Identification of the SSZ, then, is important for planning harvesting and resource development activities to minimize impact on snowmelt-generated peak flows. In British Columbia, early guidelines recommended estimating the SSZ using the H60, the elevation above which 60% of the total basin area lies. Attempts have been made to validate the H60 concept in the southern interior of B.C. using visual analysis of aerial photographs, efforts that were costly and labour intensive and therefore limited in geographic coverage. The more recent availability of modelled spatial snow cover datasets has opened the opportunity to assess the SSZ across larger areas than was possible in the past. This report describes the method developed to map the SSZ using modelled 1 km2 spatial resolution SNODAS snow water equivalent (SWE) product to identify the snow covered area (SCA) at the onset of the peak flow period. The method was applied to eight nested catchments in the Kettle River basin in the Boundary area. The nested catchments ranged in area between 148 and 9943 km2 and spanned from dry valley bottoms to forested mountains that accumulate more than 800 mm SWE per winter on average. Maps of SCA at the onset of the peak flow period were derived for 2010 to 2020, demonstrating variability in melt timing but consistent patterns in snow melt from year to year. The median SCA for the 2010-2020 period was used to delineate the SSZ for each study basin, and the elevation of the SSZ lower limit was estimated from hypsometric curves. Modelled climate normal data and a hydrologic model were used to verify aspects of the SSZ in the Kettle River basin. Elevation bands were identified where annual precipitation was dominated by rain, snow or mixed rain and snow, and the lower limit of the SSZ occurred slightly below the boundary between the rain dominated and mixed rain and snow zones, estimated at 1250 m. Output from a hydrologic model verified that snowmelt occurred at all elevations in the SSZ at the start of the peak flow period, and that the lower limit of the SSZ fell between 1100 and 1400 m. The lower limit of the SSZ was slightly lower than the H60 elevation in all but the largest watershed analysed in the Kettle River basin, and the differences increased as median basin elevation increased. This indicated that using a fixed SCA value (e.g., the H60) when assessing the hydrologic impacts of forest harvesting or disturbance in the Kettle River watershed without regard to basin characteristics may not accurately reflect potential impacts on snow accumulation and melt patterns. Further work is planned to expand this analysis across the upper Columbia River basin, to compare SSZ characteristics across a wider range of climates and topographic conditions. The methods used in this analysis provide an objective, repeatable way to identify the SSZ for watersheds that have hydrometric data. The SSZ maps can provide information to inform specialists planning of harvest activities in ways that reduce the potential for snowmelt synchronization and resultant increases in peak flow in the Kettle River basin. Snowmelt synchronization can be reduced by considering differences in snow accumulation and melt with slope, aspect and harvest practises within the SSZ. The SSZ maps can also be used by land and resource managers when evaluating the cumulative effects of multiple disturbances on watershed processes.

Report Type
  Meteorological - Rain or Snow - Snow
  Region - Kootenay
  Watershed Groups - 320 - Kettle
  Water Information - Hydrometric / Hydrology

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