Measurement of Snow Water Equivalent Kara Przeczek

Measurement of Snow Water Equivalent Kara Przeczek

Outline l Snow water equivalent l Snow pillows l Snow survey equipment & methods l Examples from 2008 snow surveys

Snow Water Equivalent “The vertical depth of water which would be obtained by melting snow” l l Snowfall measurement can be problematic Determining swe from snowpack depth can be problematic l l Assuming mean density 100 kg/m 3 Regional density variations (new snow: 35– 101 kg/m 3) Densification over time Measuring snowpack swe is the standard l by weight of snowpack sample

Snow Pillows l An antifreeze filled bladder of various shapes, sizes, and materials. l Minimum size based on expected winter swe l Pressure inside the pillow changes in response to the weight of snow l Fluid pressure changes are measured with a manometer or pressure transducer l data can be transmitted remotely

http: //watershed. montana. edu/hydrology/images/img_1053. jpg

Snow Pillows Pros: l A non-destructive sampling technique l An automatic measure of swe in remote locations l Identify snowfall and snowmelt events l Can provide rough estimates of loss of swe Cons: l A point swe measurement l l Bridging may occur l l l site representativeness important separation of pillow from overlaying snow under measurement of swe Snow pillow is a barrier to heat & moisture fluxes between snowpack and ground

Snow Pillows l B. C. Ministry of Environment River Forecast Centre l l l http: //www. env. gov. bc. ca/rfc/ Automatic Snow Pillows (ASPs) and manual snow surveys view ASP swe graphs (weekly) download temperature, precipitation, and swe data from ASP sites (near real-time) download manual snow survey data l Historic data 1935 - present 54 active ASP sites and almost 200 snow survey sites in the province

Sonic Ranger l Automatic snow depth measure l Determines the distance to a target by sending out ultrasonic pulses and listening for the returning echoes that are reflected from the target l The time from transmission to return of an echo is used to obtain the distance measurement l Air temperature correction required for variations in the speed of sound in air

Snow Surveys l l Measure snow depth, density and swe Snow course is the line of permanently marked sampling points Repeat measurements at regular intervals throughout season Location and frequency of snow survey depends on purpose: l l l 1 o as index of swe for spring runoff prediction 2 o absolute measure for hydrologic, agricultural, ecologic, transportation, recreational, and engineering functions Snow survey data are the ultimate base of comparison for other methods

Snow Surveys Considerations l What are you measuring swe for? l Where to set up snow course? l How long should the snow course be? l How many sample points per course? l swe depth sampling ratio? l When & how often to sample? l What equipment to use?

Snow Surveys Purpose l Index of swe: l l Choose high accumulation area l Show changes in swe Represent basin characteristics l Forest cover; aspect; elevation Consistent instrumentation and methodology Absolute estimate of swe: l l Account for biases l Instrument, method, site Account for variability in snow cover l Erosional/depositional areas l Stratify by landscape features

Snow Surveys Site Selection l l l Accessibility Representativeness of terrain/land cover Choose slightly sloping terrain Avoid steep slopes Avoid areas with land use disturbances l l Avoid microsite irregularities at sample points l l i. e. logging, mining, construction i. e. stumps, logs, ponding areas Avoid areas with snow removal activity

Snow Surveys Site Selection l l Snowmelt for testing melt models North and south aspect l l Open and forest cover l l Radiation differences Similar location, slope, elevation l l Radiation differences Same weather, vegetation Access

Snow Surveys Course Length l Longer in complex terrain l More samples in complex terrain l Snow course can zig-zag l Oversample initially l l Length & sample density adjusted based on selected precision level Fewer swe than depth measurements possible l Density shows least variability l Calculate swe at depth measurement points

Snow Surveys Course Length cont. l l Cumulative coefficient of variation (C. V. ) C. V. = l standard deviation mean Measure of variability independent of scale l Plot cumulative C. V. against length of course l How many samples before C. V. levels off ?

Snow Surveys Sampling Schedule l RFC: sample beginning of month Jan-Jun l l Purpose of snow course? l l l Extra mid-month sampling May & June Snow accumulation Snow melt Maximum swe Snow cover duration Purpose: melt l l l Start: April 1 st Interval: weekly End: snow melted (June)

Snow Surveys Equipment l Graduated snow tube with cutter l Various materials, cutter configurations, and sizes l Larger diameter tubes for shallow snow packs l Graduations on outside to measure snow depth l Slots in tube to view snow core l Spring balance measures swe directly l Federal (formerly Mt. Rose) snow sampler

Snow Surveys Methods l B. C. “Snow Survey Sampling Guide” l http: //www. env. gov. bc. ca/rfc/river_fo recast/snow_surveys_manual. pdf

Snow Surveys Methods - 1 l l l 1. Weigh and record empty tube swe. 2. Push tube straight into snow pack to ground. 3. Push and twist into ground to obtain soil plug.

Snow Surveys Methods - 2 l l l 1. Weigh and record empty tube swe. 2. Push tube straight into snow pack to ground. 3. Push and twist into ground to obtain soil plug. 4. Record snow depth from outside of tube. 5. Carefully pull tube straight out of snow pack. 6. Check for soil plug.

Snow Surveys Methods - 3 l l l l 1. Weigh and record empty tube swe. 2. Push tube straight into snow pack to ground. 3. Push and twist into ground to obtain soil plug. 4. Record snow depth from outside of tube. 5. Carefully pull tube straight out of snow pack. 6. Check for soil plug. 7. Remove soil plug and estimate depth of soil in tube.

Snow Surveys Methods - 4 l l l l l 1. Weigh and record empty tube swe. 2. Push tube straight into snow pack to ground. 3. Push and twist into ground to obtain soil plug. 4. Record snow depth from outside of tube. 5. Carefully pull tube straight out of snow pack. 6. Check for soil plug. 7. Remove soil plug and estimate depth of soil in tube. 8. Weigh and record filled tube swe. *If hit obstruction during sampling or if no soil plug obtained re-take snow sample.

Snow Surveys Methods – rinse & repeat l Give tube a good shake or tap it against toe of boot to get out snow core l l Don’t hit tube or cutter end against anything hard because it is easy to damage Check that no snow remains in tube between samples l No ‘significant’ snow in tube

Snow Surveys Tips l Weighing empty tube before each sample is at your discretion l l l Dependent on snow conditions; tube-clearing skill; level of accuracy desired Wax inside of tube before use (Turtle Wax works well) Bring small, slotted screwdriver and old knife to remove soil plug, or tricky snow core, from tube Bring many pairs of gloves, grippy palms are a good idea Waterproof paper and pencil for data recording

Site NF GPS 10 U 0588859 5894468 Aspect S Slope 16. 1% Date Apr 27, 2008 Start Time 13: 20 End Time 14: 15 Weather Overcast, light drizzle Sampler Kara Recorder Audrey N-S Distance (m) 7 E-W Distance (m) 20 Point Line Direction Depth w/ Soil Plug SWE Empty SWE Full NOTES 1 1 5: 00 67 5 88 113 2 1 5: 00 50 7. 5 87 100 3 1 5: 00 55 6 86 102 4 1 5: 00 67. 5 10 86 94 in ‘nasty’ tree well … … … …

l Swe increasing at one site while snow depth decreasing l Effect of site on swe SWE Depth

l l Density increasing over time Effect of site on density l l Interception, shading, wind Calculate density from swe and snow depth l Conversion between mm & m = density of water

Snow Surveys Issues l Different samplers for different conditions l l Generally overestimate swe l l l Through slots with twisting in deep snowpack Dull cutter Shrubs, branches, vegetation beneath the snow l l l Design of cutting point forces more snow inside tube Ice layers Losing water from the tube during melt Gaining extra snow l l Federal generally regarded as best all-around Air pockets Check length of core ≥ 80% snow depth; consistent density between samples Freezing of snow in the tube l Particularly when air temp > 0 o. C, snow temp < 0 o. C

l April 20 th, snow freezing in core resulted in anomalously low swe measurement. l l Sunny, cold day Strongest effect at open sites l Wear gloves l Wax the inside of the snow tube Keep the tube filled with snow between samples l

Site GPS Aspect Slope Date Start Time End Time Weather Sampler Recorder N-S Distance (m) E-W Distance (m) Point Line 1 1 2 1 3 1 4 1 Direction Depth w/ Soil Plug SWE Empty SWE Full NOTES