eduKateSG · Hydrology · The Gold Standard of Education
Treat wind-blown snow as a moving water store with an open boundary
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Full chapter index · Independent practice · Hydrology subject guide
Snowfall is not necessarily stored where it lands. Wind can erode exposed snow, carry particles by saltation and suspension, deposit them in sheltered terrain, export them from a catchment and remove mass through sublimation. A gauge total therefore cannot, by itself, describe the spatial snow water equivalent that later melts into soil, aquifers and streams.
This article follows snow from deposition through transport, loss and measurement. Its examples are invented. The aim is to keep internal redistribution separate from true basin gains or losses and to show how terrain-stratified evidence supports a defensible water balance.
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Full chapter index
See the mechanism · Chapters 1–2
Build the worked model · Chapters 3–5
Test what changes · Chapters 6–8
Check evidence and limits · Chapters 9–10
Practise and continue · Chapters 11–12
CHAPTER 1 OF 12 · See the mechanism
1. Separate snowfall, storage, transport and loss
Snowfall is an atmospheric input. Snow water equivalent, or SWE, is the liquid-water depth stored in the snowpack over a declared area. Wind transport moves that stored mass between places; sublimation changes ice directly to water vapour and removes mass from the surface store.
At a point, wind erosion can make accumulation much smaller than measured precipitation. A nearby drift can be much deeper than the storm total would imply. Across a closed basin, movement from one slope to another should cancel; export across the boundary and sublimation do not.
Write the control volume before writing the balance. State the basin outline, vertical atmosphere boundary, period, land-cover classes and whether blowing snow may cross the divide. “Redistribution” is not a residual bucket for every mismatch.
Evidence check: for Separate snowfall, storage, transport and loss, preserve the observation before the interpretation. Record snowfall, SWE, transport and sublimation separately. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
CHAPTER 2 OF 12 · See the mechanism
2. Recognise saltation, suspension and deposition
Saltating grains hop close to the surface and can dislodge other grains. Smaller particles may enter suspension and travel higher or farther. The relative contributions depend on wind, particle bonding, snow age, temperature and surface roughness.
Transport begins only when aerodynamic forcing overcomes resistance, but there is no universal wind-speed threshold. Fresh cold snow, crusted snow and wet snow behave differently. Measurement height and exposure also change the wind value used in a model.
Deposition occurs when carrying capacity falls, often behind a ridge, shrub belt, forest edge or snow fence. Terrain curvature and vegetation alter wind fields, so straight-line distance from an exposed source is rarely enough to predict a drift.
Evidence check: for Recognise saltation, suspension and deposition, preserve the observation before the interpretation. Record saltation, suspension and deposition conditions. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
CHAPTER 3 OF 12 · Build the worked model
3. Write the basin mass balance before mapping
For a chosen period, a useful accounting form is change in snow storage = snowfall and other deposition − sublimation − melt and runoff − wind export, with internal wind transfers cancelling when summed across all cells. Rain-on-snow, canopy interception and ground melt may require additional terms.
Keep every term in common water-equivalent units. Depth in millimetres becomes volume only after multiplying by area. Snow depth is not SWE unless density is measured or defensibly estimated.
Closure error is information. It can reflect gauge undercatch, missed drifts, density bias, an incorrect boundary or an omitted flux. Report it rather than forcing one uncertain term to absorb the difference.
Evidence check: for Write the basin mass balance before mapping, preserve the observation before the interpretation. Record control volume, period and common units. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
CHAPTER 4 OF 12 · Build the worked model
4. Work a terrain-stratified redistribution example
Consider a 1.0 km² basin beginning with a uniform 120 mm SWE. An exposed 0.40 km² upland loses 40 mm, contributing −16 basin-mm. A sheltered 0.30 km² lee zone gains 30 mm, contributing +9 basin-mm. The remaining 0.30 km² changes little.
The mapped storage change is therefore −7 basin-mm, leaving 113 mm before melt. Field and model evidence attribute 5 mm to wind export across the divide and 2 mm to blowing-snow sublimation. Internal deposition is already included in the +9 mm and must not be counted again.
Because one basin-mm over 1 km² equals 1,000 m³, the example represents 5,000 m³ exported and 2,000 m³ sublimated. The arithmetic closes, but the attribution remains conditional on sampling and flux assumptions.
| Terrain unit | Area share | SWE change | Basin contribution |
|---|---|---|---|
| Exposed upland | 0.40 | −40 mm | −16 mm |
| Sheltered lee | 0.30 | +30 mm | +9 mm |
| Other terrain | 0.30 | 0 mm | 0 mm |
| Whole basin | 1.00 | — | −7 mm |
Evidence check: for Work a terrain-stratified redistribution example, preserve the observation before the interpretation. Record every area-weighted term in the worked balance. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
Classify terrain by exposure, slope, aspect, curvature and distance to aerodynamic breaks. Ridge crests and broad windward slopes may erode; lee slopes, gullies and forest edges often accumulate. The pattern can reverse when storm direction changes.
Vegetation both traps moving snow and alters sublimation and interception. Shrubs protruding through shallow snow create roughness, while dense canopies intercept snowfall and change wind below. A land-cover class is not a fixed drift multiplier across all winters.
Use storm histories rather than a single prevailing-wind arrow. Weight directions by periods when snow was erodible and transport actually occurred. Calm winds after crust formation contribute little even if their seasonal frequency is high.
Evidence check: for Map terrain and vegetation controls, preserve the observation before the interpretation. Record terrain, vegetation and storm direction. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
CHAPTER 6 OF 12 · Test what changes
6. Measure erosion, transport and deposition with matched footprints
Snow stakes and repeated transects show local depth change. Density cores convert depth to SWE. Terrestrial or airborne lidar can map surface elevation change, while remote sensing expands coverage subject to resolution, vegetation and retrieval limits.
Blowing-snow particle counters and flux instruments sample transport at particular heights and locations. Wind observations need exposure and height metadata. A sheltered weather station may not represent the ridge that supplies most transported snow.
Match measurement footprints. Comparing a point gauge with a kilometre-scale grid cell can create a false process discrepancy. Use co-located observations for calibration and independent terrain strata for validation.
Evidence check: for Measure erosion, transport and deposition with matched footprints, preserve the observation before the interpretation. Record matched footprints for depth, density and wind. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
CHAPTER 7 OF 12 · Test what changes
7. Estimate sublimation without turning it into a plug term
Blowing particles have high surface area and exchange heat and vapour with air. Sublimation depends on humidity, temperature, ventilation, particle size and time aloft. Models couple particle transport with thermodynamic exchange rather than applying one percentage to every storm.
Direct basin-scale measurement is difficult. Eddy-covariance, lysimeter, snow-mass and atmospheric observations each capture different components and footprints. Model estimates should be tested against storage change and vapour-flux evidence where possible.
Do not label every eroded millimetre as sublimated. Much of it may deposit downwind or leave the mapped domain. Conversely, an apparently stable drift can lose mass to vapour while receiving new particles.
Evidence check: for Estimate sublimation without turning it into a plug term, preserve the observation before the interpretation. Record sublimation evidence rather than a residual plug. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
Stratify the basin into process-relevant units such as exposed upland, lee slope, valley floor and forest. Within each stratum, estimate mean SWE and uncertainty from probability-based or spatially balanced samples.
Multiply each stratum mean by its mapped area share, then sum. Preserve covariance when the same storm or density model affects several strata. A precise area-weighted mean can still be biased if the drift tails were never sampled.
Conservation offers a check: mapped erosion plus atmospheric and boundary losses should be consistent with mapped deposition and storage change. Disagreement should trigger a search for missing area, density error or gauge undercatch before parameter tuning.
| Stratum | Likely process | Priority measurement | Validation check |
|---|---|---|---|
| Exposed upland | Erosion and export | Repeated SWE and ridge wind | Event mass loss |
| Lee slope | Deposition and delayed melt | Dense transect or lidar | Drift volume |
| Valley floor | Mixed deposition and melt | SWE course and density | Storage timing |
| Forest | Interception and sheltered storage | Canopy-aware SWE samples | Independent winter |
Evidence check: for Upscale with conservative terrain strata, preserve the observation before the interpretation. Record stratum areas, uncertainty and conservation. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
CHAPTER 9 OF 12 · Check evidence and limits
9. Test models with events and withheld winters
A transport model may reproduce seasonal SWE while getting storm pathways wrong. Evaluate erosion and deposition after individual events, cumulative seasonal storage and melt timing. Different failures appear at different scales.
Calibrate with some winters and test with others, especially winters with different wind directions, crust histories and snowfall temperature. Spatial cross-validation should withhold whole terrain units rather than random neighbouring points that share the same drift.
Inspect mass conservation separately from pattern skill. Correlation can be high even when basin storage is biased, and a balanced total can hide misplaced water that changes melt timing and runoff connectivity.
Evidence check: for Test models with events and withheld winters, preserve the observation before the interpretation. Record event, spatial and withheld-winter validation. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
CHAPTER 10 OF 12 · Check evidence and limits
10. Know the limits and common misconceptions
Deep drifts do not prove extra precipitation. They may be imported snow. Scoured ground does not prove sublimation; snow may have moved beyond the plot. Gauge snowfall is also uncertain because wind causes undercatch.
Redistribution can change streamflow timing even when basin mass changes little. Thick lee drifts persist later, while scoured areas expose soil early. The spatial arrangement affects radiation, infiltration and runoff pathways.
Parameters are not universally transferable. Snow climate, grain properties, topography, vegetation and wind exposure differ. A model that works on an Arctic plain may require different structure or scales in an alpine forest basin.
Evidence check: for Know the limits and common misconceptions, preserve the observation before the interpretation. Record limits from undercatch, crusts and model transfer. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
CHAPTER 11 OF 12 · Practise and continue
11. Independent practice and changed-context transfer
Practice case: a 2 km² basin has 150 mm initial SWE. An exposed half loses 30 mm, a lee quarter gains 40 mm and the remaining quarter is unchanged. Calculate the net basin storage change and volume. List two combinations of export and sublimation that could explain it.
Changed context: rotate the dominant storm wind by 90 degrees and add a breakable crust after rain-on-snow. Predict which terrain classifications and transport thresholds must be revisited. Explain why the previous seasonal drift map cannot simply be reused.
Finish with a balance table that separates observed storage, internal transport, boundary export, sublimation, melt and residual error. Name the measurement or model owner for every non-zero term.
Evidence check: for Independent practice and changed-context transfer, preserve the observation before the interpretation. Record the rotated storm and crust changed context. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.
Does wind transport change basin-average SWE?
Internal redistribution alone does not. Basin-average SWE changes when snow crosses the boundary, sublimates, melts or when other inputs and losses occur.
Can snow depth map wind redistribution?
It helps, but depth must be paired with density for SWE, repeated surveys for change and terrain-aware sampling for basin inference.
Why are late drifts hydrologically important?
They concentrate water and often melt later, changing the timing and location of soil wetting, groundwater recharge and runoff.
Continue with snow water equivalent and basin upscaling, snowmelt, degree-day models and rain-on-snow limits, and snow albedo, dust and earlier melt timing.
Authoritative background includes Pomeroy’s field analysis of transport and sublimation in wind-scoured alpine terrain, the U.S. Forest Service study of snow distribution and basin hydrologic response, and the peer-reviewed 2024 evaluation of snow redistribution in an intermediate-complexity model. Sources and links were checked on 22 October 2026.
Evidence check: for FAQ, sources and connected reading, preserve the observation before the interpretation. Record the bounded basin mass-balance claim supported. Then state the inference, one credible alternative, and the additional observation that would distinguish them. This compact audit keeps a fluent explanation tied to evidence that another reader can inspect.
Changed-context transfer: repeat this chapter’s decision after changing one consequential condition while holding the others fixed. Calculate or predict what should change, name the threshold or mechanism, and compare with the original case. If the conclusion survives, state the range tested; if it reverses, explain why. Transfer is demonstrated by the changed case, not asserted from the first example.
Practical decision record: write one sentence for what is known, one for what remains uncertain, and one for the next safe action. Include the evidence owner, the latest useful decision time, and the condition that would stop or reverse the action. This turns a technical result into a reviewable choice without pretending uncertainty has disappeared.