Lecture 31 Snow Hydrology

Introduction to Snow Hydrology

• Speaker: Manuela Girotto, Associate Professor at Berkeley

• Date: November 7th, 2025

Background

Geographic References

• Various locations in relation to the topic of snow hydrology:

  • Vancouver

  • Portland

  • Seattle

  • Spokane

  • Boise

  • California cities: Los Angeles, San Diego, San Francisco, San Jose, etc.

  • Areas in Europe: Toronto, Montreal

  • Areas across the USA including Chicago, Denver, etc.

• References to organizations like NASA Goddard Space Flight Center

Learning Objectives

1. Understand why snow is important globally and particularly in California.

2. Identify key state variables that characterize snowpacks.

3. Explore what governs the accumulation and melting of snow in seasonal snowpacks.

4. Learn how to estimate snow.

5. Analyze the fate of snow under climate change.

The Water Cycle

Moisture Transport

• Water is cycled through various forms and pools on Earth:

  • Pools: Locations where water is stored such as oceans, lakes, etc.

  • Fluxes: Mechanisms of moving water between pools, e.g., evaporation, precipitation, runoff.

• On Earth, water can be fresh, saline, or brackish; 96% of all water is stored in oceans.

Key Processes

• Water is transported from oceans to land and eventually returns to the ocean.

• Different types of water use:

  • Domestic

  • Agricultural

  • Industrial

• Human activities impact the water cycle by redirecting rivers and draining wetlands, affecting both water quantity and quality.

Importance of Snow in Runoff

• Regions with significant snow provide water for nearly 3 billion people downstream.

• Snow-derived water is especially vital in areas with high population density and agricultural needs.

• The masked regions are significant but often poorly monitored.

Snow Socio-Climatic Risks

Water Towers

• Major mountain ranges referred to as 'water towers':

  • Alps, North America, Andes, Himalayas, Alaska Range

  • Vulnerable due to combined water stress and climate/socio-economic changes.

Global Radiation and Albedo

• Albedo: Measure of how much sunlight is reflected back into space.

  • Snow cover reflects solar radiation, thereby affecting the energy budget on Earth.

  • Positive feedback loop where less snow leads to more warming.

Importance of Snow for California

• California relies on snowmelt runoff for approximately 75% of its water resources, especially from the Sierra Nevada and Colorado River basin.

Metrics for Snow

Key Metrics

• Snow Depth: Measurement of the thickness of the snow layer.

• Snow Water Equivalent (SWE): Amount of water stored as snow.

  • Formula:
    $SWE = ds * <br>ho<em>s / ho</em>w$
    where:

  • $ds$ = snow depth

  • $<br>ho_s$ = snow density (150-500 kg/m³)

  • $<br>ho_w$ = water density

Comparisons

• Snow depth may exceed SWE, denoting that snow has variable densities.

• For a given snowpack:
  hos < how
  ightarrow SWE < ds

Seasonal Evolution of Snow

Snow Accumulation Season

• High snowfall and low melt energy, with snow temperature below 0°C.

• Snow Mass Balance formula:
  $rac{dSWE}{dt} = Precipitation - Evaporation - Melt$

• Snow Energy Balance:
  $Ground ext{ }Heat = Net ext{ }Radiation - Latent ext{ }and ext{ }Sensible ext{ }Heat$

Ablation Season

• Low or zero snowfall and high melt energy during warmer temperatures, while still remaining at or below 0°C.

Monitoring and Estimating Snow

Ground Observations

• In-Situ Observations: Includes using snow pillows and courses.

  • Challenges: Sparse observations, cost, complex environments.

Remote Sensing of Snow

• Use of technologies like Landsat and MODIS for snow monitoring.

  • Spatial and temporal challenges exist with satellite data.

Models in Snow Hydrology

• Importance of combining in-situ observations, models, and remote sensing.

  • To limit uncertainties and improve predictions regarding snowpack changes due to climate change.

Climate Change Implications

Effects on Snow

• Warmer air holds more water vapor, leading to changes in precipitation patterns.

• Shift in the rain-to-snow line due to temperature increases, affecting snow availability.
  ## Hydrological Impact

• Less snowpack can be associated with early snowmelt timing, leading to water shortages in dry seasons.

• Increased rain events may lead to flooding as snowlines rise.

Summary

• Snow is a crucial part of the global water cycle, with significant implications for water resource availability.

• Snow Water Equivalent, snow depth, and snowfall are important metrics to understand snow behavior.

• The impacts of climate change are evident in snow dynamics, affecting availability and quality.

• Recommendations for further learning:

  • ESPM C130 Terrestrial Hydrology

  • ESPM 172 Remote Sensing of the Environment

Contact Information

• Manuela Girotto's email: mgirotto@berkeley.edu