Lecture 31 Snow Hydrology

Introduction to Snow Hydrology

  • Instructor: Manuela Girotto (mgirotto@berkeley.edu)

  • Date: November 7th, 2025

Background

  • Geographic references and importance of locations such as:

    • Mionetto Prosecco

    • Various U.S. states and cities including Vancouver, Portland, Seattle, Spokane, Los Angeles, San Diego, and notable areas like the Sierra Nevada, which is central to the discussion on snow and water resources.

Learning Objectives

  • Understand the importance of snow globally and specifically in California.

  • Identify key state variables characterizing snowpacks.

  • Analyze what governs accumulation and melt of snow within seasonal snowpacks.

  • Learn methods to estimate snow.

  • Assess the impact of climate change on snow.

The Water Cycle

  • Water cycle components:

    • Pools: Locations where water is stored, such as oceans, lakes, glaciers, and snowpacks.

    • Fluxes: Processes through which water moves between pools (e.g., evaporation, precipitation, runoff, discharge).

  • Storage breakdown:

    • 96% of the world’s water is stored in oceans.

    • Other reservoirs include lakes, rivers, and aquifers.

  • Interaction between water in various states:

    • Water can exist as solid (ice/snow), liquid (fresh/saline), or gas (water vapor).

Snow as Water Towers

  • Definition: Mountain ranges that act as crucial reservoirs of water due to their snowpacks.

  • Five Major Mountain Ranges recognized as water towers:

    • Alps (Europe)

    • Rocky Mountains (North America)

    • Andes (South America)

    • Himalayas (Asia)

    • Alaska Range

  • Importance related to socio-climatic risks:

    • Vulnerability due to water stress, governance issues, hydro-political tensions, and future climate and socio-economic changes.

Importance of Snow in California

  • Approximately 75% of California’s water resources derive from snowmelt runoff from the Sierra Nevada and the Colorado River Basin.

Snow Measurement Metrics

  • Key metrics include:

    • Snow Depth: Measurement of the depth of snow present on the ground.

    • Snow Water Equivalent (SWE): Total volume of water stored in the snow. Calculated as:
      SWE=snow density×snow depth\text{SWE} = \text{snow density} \times \text{snow depth}, where snow density (ρs\rho_s) typically ranges from 150-500 kg/m³.

    • Snowfall: Total amount of precipitation that falls as snow over a defined period.

  • Example calculations:

    • For peak SWE in the Sierra Nevada, total volume noted around 20 km³, equating weather events to significant infrastructure like the Transamerica Pyramid.

Seasonal Evolution and Dynamics of Snow

  • Different phases of snow dynamics during the season include:

    • Snow Accumulation: Characterized by high snowfall, low melt energy, and temperatures below 0°C.

    • Melt Season: Marked by lower or zero snowfall, high energy for melt, and temperatures reaching melting point.

  • Factors influencing snow mass and energy balance:

    • Snow Mass Balance: Change in SWE is modeled as:
      dSWEdt=Precipitation−Evaporation−Melt\frac{dSWE}{dt} = \text{Precipitation} - \text{Evaporation} - \text{Melt}

    • Snow Energy Balance: Ground heat relates to the net radiation minus latent and sensible heat losses.

Estimation of Snow

  • Challenges in estimating snow include:

    • Ground Observations: Local measurement difficulties (slope/aspect variations, vegetation cover) leading to sparse spatial and temporal data.

    • Remote Sensing Techniques: Utilization of satellite imagery and airborne sensors to assess snow coverage and conditions, though limited by atmospheric interference and observational frequency.

  • Importance of integrating various methodologies (in-situ, remote sensing, modeling) to enhance understanding and accuracy in snow estimation.

Climate Change Implications on Snow

  • Climate change’s influence on snow found in:

    • Increase in temperature correlates with an increase in the atmosphere's capacity to hold water vapor.

    • This leads to shifts in precipitation patterns whereby:

      • Warm oceans lead to higher water vapor availability.

      • Less frequent but more intense precipitation events.

    • Changes in the rain-snow line:

      • Rising temperatures may elevate the rain-snow line, resulting in reduced snowpack at lower elevations.

      • Potential water shortages during dry seasons due to earlier snowmelt and skewed precipitation distribution.

Summary

  • Snow serves as a critical component of global water cycles and is essential for water resource management in California.

  • Around 3 billion people rely on waters downstream of regions affected by snowmelt.

  • The variability of SWE shows significant fluctuations both spatially and temporally, necessitating precise scientific observation and modeling to predict future trends and manage resources effectively.

Further Learning Opportunities

  • Suggested courses for more in-depth knowledge:

    • ESPM C130: Terrestrial Hydrology

    • ESPM 172: Remote Sensing of the Environment

  • Contact: Manuela Girotto (mgirotto@berkeley.edu) for inquiries or additional resources.