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Snow Levels

Snow is vital to the environmental health of the western United States. Unlike rainfall, it doesn't come as one burst that's here and then quickly gone. Instead, it tends to accrue over the winter and slowly melt as the temperatures rise in the spring. The melt typically continues into the early summer, providing a steady, consistent level of water in mountain watersheds. This is particularly important for a state like California, which receives almost no summer rainfall.

Snow comes in a variety of forms. It can be light and airy, almost dry, or it can heavy and dense. For example, light, fluffy snow that measures four feet deep might actually have the same volume of water as heavier, denser snow that is two feet deep.To account for this variation a measure called snow water equivalence is used to normalize the amount of liquid water in a given level of snow. The graph below uses SNOTEL data; a series of automated weather stations distributed across the west to measure things such as snow levels and temperature. Notice how temperature and snow levels change with elevation. Compare the average snow/water levels by year and elevation. The general trend is towards drier conditions, but wet years are interspersed with drier years.

Temperature (degrees Fahrenheit)

Drought Level (Region Wide Palmer Drought Index)

The Palmer Drought Index is a measure of long term drought. It calculates soil moisture based on recent precipitation and temperature. 0 is considered a normal level of soil moisture, while increasingly negative numbers indicate progressively severe levels of drought. Positive numbers represent increasingly wet soil conditions. Compare the Palmer Index to the temperature and precipitation levels below. Recent droughts have tended to have hotter temperatures than usual. This results in increased levels of evapotranspiration from the soil and plants, leading to more severe droughts and worse Palmer Index numbers.

Note: Hover over a bar to see the value for a particular month.

Departure from Average (Avg Anomaly)

Temperature Level (Region Wide)

Departure from Average (Avg Anomaly)

Precipitation Level (Region Wide)

Departure from Average (Avg Anomaly)

Percentage of State at a Particular Drought Level

Smaller Greater

Interpreting the Drought Circles

Abnormally Dry (D0) Moderate Drought (D1) Severe Drought (D2) Extreme Drought (D3) Exceptional Drought (D4)

Drought Effects

The United States Drought Monitor defines the effects of various drought levels as follows:

  • DO
    • Short-term dryness slowing planting, growth of crops or pastures
  • D1
    • Some damage to crops, pastures
    • Streams, reservoirs, or wells low, some water shortages developing or imminent
    • Voluntary water-use restrictions requested
  • D2
    • Crop or pasture losses likely
    • Water shortages common
    • Water restrictions imposed
  • D3
    • Major crop/pasture losses
    • Widespread water shortages or restrictions
  • D4
    • Exceptional and widespread crop/pasture losses
    • Shortages of water in reservoirs, streams, and wells creating water emergencies

If you have any questions or comments feel free to contact me via Twitter at @farrelldlfarrel, or email at farrelldlfarrell at gmail.com