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How the City of Boulder Determines Whether to Impose Water Restrictions

How the City of Boulder Determines Whether to Impose Water Restrictions

Each year, on or around May 1, the City of Boulder makes a declaration about its water supply status, determines whether or not to declare a drought stage, and decides if water restrictions are necessary.

Waiting until May 1 is beneficial for two reasons:

  1. March and April are typically the two snowiest months in Colorado and measuring earlier could result in a lower peak snowpack reading, and
  2. Measuring on the same day each year allows the city to accurately compare readings from year to year.

Historically, April provides around 19 percent of the annual snowpack in the city’s watersheds. In addition, water allotments from the Colorado Big Thompson (CBT) project, managed by Northern Colorado Water Conservancy District, are typically announced in April.

Deciding Factors

The primary factors in the city’s annual May 1 water supply evaluation include the:

  • high-mountain snowpack measurements in the city’s watershed,
  • water storage levels in the city’s reservoirs, and
  • available amount of CBT water.


Snowpack is important because the city relies heavily upon snowmelt runoff to fill and store water in its upper Boulder Creek basin reservoirs each year. During the winter and early spring, city staff measures snowpack levels in the watershed that supplies much of the city’s water. The peak snowpack is measured on May 1of each year.

However, snowpack isn’t the only measurement for drought. Even with lower-than-average snowpack, it’s possible to have enough water stored in reservoirs, combined with a decent allotment of CBT water, to provide water to customers for the year in question.


Water stored in the city’s upper Boulder Creek basin reservoirs is also an essential component in determining the city’s annual water supply. As snowmelt wanes later in the summer and the city can no longer treat runoff directly from its diversion points on the streams, the city must start treating water from reservoir storage to distribute. The city continues to use reservoir water until the following spring when snowmelt begins again. The city manages its reservoirs so that they are not completely drained in a single year in order to assure adequate water supply during subsequent dry years.

Colorado-Big Thompson (CBT) Water

Water supplied by Northern Colorado Water Conservancy District through the CBT system is another significant factor in the city’s annual water supply, particularly during dry years. CBT water originates in the Colorado River Basin on the West Slope and is delivered to the East Slope through a 13-mile tunnel. Each year, the Northern Water Board of Directors sets an allotment or “quota” of CBT water as a supplemental supply for eastern slope members.

Higher quotas are typically set when East Slope runoff is projected to be lower than normal and West Slope reservoirs are at or above average storage levels. Lower quotas are typically set when East Slope runoff is expected to be above normal or when West Slope reservoirs are significantly below normal storage levels.