2021 March water supply outlook summary

County 10 Photo - Boysen Reservoir, January 2021

Wyoming NRCS Hydrologist Jim Fahey shared the 2021 March water supply outlook summary hydrological discussion.

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General Summary

  • Wyoming continues to see below median percent of snowpack and/or snow water equivalents (SWEs) through late February. 
  • Precipitation totals across Wyoming for February were well above average. Water year precipitation continues to be below average.
  • Reservoirs across Wyoming were averaging near 73% of capacity-down from 80% reported last year. Overall reservoir storages for late February continue to be above average.
  • Stream flow snowmelt volumes are forecasted to be below average for almost all major drainages across Wyoming

Hydrologic Discussion:

Many basins across Wyoming had a 15 to 25 percent increase in snow water equivalents (SWEs) during the month of February. Basins in far western Wyoming and across northern Wyoming are seeing SWE numbers near to above medians for late February; while watersheds in central-southern-eastern Wyoming still have SWE numbers below to well below medians.  Notably, there was also a lack of significant low elevation snow (6500-8000 feet) across many basins in central through southern Wyoming.

Several basins across Wyoming had 120 to near 180 percent of average precipitation totals for February. However, current water year precipitation totals are still below average for majority of basins in Wyoming—especially basins east of the continental divide.

Reservoirs across Wyoming were averaging near 75% of capacity. Last year at this time Wyoming reservoirs were 80% of capacity. Reservoir storages have remained around 110% of average during the winter season.

Extreme to severe drought conditions continue for several basins east of the continental divide. Water Year 2021 started out with dry to very dry antecedent soil conditions throughout most of Wyoming. There was also below normal baseflows for several streams in central through southern Wyoming in early Water Year 2021. The latest spring outlook indicates that there will be a warmer than average as well as a drier than average spring—especially during late spring and into early summer. As a result of current hydrological and expected climate conditions, there is very good chance that there will be an earlier than normal runoff with an overall below average streamflows. Runoff volumes are expected to be near average for drainages in far western and northern Wyoming and below average for the rest of the basins in Wyoming.

Wyoming snowpack and basin hydrological conditions—especially for several basins east of the continental divide—are very similar to what occurred in Water Years 2012 and 2013. Spring runoff volumes during those water years were the lowest in the past decade.

There is still much uncertainty in the final snowmelt runoff volume forecasts due to the uncertainty of the timing and the amount of the upcoming spring precipitation. Many locations along east of the continental divide receive up to 60% of their annual precipitation during late March into early June. Much higher flows and drastic increases in volumes can be expected during a rapid warmup followed to by rain on a melting snowpack. Water planners need to keep abreast of the latest spring runoff forecasts as well as the latest weather trends during the upcoming spring.


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