Wyoming Drought Condition update
by Jim Fahey, Wyoming NOAA hydrologist
October 31, 2010
Below is the latest information on drought conditions across Wyoming (as of April 30th, 2010)
- Moderate hydrologic drought conditions continue across the Upper Green and Shoshone Watersheds.
- Normal to above normal precipitation totals for rangelands/basins across almost all of Wyoming for water year 2010 (October 2009 - September 2010.
- Normal to above normal mountain snowpack averages across basins east of the continental divide during water year 2010. Below normal mountain snowpack average across watersheds west of the Continental Divide.
- Above to well above normal stream flows during the Spring runoff. Near normal to slightly above river flows into the summer and early fall.
- Reservoir storages across Wyoming continue to remain slightly higher than water year 2009.
Three key ingredients define the overall drought picture for Wyoming:
Precipitation across Wyoming’s pasturelands/rangelands during the water year 2010 (October 2009 - September 2010) was normal to above normal for almost all of Wyoming. Precipitation across the major river basins across Wyoming was also near normal to above normal. Specifically, precipitation averages across Wyoming’s major watersheds varied from 84 to 133 percent of average during water year 2010.
Reservoir storages at the end of water year 2010, at a majority of the major reservoir, continue to be slightly above water year 2009 averages. Storages at the big reservoirs along the North Platte River had very sharp gains during a record spring runoff; but by the end of the water year, storages were still higher than at the end of water year 2009. Seminoe and Pathfinder Reservoirs, respectfully, ended up at 84 and 73 percent of capacity by the end of the water year.
Streamflows across Wyoming during the runoff were above to well above normal. Streamflow trends into the summer and into early fall were normal to slightly above normal.
Snow water equivalents (SWEs) for water year 2010 were near normal to above normal across all major watersheds east of the continental divide; however, mountain snowpack averages were below normal across major watersheds west of the Continental Divide.
Overall Drought Picture and What Does the Future Hold?
Lack of adequate snowpack for the Upper Green and Shoshone River Basins was the main driving force that kept these watersheds in moderate hydrologic drought during most of water year 2010. The above average precipitation during the spring and subsequent record spring runoff for many locations, especially east of the Continental Divide, kept the rest of Wyoming out of hydrologic drought during water year 2010. However, even though the summer of 2010 had near normal temperatures, it became very dry after the middle of June. September and October were also very dry and temperatures were above normal during the same time. The dry conditions during summer and into early fall has caused a significant soil moisture deficits across many rangeland locations across Wyoming.
The above average runoff, and even record runoff at several watersheds, quickly filled many major reservoir across Wyoming. However, due to the dry summer and early fall, many reservoirs showed significant decreases in storages by the end of the irrigation season. Even so, reservoir storages across Wyoming in water year 2010 remained slightly above water year 2009 reservoir storages. Most importantly, the big reservoirs (Boysen, Seminoe, and Pathfinder) have kept storages at greater than 70 percent of capacity.
Wyoming’s mountain snowpack averages, especially for basins east of the Continental Divide, were near normal to above average for the third straight water year. However, late fall and early winter snowpack averages across Wyoming during the past three water years have continued to be below average. It is hoped that with the soil moisture deficits that have shown up during the early fall, an adequate late fall/early winter snowpack will reverse the "dry" antecedent soil conditions.
Bottom line is that current precipitation trends (during the last four months) have caused short-term hydrologic condition concerns. However, current water supply and streamflow trends are not pointing to any development of another significant long-term hydrologic drought. Wyoming is the 5th driest state in the country, so drought is always ‘knocking-on’ on Wyoming’s backdoor.
For the complete drought report with graphics (in PDF format) please go to:
For additional drought graphics and information, please refer to:
Source: Jim Fahey, Wyoming NOAA hydrologist, 307-857-3898 ext 493.