06-12-12 *NRCS-CO* The June 1, 2011 Colorado Basin Outlook Report is now available…
Posted by Brian Allmer on June 12, 2012
United States Department of Agriculture - Natural Resources Conservation Service
For more water supply and resource management information, contact:
Michael A. Gillespie, Data Collection Office Supervisor, USDA, Natural Resources Conservation Service, Denver Federal Center, Bldg 56, Rm 2604, PO Box 25426, Denver, CO 80225-0426, Phone (720) 544-2852
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Basin Outlook Reports and Federal – State – Private Cooperative Snow Surveys
How forecasts are made
Most of the annual streamflow in the western United States originates as snowfall that has accumulated in the mountains during the winter and early spring. As the snowpack accumulates, hydrologists estimate the runoff that will occur when it melts. Measurements of snow water equivalent at selected manual snow courses and automated SNOTEL sites, along with precipitation, antecedent streamflow, and indices of the El Niño / Southern Oscillation are used in computerized statistical and simulation models to prepare runoff forecasts. These forecasts are coordinated between hydrologists in the Natural Resources Conservation Service and the National Weather Service. Unless otherwise specified, all forecasts are for flows that would occur naturally without any upstream influences.
Forecasts of any kind, of course, are not perfect. Streamflow forecast uncertainty arises from three primary sources: (1) uncertain knowledge of future weather conditions, (2) uncertainty in the forecasting procedure, and (3) errors in the data. The forecast, therefore, must be interpreted not as a single value but rather as a range of values with specific probabilities of occurrence. The middle of the range is expressed by the 50% exceedance probability forecast, for which there is a 50% chance that the actual flow will be above, and a 50% chance that the actual flow will be below, this value. To describe the expected range around this 50% value, four other forecasts are provided, two smaller values (90% and 70% exceedance probability) and two larger values (30%, and 10% exceedance probability). For example, there is a 90% chance that the actual flow will be more than the 90% exceedance probability forecast. The others can be interpreted similarly.
The wider the spread among these values, the more uncertain the forecast. As the season progresses, forecasts become more accurate, primarily because a greater portion of the future weather conditions become known; this is reflected by a narrowing of the range around the 50% exceedance probability forecast. Users should take this uncertainty into consideration when making operational decisions by selecting forecasts corresponding to the level of risk they are willing to assume about the amount of water to be expected. If users anticipate receiving a lesser supply of water, or if they wish to increase their chances of having an adequate supply of water for their operations, they may want to base their decisions on the 90% or 70% exceedance probability forecasts, or something in between. On the other hand, if users are concerned about receiving too much water (for example, threat of flooding), they may want to base their decisions on the 30% or 10% exceedance probability forecasts, or something in between. Regardless of the forecast value users choose for operations, they should be prepared to deal with either more or less water. (Users should remember that even if the 90% exceedance probability forecast is used, there is still a 10% chance of receiving less than this amount.) By using the exceedance probability information, users can easily determine the chances of receiving more or less water.
Colorado Water Supply Outlook Report - June 1, 2012
The 2012 water year is definitely one that will be remembered for quite some time. Coming on the heels of one of the wettest and snowiest years in recent memory it began with great expectations and with well above average precipitation in October and significant early season snow storms across the state. Unfortunately hopes were thwarted by a very dry December which left our first snow surveys of the year measuring a below average snowpack. From that point, with the exception of some decent snow accumulation in February, the weather managed to conjure up some painful memories of the 2002 drought. In the end this season saw the lowest statewide snowpack accumulation since 2002 and in some basins, this year became the new minimum on record. This spring the entire state has experienced persistent warm and dry weather patterns contributing to dry soils and the early melt of an already anemic snowpack. The only part of the equation separating this year from conditions in 2002 is reservoir storage. Across the state storage volumes remain very close to average thanks to the abundant snowfall and runoff from the 2011 winter.
May ended up being another dry and warm month across Colorado. The continuation of this abnormally warm weather caused the snowpack to continue to melt out at a nearly uninterrupted pace. By June 1, the snowpack was nearly nonexistent in all of Colorado’s major basins with only 4 out of the 92 SNOTEL sites used in this report, measuring any snow. The statewide snowpack as of June 1 was a negligible 2 percent of average, and 1 percent of last year’s report on this date. Basin by basin only the Colorado, South Platte, Arkansas and combined Yampa, White and North Platte basins had any snow remaining and only at the higher elevations in the basins. Most SNOTEL sites were completely melted out about a month earlier than normal. The warm weather this spring in combination with dry winds and dry soils really decimated what little snowpack we had received. These conditions also result in a fair amount of sublimation which will likely have an impact on streamflow volumes.
Precipitation at Colorado’s SNOTEL sites was well below average this May. Six out of the last seven months have recorded below average precipitation across the state. Statewide totals for May were just 42 percent of average which dropped the water year to date precipitation to 71 percent of average and 59 percent of last year’s totals. The combined San Miguel, Dolores, Animas and San Juan basin received precipitation that was only 14 percent of average for the month and the Gunnison basin was also very dry at just 23 percent of average. The South Platte basin fared somewhat better with monthly totals at 62 percent of average. Water year totals range from a high of 85 percent of average in the Upper Rio Grande basin to 66 percent of average in the Colorado basin.
Reservoir storage in the state is slightly below average. In an average year the state’s reservoirs store 3786 kilo acre feet (KAF) of water at this time of year. This year storage levels are at 3716 KAF, which equates to 98 percent of average. Since May 1 storage volumes statewide have declined by 24 KAF. Typically storage volumes increase during May but water managers filled their reservoirs early due to low streamflow predictions and demand has likely increased with above normal temperatures. As of June 1 the Arkansas and Upper Rio Grande reported the lowest storage volumes in the state at 78 and 57 percent of average respectively. All other major basins in the state are reporting near or above average storage. The Gunnison River basin’s reservoirs are storing 823 KAF which is 103 percent of average. The Upper Colorado basin is storing 905 KAF which is 113 percent of average. The South Platte basin is storing 930 KAF which is 95 percent of average. The combine San Miguel, Animas, Dolores and San Juan basins are storing 526 KAF which is 106 percent of average. And the Yampa basin is storing 34 KAF in Stagecoach reservoir which is 113 percent of average; Yamcolo reservoir was not included in this report due to a broken gauge.
Another dry month in May brought additional decreases to the streamflow forecasts across western Colorado. As a general rule, forecasts for the western basins range from about 25 to 50 percent of average. In the Upper Rio Grande, Arkansas, and South Platte basins seasonal predictions improved by a couple of percentage points for most forecasts due to higher than expected observed flows so far this spring. These basins can expect volumes of 20 to 60 percent of average. The lowest forecasts, as a percent of average, are in the headwaters of the Gunnison River, where the forecasted flow for the Tomichi Creek tributary is just 7 percent of average. The state’s best outlook, while still quite dismal, is for the Upper Rio Grande basin as a whole. Streams in this basin are expected to run at 40 to 60 percent of average from April to September. In summary, across the state, early snowmelt has translated to earlier than normal peak flows, which will likely be followed by an earlier than normal return to base flows in mid-summer.