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COLORADO WATER SUPPLY CONDITIONS UPDATE FROM THE OFFICE OF THE STATE ENGINEER; DIVISION OF WATER RESOURCES July, 1991 ROOM 818,1313 SHERMAN ST.,DENVER,CO 80203; (303)-866.3581 JOHN R.KALISZEWSKI;EDITOR Statewide water supplies have improved significantly compared to a year ago, especially on the west slope. A series of precipitation events in the first half of June lowered irrigation demands and provided for increased reservoir storage. The second half of the month brought hot, dry, windy conditions resulting in diminished streamflow and increased demands. Reservoir storage remains above average. Soil moisture conditions are generally much improved over last year's conditions. Water shortages, however, are likely to occur by late summer in those areas of the state with less than adequate storage. Statewide precipitation for the month of June was 126% of average ranging from a low of 99% of average in the Gunnison basin to 149% of average in the South Platte basin. Water year totals are slightly above normal across the state with the Rio Grande basin the highest at 127% of average and the South Platte basin the lowest at 90%of average. The statewide reservoir storage was 110%of average on July 1,which is 109%of last year's July 1 amount. The Gunnison basin continues to have the highest storage rate at 124%of average while the Arkansas basin has the lowest storage rate at 95% of average. Statewide storage is currently 53%of capacity (approximately 9 million acre-feet). The National Weather Service 30 day forecast (starting July 1) for Colorado is for below average precipitation and above average temperatures. The 90 day forecast (starting July 1) is for below average precipitation and above average temperatures for the east slope and above average precipitation and below average temperatures for the west slope. Going into 1991, 25% of the contiguous U.S. was experiencing drought conditions while 10% of the U.S. was experiencing extremely wet conditions. Drought conditions are expanding in the western U.S.while wet conditions are expanding in the eastern U.S. The Palmer index computed by the NOAA/USDA joint Agricultural Weather Facility identifies areas of prolonged abnormal dryness or wetness. The primary component evaluated is soil moisture. The index indicates extremely dry conditions occupying central and southern California,south central Washington,eastern Oregon,northeastern North Dakota, northwestern Minnesota, and the Colorado River basin in west central Colorado and east central Utah. Extremely wet conditions prevail in the upper midwest extending from southern South Dakota through Iowa,Illinois, and Indiana, and in the south central region of the U.S. including Louisiana, Arkansas, Mississippi, and Alabama. The Surface Water Supply Index (SWSI) developed by this office is used as an indicator of water supply conditions in the state. It is based on reservoir storage, streamflow, and precipitation for the summer period (May 1st through November 30). Weight factors are applied to each of the measured hydrologic factors in computing the SWSI values for each basin. During the summer period, streamflow is the primary component in each basin except the South Platte where reservoir storage is the primary component. The following SWSI values were computed for each of the seven basins on June 1, 1991 and 1990: July 1, 1991 Change From July 1, 1990 Basin SWSI Value June 1. 1991 SWSI Value South Platte +1.7 +1.0 +0.3 Arkansas +0.5 +0.3 +0.6 Rio Grande +0.1 -1.6 -0.5 Gunnison +1.2 +0.1 -1.9 Colorado +0.2 +0.7 -2.1 Yampa/White +0.1 +2.5 -2.4 San Juan/Dolores -0.4 -0.8 -2.7 SCALE -4 -3 -2 -1 0 +1 +2 +3 +4 Severe Moderate Near Normal Above Normal Abundant Drought Drought Supply Supply Supply 1