Development of a surface water supply index (SWSI) to assess the severity of drought conditions in snowpack runoff areas

Colorado has experienced recent periods of drought in the winters of 1976-77 and 1980-81 which were detrimental to the state's economy. To deal with these conditions, Colorado has developed a drought response plan which relies upon a water availability assessment. A Surface Water Supply Index (SWSI) has been developed which when combined with the Palmer Drought Index, acts as both a triggering mechanism to activate the state's drought response plan as well as a yardstick to determine when a drought has effectively terminated. A single SWSI number is generated for each major basin on a monthly basis throughout the year. The index is derived from the cumulative probability distributions of snow course, precipitation, reservoir, and streamflow data. Each of the constituent variables in the index is weighted in proportion to its probable impact on total surface water resource of the basin. A SWSI map of Colorado is developed to display the information. The development of SWSI has been a joint endeavor of the Colorado State Engineer's Office and the U.S. Soil Conservation Service.