Percent Bias Assessment of Water-Supply Outlooks in the Colorado River Basin

TitlePercent Bias Assessment of Water-Supply Outlooks in the Colorado River Basin
Publication TypeConference Proceedings
Year of Conference2014
AuthorsHarrison, Brent, and Bales Roger
Conference Name82nd Annual Western Snow Conference
Series TitleProceedings of the Western Snow Conference
Date Published2014
Conference LocationDurango, Colorado
Keywordsbias, Colorado River, forecast, forecast skill, runoff

Water-supply forecasts on various watersheds are intended to predict the April through July (snowmelt) runoff and assist in estimating the total water-year runoff, and are thus are very important to users of water from those watersheds. Water-supply outlooks, a type of forecast, are made on major contributing watersheds of the Colorado River. This study reviewed the characteristics of twenty-eight watersheds on the Colorado River. During that review, a strong linear relationship was found between watershed elevation and yield. As elevation increased, the runoff yield increased in a linear fashion. When studying the relationship between runoff and area, it was found that there was a non-linear relationship between increasing area and increasing runoff. The skill level of April to July forecasts was examined using percent bias as a representative summary measure of forecast skill. Review of percent bias of forecasts during dry, near normal and wet years indicates that in dry years the forecasts have a positive bias while those in wet years have a negative bias. Forecasts for near normal runoff years show limited or no bias toward over or under prediction. Seventy percent of the values for the absolute value of percent bias for individual forecasts were 40 percent or less.