Climate change and the detection of trends in annual runoff

This study examines the statistical l~kelihood of detecting a trend in annual runoff given an assumed change in mean annual runoff the underlying year-to-year variabihty in runoff, and serial correlatlon of annual runoff Means, standard deviations, and lag-l senal correlations of annual runoff were computed for 585 stream gages in the contermlnous Unlted States and these statlstlcs were used to compute the p~obablll ty of detecting a prescnbed trend in annual runoff Assuming a h e a r 20% change in mean annual lunoff over a 100 yr perlod and a significance level of 95"/0, the average probability of detecting a significant trend was 28% among the 585 stream gages The largest probability of detecting a trend was in the northwestern U S the Great Lakes reglon, the northeastern U S the Appalachian h4ountalns and parts of the northeln Rocky hdountalns The smallest probability of trend detec t~on was in the central and southwesteln U S and In Florida Low probabilities of trend detection were assoc~ated with low ratlos of mean annual runoff to the s t a n d a ~ d devlatlon of annual runoff and with hlgh lag-l s e ~ i a l correlatlon in the data