PREDICTION OF RIVER DISCHARGE AT UNGAGED SITES WITH ANALYSIS OF UNCERTAINTY

A technique is outlined for assigning uncertainty to the estimation of cumulative probability distribution functions (flow-duration curves) for river discharge at ungaged sites. Data from existing stage-dischar ge gaging stations are used to determine a best-fit exponent for the a djustment of gaged data to a site within the watershed, and to estimat e the uncertainty associated with said adjustment. Average annual disc harge is shown to be a good indicator of the required adjustment and u ncertainty of cumulative probability distribution functions, especiall y when applied to mean expected discharge through a water-development project. The distribution of the actual values around the predicted is log-normal, with a simple power relationship between discharge and dr ainage area describing the median values. In this paper, the applicati on is to flow-duration curves for the addition of hydropower capacity.