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-discharge gaging stations are used to determine a best-fit exponent for the adjustment of gaged data to a site within the watershed, and to estimate the uncertainty associated with said adjustment. Average annual discharge is shown to be a good indicator of the required adjustment and uncertainty of cumulative probability distribution functions, especially 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 drainage area describing the median values. In this paper, the application is to flow-duration curves for the addition of hydropower capacity.