Analytical results for operational flash flood guidance

The theoretical basis of developing operational flash flood guidance systems is studied using analytical methods. The Sacramento soil moisture accounting model is used operationally in the United States to produce flash flood guidance estimates of a given duration from threshold runoff estimates. Analytical results to (a) shed light on the properties of this model's short-term surface runoff predictions under substantial rainfall forcing and (b) to facilitate flash flood computations in real time. The results derived pertain to the computation of pervious-area surface runoff from the upper zone tension and free water elements of the model, and to surface runoff from both the permanently impervious and the variable additional impervious area of the model that provides spatially distributed characteristics in the production of surface runoff. The time to the production of surface runoff is also computed on the basis of the model parameters and initial water element conditions and is related to the flash flood guidance duration. Analytical solutions for time to saturation and surface runoff are also derived for the case of saturation-excess models, and these results are compared to those derived for the Sacramento model. Various characteristics of the flash flood guidance to threshold runoff relationship are discussed and considerations for real-time application are offered. Uncertainty analysis of the threshold runoff to flash flood guidance transformation is also performed.