Derivation of formulae for variable lag time

Abstract

Formulae are developed which permit the computation of lag time for a catchment as a variable dependent on flood magnitude. Nonlinearity, as demonstrated by the variation in lag, differs little from catchment to catchment, and what difference there is can only be weakly related to the catchment's characteristics. A highly simplified analysis of the stream channels indicates that they are subject to a nonlinear hydraulic response of the same order of magnitude as that measured in the catchments as a whole. It is therefore suggested that the nonlinearity associated with the catchment areas is chiefly the result of that to be found in their channel systems. The absolute magnitude of lag time is highly correlated with catchment area and slope. This correlation leads to the development of three formulae for use with ungaged catchments which closely resemble those developed in the past for the estimation of a constant lag time.