A framework for detecting stage-discharge hysteresis due to flow unsteadiness: Application to France's national hydrometry network.

• The method aims to detect stage-discharge hysteresis at gauging stations. • The method can be applied easily to a large set of gauging stations. • The hysteresis is quantified based on simple hydraulic concepts (Jones equation). • The use of bed slopes indicative of hydraulic controls is a key for good diagnosis. A generic framework is proposed to evaluate the relative discharge error made when ignoring stage-discharge hysteresis due to transient flow over large gauging station networks. The diagnosis is conducted using the Jones equation, a simple hydraulic concept relating discharge to stage and its time-gradient. The input data used for the method are the flow resistance coefficients, the temporal stage gradients, and the bed slopes. The hysteresis effect is quantified for each gauging station and mapped using the relative discharge error. The method was applied to 2618 gauging stations of France's national hydrometry network using data extracted from the national hydrological archive and from Digital Terrain Models. The diagnostic results highly depend on slope estimates used as inputs. Substantial hysteresis effects were found at stations with low bed slope combined with a fast flood regime. The application to France shows the difficulty of drawing a firm conclusion about stations prone to hysteresis due to the slope data uncertainty. This issue is not specific to France; slope estimates over a network of hydrometric stations are difficult to obtain in many countries. The use of local bed slope estimates is recommended to approach the slopes of the reaches controlling the station stage-discharge relation. [ABSTRACT FROM AUTHOR]