The main protocols for continuous discharge estimation are stage-discharge, index-velocity, and slope-area methods. The continuous monitoring methods are based on semi-empirical relationships built around elementary hydraulics formulae and simplifying assumptions that cannot encompass all the flow complexities. Most monitoring systems are based on the assumption of a steady and uniform regime for the channel flow.

For unsteady flows the relation-ships between variables are more complicated as the relationships between variables are not unique being distinct for the rising and falling stages of a time-varying event (e.g., [Schmidt, 2002]; [Nihei, 2006]; [Perumal, 1999]).

Frequently, estimation of stream discharges is based on the stage-discharge rating method ([Rantz, 1982]). Advances in acoustic and other sensing technologies, allowed the development of new methods and the continuous improvement of the existing ones. This is the case with the index-velocity and the continuous slope-area methods that are driven by various technologic advancements after the 1980’s ([Levesque, 2012]; [Smith, 2010]).

The main methods are shortly described in the following.

The Stage-discharge method (HQRC) is based on unique stage-discharge empirical relationships relating stage to discharge. Shifting and loops in the steady rating curves (RCs) may result from a number of physical factors such as in-channel modifications, presence and growth of vegetation, unsteady flow due to flood wave propagation, backwater, etc. Therefore, there is a need for adjusting HQRCs whenever deviations are observed during operations.

In addition to continuously measure the stage, the Index-velocity method (IVRC) method requires measurement of the stream velocity over a portion of a cross section. The two direct measurements are associated with rating curves that provide mean velocity and the area of the channel at the gaging site. The construction of the index-velocity rating uses regression technique applied to calibration data obtained from field measurement campaigns. Preliminary analysis is applied to the data acquired to construct the index-velocity rating curve to decide if the regressions are functions of one or multiple variables. The stage-area rating is developed analytically using the surveyed cross-section as input. The outputs from the two ratings are multiplied to compute a discharge for each stage and index-velocity measurement pair. New guidelines for IVRC implementation using acoustic instruments have been recently developed (e.g., [Levesque, 2012]). The performance of IVRC method in steady and unsteady flows is still under scrutiny ([Kastner, 2015], [LeCoz, 2014]).

Continuous slope-area method (CSA). The conventional slope-area (SA) method is typically used to extend the stage-discharge rating curve to high flows using high water marks produced during flood events. The method requires measurements of the stream stage at minimum two locations where the stream cross section is surveyed. The availability of low-cost recording pressure transducers, the method is used for continuous measurement of stream flows [Smith, 2010].