The Gauckler–Manning coefficient, often denoted as , is an empirically derived coefficient, which is dependent on many factors, including surface roughness and sinuosity. When field inspection is not possible, the best method to determine  is to use photographs of river channels where  has been determined using Gauckler–Manning's formula.
The friction coefficients across weirs and orifices are less subjective than  along a natural (earthen, stone or vegetated) channel reach.  Cross sectional area, as well as , will likely vary along a natural channel.  Accordingly, more error is expected in estimating the average velocity by assuming a Manning's , than by direct sampling (i.e., with a current flowmeter), or measuring it across weirs, flumes or orifices. 
In natural streams,  values vary greatly along its reach, and will even vary in a given reach of channel with different stages of flow. Most research shows that  will decrease with stage, at least up to bank-full. Overbank  values for a given reach will vary greatly depending on the time of year and the velocity of flow. Summer vegetation will typically have a significantly higher  value due to leaves and seasonal vegetation. Research has shown, however, that  values are lower for individual shrubs with leaves than for the shrubs without leaves.
This is due to the ability of the plant's leaves to streamline and flex as the flow passes them thus lowering the resistance to flow. High velocity flows will cause some vegetation (such as grasses and forbs) to lay flat, where a lower velocity of flow through the same vegetation will not.
In open channels, the Darcy–Weisbach equation is valid using the hydraulic diameter as equivalent pipe diameter. 
It is the only best and sound method to estimate the energy loss in human made open channels. For various reasons (mainly historical reasons), empirical resistance coefficients (e.g. Chézy, Gauckler–Manning–Strickler) were and are still used. The Chézy coefficient was introduced in 1768 while the Gauckler–Manning coefficient was first developed in 1865, well before the classical pipe flow resistance experiments in the 1920–1930s. Historically both the Chézy and the Gauckler–Manning coefficients were expected to be constant and functions of the roughness only. But it is now well recognised that these coefficients are only constant for a range of flow rates. Most friction coefficients (except perhaps the Darcy–Weisbach friction factor) are estimated 100% empirically and they apply only to fully rough turbulent water flows under steady flow conditions.
One of the most important applications of the Manning equation is its use in sewer design. Sewers are often constructed as circular pipes. It has long been accepted that the value of  varies with the flow depth in partially filled circular pipes. A complete set of explicit equations that can be used to calculate the depth of flow and other unknown variables when applying the Manning equation to circular pipes is available. These equations account for the variation of  with the depth of flow in accordance with the curves presented by Camp.