To determine the stage-discharge relationship, assemble all stage-discharge information, plot the measurements on Form AQU-09, then determine the best fit curve or curves. This procedure has been extracted and modified from the Manual of Hydrometric Data Computation and Publication Procedures, Inland Waters Directorate, pp. 27-28.
Proceed as follows:
Select scales on the curve sheet so that the significant figures, as required for the stage-discharge table, can be read with reasonable accuracy. Gauge heights are plotted on the vertical scale and the discharges on the horizontal scale. Suggested scales for the gauge height are 1 cm = 0.1, 0.2, 0.5, 1.0, or 2.0 m and for the discharge, 1 cm = 1, 2, 5, 10, 50, 100, 200, or 500 m3/s. Make adequate provision for the entire range in stage which is known to have occurred during the history of the station. The stage-discharge relationship may have to be shown in one or more curves to obtain the degree of accuracy required for the computation of the stage-discharge table. Designate each curve as "low flow curve," "high flow curve," etc.. Where feasible, carry each curve down to or near the "zero flow" stage. Allow for at least 0.3 m of overlap between curves and use more than one sheet, if necessary, to avoid cramping and confusion.
When a new curve sheet becomes desirable, first plot all extreme high or low discharge measurements from former years on the new sheet. Then on this new sheet, plot the latest applicable stage-discharge curve. Finally, plot all new open water measurements and, if necessary, plot a new stage-discharge curve.
Field discharge measurements will have been computed in the field and plotted in the Field Data Book. Any measurement that is considered an outlier should have a supplemental measurement completed before leaving the site. The supplemental measurement will confirm an error in the first or indicate a shift.
Indicate the plotted point for each discharge measurement by a dot surrounded by an open circle about 2 mm in diameter. Circles that indicate measurements for previous years should be color coded to distinguish them from the measurements made during the current year. Designate a discharge measurement by its date (e.g., June 12, 1997) with a diagonal line from the plotted point (use the same angle, say 60°, on each sheet or draw the diagonal line about perpendicular to the curve).
Any set of points which lie within 7% of a selected curve can be defined as Curve 1. Calculate deviations in Form AQU-05. Positive and negative deviations should balance out, and all should be within 7% (of discharge). At the curves extremities, deviations in excess of 7% may be acceptable. An example of this tabulation is shown in Appendix III.
Measurements known to be affected by backwater may be plotted in pencil or by use of a distinctive symbol. To identify measurements made by another organization, use a different symbol (e.g., a triangle, square, or cross) with an explanatory note in the lower right-hand corner of the curve sheet.
The presence of shallow flow and low gradients, often make it difficult to obtain discharge measurements at very low flows. Extending the low end of the stage-discharge curve to zero flow is desirable, but problematic, because of the lack of accurate measurements. A graphical technique for determining the gauge height for zero flow is explained below (Figure F-2).
(a) Refer to the current stage-discharge curve (example in Figure F-3).
(b) Take three discharges that are in a geometric progression. (For example, 2, 4 and 8 m³/s).
(c) Plot up from 2 m³/s and across from 4 m³/s to obtain point (1).
(d) Plot up from 4 m³/s and across from 8 m³/s to obtain a second point (2).
(e) Draw a line through these two points.
(f) Draw a line through the points where 2 m³/s and 8 m³/s intersect the stage-discharge curve (points (3) and (4)).
(g) The intersection of these two lines identifies the best estimate of the zero-flow gauge height.
Note: For a stream where flow actually decreases to zero, it is possible to verify the estimate. This is more likely where there is a good control, however, it must be remembered that the water level can and does fall below the actual zero flow level.
Figure F-2. Graphical method for obtaining zero flow gauge height.
Figure F-3. Typical stage-discharge table as extracted from the relevant curve.
