One of the basic objectives of a hydrometric technician's work is to gather data for the determination of daily discharge. A detailed knowledge of the procedures involved in preparation of these data to publishable standards is essential.
While several automated procedures have been developed for the computation of open water discharges, the following sub-sections explain the procedures used in the manual preparation of Daily Discharges. Procedural mistakes could well arise if one is not aware of the functions the computer is performing during automatic computations. To truly understand the basic concepts of discharge computations, the procedures for manual data processing must be understood.
Figure F-5 illustrates the steps involved from data collection through to data archiving. This chart, taken from Water Survey of Canada (Hydrometric Technician Career Development Program. 1990. Vol. 1-5), an Environment Canada training manual, demonstrates the complexity and large number of components involved in the process. It can be seen that gauge heights must be compiled. In the simplest case, a daily mean stage (or gauge height) is determined from the stage records. This is then used with the stage-discharge table to determine the daily mean discharge.
The daily mean gauge height is often used to compute the daily mean discharge, as described above. However, a daily mean discharge determined directly from the daily mean gauge height may be in error for a number of reasons.
These reasons include:
(b) the relative curvature in the stage-discharge curve in the range of stage recorded during the day.
To obtain a more accurate determination of the daily discharge, it may be necessary to subdivide the day into two or more parts, determine the mean gauge height for each part, and determine the discharge for each mean gauge height. From these, compute the weighted mean discharge for the day. If the resultant weighted mean discharge differs from that determined using the mean gauge height by more than the selected allowable limit of 2% for discharge above a predetermined amount, then subdivision is necessary for all similar conditions.
To determine whether it is necessary to subdivide, examine the data and select a few sample days that may be critical because of the conditions listed in (a) to (b). Compute the daily mean discharge for these days: (1) from the daily mean gauge height, and (2) by subdivision. A few tests of this nature will provide the necessary experience for the particular station upon which to base future decisions regarding the necessity of subdivision.
Allowable range tables may be used to determine if a day for a particular station needs to be subdivided. The following trial and error procedure is used for drawing up allowable range tables. A hypothetical example extracted from the Manual of Hydrometric Data Computation and Publication Procedures, page 27 (Environment Canada 1980), is used to illustrate the procedure:
(a) From the stage-discharge table, select a range in stage during medium flow, for example, from 3.0 to 4.0 m.
Suppose that the discharge at gauge height 3.0 m equals 186 m³/s and the discharge at gauge height 4.0 m equals 339 m³/s. The mean discharge for this range in stage would then equal 262 m³/s.
However, you observe that at a mean gauge height of 3.5 m, the discharge is only 252 m³/s. This represents a difference of 4% (10 divided by 262 x 100) which is not allowable.
(b) Select a smaller range, for example from 3.0 to 3.4 m. Calculate the mean discharge for this range in stage. Compare the mean discharge with the actual discharge at the mean gauge height for this range. Now you get a difference of 1%. This is too low, but 3.0 to 3.6 m gives 2%, which is satisfactory.
(c) Now try a range between 4.0 and 4.6 m. This gives a 1% difference, which is too low. Try between 4.0 and 5.0 m, which gives a 3% difference. Therefore, an allowable range of 0.8 m is about right.
(d) The range from 6.5 to 7.5 m will give 2%.
(e) After several such attempts, you will develop an approximate allowable range table. When in doubt, subdivide.
Figure F-4. First page of typical expanded stage-discharge table.
Figure F-5. Flowchart for manual computation of streamflow data.
Sometimes the discharge for a given stage at a particular station is greater when the stream is rising than when it is falling. This produces a loop (or hysteresis) curve. On a simple stage-discharge curve, it will be found that measurements made on a rising stage tend to plot to the right of the curve, while those made on a falling stage tend to plot to the left. As stated in Rantz et al. (1982, page 414):
The discharge measurements for individual flood waves will commonly describe individual loops in the rating. In other words, "there will be a different loop for each flood". The departure of measurements from the rating curve for steady flow is of significant magnitude only if the slope of the stream is relatively flat and the rate of change of discharge is rapid. For gauging stations where this scatter of discharge measurements does occur, the discharge rating must be developed by the application of adjustment factors that relate steady flow to unsteady flow. (Unsteady flow refers to discharge at a site that changes appreciably with time, as in the passage of a flood wave.) An example of a stage-discharge curve of this type is shown in Figure F-6.
The detection and application of loop curves is limited to large rivers and therefore beyond the scope of this RIC Manual.
Figure F-6. Example of a stage-discharge loop.
