Control structures such as V-notch weirs and sharp-crested weirs are commonly used in irrigation canals to measure flow. The stage, measured a short distance upstream of the control structure, is functionally related to discharge, though each weir must be calibrated to account for conditions such as weed growth, obstructions and algal growth, that can affect this relationship. If measured with electronic pressure transducers or shaft encoders and relayed as digital signals to a data logger, the reading can be telemetered. In the case of the GBP sites, only site A is measured using a sharp crested weir. The discharge measured at Site A should have an accuracy of better than +/- 5%.

GWD gauges flow at canal check structures by measuring the height of the flow above the boards. This method of flow gauging has been sufficiently accurate for the District's accounting purposes. Flow is typically measured once per day using a graduated rule placed upon the top weir board of the control structure. Accuracy of flow measurement has not been determined at all sites, although monitoring by both GWD and LBNL along the Agatha Canal during 1994 and 1995 suggests that flows are within 15% of those measured with the acoustic Doppler and pressure transducer technologies. Although the GWD will continue to measure flow at Sites J and M, flow measurements at these sites are not critical for the GBP compliance monitoring program since compliance with the Use Agreement requires that selenium concentrations in wetland supply channels be lower than 2 ppb. Calculations of loads at these sites is not required for purposes of compliance monitoring, but suitable for mass balance calculations.

In large streams such as Mud and Salt Slough and the San Joaquin River, where control structures are not available, flow measurements are made by using direct stage measurements and a stream rating curve. The rating curve for a gauging station is a graphical depiction of the relation between stage and discharge. Each station's rating curve represents the individual characteristics of each site which, in the case of a stream, may change from time to time after flood events, seasonally, or as a result of sedimentation or stream bed erosion. These changes result in a correction or "shift" in the fixed relationship between stage and discharge. Occasionally, downstream conditions may control the discharge, creating a "backwater" condition at which time the rating curve is no longer valid. During these episodes, flow measurements must be made directly. It is necessary to visit this type of flow monitoring station regularly to develop an accurate rating curve and to check the current stream rating. This subject is discussed at length in the USGS Quality Assurance Program for the GBP, found in Appendix D of the Quality Assurance Project Plan (USBR, 1996). Both Mud and Salt Slough stations are rated "good" by the USGS and should produce flow measurements that are accurate to within +/- 10 % during those times of the year when the Sloughs are not in backwater (i.e. stage is influenced by a downstream control).

Site N at Crows Landing Bridge has only recently been rated and hence has insufficient recent flow record to be assessed for reliability. Until this data is available the station should be designated "poor" at the present time with an expected discharge measurement error of more than +/- 15%.

Another measurement system, deployed at Site B, on the SLD, is an acoustic Doppler sensor, which measures flow velocity directly within the canal. The flow sensors are mounted on adjacent bridge piers at 2/10 ths and 8/10 ths of the average flow depth in the SLD, aligned to face each other across the canal. The sensors make measurements every minute and report a mean velocity every 15 minutes. The velocity measurements are combined with stage measurements, also taken every 15 minutes, to produce a discharge measurement. This measurement system is very accurate and is anticipated to produce flow measurements with an accuracy of between 1% and 5%.

Table 4.2.1 summarizes the field measurement techniques at each of the primary sites at which flow monitoring will be performed.