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. .- 4 Losses and Gains for Eight Unlined Canals Along the Purgatoire River near Trinidad, Colorado, 200ll-2llll4 2-mile reach (located from about 1.5 miles to 3.5 miles down- stream from the Picketwire headgate) of Picketwire Canal and deliver water directly to Baca lands. El Mora Canal water is delivered through a single lat- eral turnout from the Picketwire Canal, measured through a Parshall flume, and then distributed through a discrete ditch to the EI Moro lands. Similarly, the John Rood Canal water is diverted at the Model Canal head gate, carried in Model Canal about 1.5 miles, and then diverted into John Flood Canal through a turnout from Model Canal. The John Rood Canal water is measured using a Parshall flume and then distributed to John Flood lands (fig. I). Acknowledgments The author wishes to thank many landowners within the study area for allowing access to their property to make discharge measurements along the canals. In addition, the author would like to thank Thelma Lujan from the Purgatoire River Water Conservancy District for her assistance in coordinating site visits and providing contact information and Danny Marques from the Colorado Division of Water Resources for providing detailed information on the operation of the canal system and answering many questions. Methods for Loss and Gain Investigations Field reconnaissance of diversion structures and loca- tion of measurement sites, discharge measurements, and loss and gain computations are discussed in this section of the report. Field Reconnaissance Field reconnaissance was conducted to locate divcrsion structures, select measurement locations, and arrange access to sites. All diversion structures along each main canal were located, and their latitude and longitude coordinates were determined using a global positioning system (table I). Parshall flumes used to measure inigation diversion at the canal headgates ranged in size from 9 inches (9-ft3/S free-flow capacity) to 10 feet (300.ft3/S free.tlow capacity). Figure 2 shows a photograph of the water-stage recorder and Parshall tlume at Ihc Hoehne Canal headgate (HOHG). Lateral turnouts with Parshall tlumes and (or) splitter boxes are used to measure and divert water out of the main canal to shareholders along the L:anal for irrigation. Figure 3 shows an example of a splitter box located at site SS07 on Enlarged Southside Canal. Discharge Measurements Discharge measurements were made along the eight canals during steady-state conditions to identify subreaches with losses or gains. Steady-state conditions were defined as periods with little or no precipitation and stable canal inflows at the headgates for at least 4 to 7 days before measurements were made. Inflows were considered stable if flow did not vary by more than 5 percent at the canal headgates. Discharge mea- surements were made at main canal and lateral diversion sites using either an Acoustic Doppler Current Meter (ADCP) or a vertical-axis mechanical current meter. The ADCP measurc- ments were made following methods described by Simpson and Oltmann (1993). Vertical-axis current-meter measure- ments were made following methods described by Rantz and others (1982). The accuracy of a discharge measurement is affected by various factors such as condition of measuring equipment, characteristics of the measurcment section, spacing of obser- vation verticals, changing stage, measurement of depth and velocity, and other factors. Four accuracy classifications are used by the USGS to rate discharge measurements. A mea- surement rated excellent means that the measured discharge is probably within 2 percent of the true discharge; good, within 5 percent; fair, within 8 percent; and a measurement rated poor may be more than 8 percent different from the true discharge (Rantz and others, 1982). The accuracy classification of a dis- charge measurement is a somewhat subjective evaluation made by the field technician making. the measurement. The tield technician considers characteristics of the measurement sec- tion (type of channel bottom, flow distribution, and so forth), instrument performance, and other factors and then assigns a rating to the measurement. Discharge measurements made during this study were generally rated good or fair, and a small number of measure- ments were rated poor. Because of measurement uncertainty, a single pair of measurements may not be sufficient to determine if a loss or gain occurs in a subreach. Measured losses or gains must be greater in magnitude than the uncertainty associ- ated with the measurement to be considered meaningful. The uncertainty of a measurement is calculated on the basis of the measurement rating. For example, a measurement of 20 ft3/s with a rating of good has an assumed uncertainty of 5 percent, or ;t 1 ft3jS. Therefore, a loss or gain must be greater than 1 ft3/S to be considered meaningful. Discharge measurements began at the headgate flume in the main canal and were repeated, moving downstream, at multiple locations in the main canal. generally during a I-day period. Discharge measurements were made over several days along Enlarged Southside Canal due to the canal length and number of measurement sites. When it was necessary to extend loss and gain discharge measurements in a canal over more than one day, the last site measured in the main canal