Hydrography: The science of measuring streamflow
A couple of weeks ago, this column discussed the Rio Grande Compact.
For the Rio Grande Compact, seven streamflow gaging stations are operated in the San Luis Valley to account for the water originating in the Conejos and Rio Grande systems and the water delivered from these systems to New Mexico. The Hydrographic Branch of the Colorado Division of Water Resources operates these seven gaging stations along with 73 other gaging stations across the San Luis Valley.
In addition to the Compact gages, stations are operated on natural streams and creeks to help water commissioners allocate the available water to water users, maintain a historic record of water in the stream, account for trans-mountain water brought into the basin, record water diverted at critical diversion structures, and provide valuable information to recreational enthusiasts such as kayakers and fishermen.
The data from these sites may be later used for water supply planning, flood warning, environmental studies, and basin modeling, such as the Rio Grande Decision Support System (RGDSS).
Gaging stations are located next to streams and are typically small metal or aggregate enclosures with an antenna protruding out the top. They operate by recording the water level, also called water stage or gage-height. The water stage is then converted manually or by computers to streamflow, also referred to as discharge, using equations and tables developed by Hydrographers (hydros). Hydros visit each of the sites regularly and measure the streamflow and water stage to develop the stage-discharge relationship that is then used to compute the streamflow. Once the relationship has been developed, hydros continue to measure the stage and streamflow to verify that the relationship remains valid and to make adjustments, called shifts, as needed.
Hydros most frequently measure the streamflow with a current meter. These measurements are performed by stringing a tape measure across the stream perpendicular to the flow and then dividing the cross-section into 25 to 30 subsections with approximately five percent of the total flow falling within each subsection. In each subsection the depth, width, and velocity are measured and the discharge is calculated by multiplying the width, depth, and velocity. The discharge for each subsection is then added up to calculate the total streamflow.
Low flow streamflow measurements are usually performed by wading the stream and using a current meter attached to a specially graduated wading rod. When it is not possible to wade the streams, cableways or bridges are used with the current meter suspended from a cable system with a heavy weight to hold the meter in the desired position.
Recently, the hydros have started using ADCPs (Acoustic Doppler Current Profilers) to measure the streamflow. These devices are attached to a float which is pulled across the river reducing the hazard of snagging submerged debris. ADCPs are able to collect more data than the current meter in less time when conditions permit.
In the winter, when ice has completely changed the channel and the stage-discharge relationship cannot be used, hydros continue to measure the flow by removing the ice or drilling holes in the ice so that estimates of daily streamflow can be made through the winter months. Hydros also assist water commissioners by making streamflow measurements at diversion structures to calibrate flumes and weirs.
The hydrographic principles used today are very similar to those developed in the late 1800’s. In 1883, Colorado’s second State Engineer, S.E. Nettleton, developed the Colorado Current Meter which is a close predecessor to the Price AA and Price Pygmy current meters used today. S.E. Nettleton also installed one of the first chart recorders on the Cache la Poudre River in 1884. Chart records are used to continuously record stream stage on graph paper. The first chart recorder required someone to change the paper every week.
Today electronic data loggers record most of the data used and transmit the data to the internet so it is available to all in near real time. Computers are used to perform much of the tedious work of converting recorded stage to streamflow, but the concepts are still very much the same as they were over 100 years ago.
The streamflow records published by the hydrographic branch are checked and reviewed for accuracy to standards that meet or exceed the standards produced by our federal counterpart, the U.S. Geological Survey. All of the gages operated for the Rio Grande Compact are reviewed by U.S.G.S. personnel. The real-time streamflow data may be found at http://www.dwr.state.co.us/.
For more information on Water 2012 in the Rio Grande Basin, please visit our website at www.water2012.org or www.rgwcei.org.
This week, Water 2012 has some very exciting events. Tune into KSLV on Wednesday, February 22nd at 8 a.m. to hear Mike Gibson, manager of the SLV Water Conservancy District, Chair of the Rio Grande Roundtable, and President of the Colorado Water Congress, along with Leah Opitz, Coordinator of Water 2012 in the Rio Grande Basin, to discuss the role of the Rio Grande Interbasin Roundtable with Water 2012. Also on Wednesday the 22nd at 7 p.m. in Porter Hall Room 130 at Adams State College, the Colorado Field Institute will host Angie Krall for their winter lecture series. She is the Heritage Program Manager for the Rio Grande National Forest and San Luis Valley Field Office of the BLM and she will give a presentation on the human relationship with water through time and how water has shaped the archaeology of the San Luis Valley and its public lands entitled “Water is Life: Water Histories and Archaeology in the San Luis Valley.”
For more information on this event, please visit their website at www.coloradofieldinstitute.org. On February 24th, all entries are due for the SLV Irrigation District’s poster and original artwork competition. For more information on this event, please visit their website at www.slvid.org.