Streamflow Measurements: The USGS Streamgage Network, A Vital Earth Observing System (Invited Presentation)

Many meteorologists know that the U.S. Geological Survey (USGS) streamgaging network provides both historic and real-time water level and streamflow information that is crucial for National Weather Service (NWS) river forecasting.  Streamflow data reported in near-real fashion (generally hourly) provide information on pre-storm river levels, and together with rainfall data, inform models and forecasters about rainfall-runoff yields, times-of-travel, and other hydraulic and hydrologic conditions that affect the timing and magnitude of recent and projected flows.  Ultimately, the stage-discharge relations (ratings), the basis of conventional USGS streamgaging techniques, provide the means for the NWS to convert forecasted streamflow into forecasted river stage, especially in relation to “flood stage”, the abiding focus of many flood-threatened communities and the general public.

What many meteorologists may not know is that the same network provides the scientific basis for protecting, managing, and sustaining freshwater that is safe and available for drinking and for other competing water demands, including for agricultural irrigation, energy production, industrial manufacturing, transportation and shipping, recreation, and ecosystem health. Statistical summaries of flow extremes, durations, magnitudes, and frequencies are the basis for the design, management, and regulation of hydropower facilities, water- and wastewater-treatment plants, flood-control-dams, bridges, culverts, and levees.  Federal Emergency Management Agency (FEMA) floodplain maps also are founded on annual-peak streamflow data extracted from the USGS streamflow database.  Many meteorologists may not know that tracking water-quality conditions and quantifying the chemical and sediment constituent loads carried by the Nation’s rivers and streams requires complementary streamflow information; indeed, robust and reliable characterization of water-quality and assessment of landuse and other anthropogenic factors affecting water quality greatly depends on the availability of streamflow records.

The Network Design

Of the 8,100 USGS streamgages, about 3,100 meet targeted, long-term Federal Information Priorities (goals) established initially by the USGS National Streamflow Information Program (NSIP) and its current (2016) successor, the USGS Groundwater-Streamflow Information Program (GWSIP). The Federal-priorities streamgage network is designed to meet federal responsibilities associated with forecasting floods; monitoring flows across international, interstate, and tribal borders; tracking flow in major river basins; and assessing long-term climatic, land use, and man-made impacts on streamflow and water quality in different environmental settings across the Nation.  

The remaining 5,000 streamgages in the USGS network are not specifically included in the NSIP design, but help address Federal informational needs and other national priorities related to hazard mitigation and water availability. These streamgages facilitate decisions, operations, and responsibilities of localities, States, Tribes, and other Federal agencies, including management of reservoirs, drinking-water intakes, groundwater pumping, and water-quality permitting. These streamgages also fill in network gaps by monitoring a broader range of watersheds and hydrologic conditions than can be covered by the Federal-needs streamgages alone.  The sites are also used to develop important USGS regional-regression equations that relate various statistical streamflow parameters (such as the 1-percent chance flood) to watershed characteristics.  For example, approximately ¼ of FEMA floodplain maps are based on such equations, as are the designs of countless highway bridges and culverts.

Improving the Network

The vast majority of USGS streamgages depend increasing on hydroacoustic technologies to measure streamflow.  Acoustic Doppler Current Profilers (ADCPs) and Acoustic Doppler Velocimeters (ADVs) now account for over 70 percent of USGS streamflow measurements.  Side-looking ADCPs are also used increasingly to continuously monitor streamflow in backwater and tidal conditions.  Such technologies provide rapid and extraordinary accurate streamflow measurements, but they continue to require travel to the gage, immersion in sometimes swift and dangerous currents, and skilled handling and complex hydraulic computations.   New technologies such as sophisticated cameras and radars coupled with advanced imagery analysis techniques, such as large-scale particle image velocimetry (LSPIV), may permit more rapid streamflow measurement under certain conditions, such as might exists for some smooth, stable channels.  In fact, it is possible that some streams may be continuously monitored by on-site LSPIV.

Often times, particularly during floods, streamflow information is needed where no streamgage currently exists.  The USGS has developed a “mobile networks” concept in response to this need.  New rapid-deployment gages (RDGs) can now be deployed and setup up within a few hours to provide water-level data based on surface radars.  Depending on channel conditions it may be possible to “rate” some sites with relatively few direct flow measurements and to improve or extend the rating with LSPIV techniques. 

Finally, the USGS is supporting the National Atmospheric and Space Administration (NASA) Surface Water and Ocean Topography (SWOT) mission to derive streamflow information using remote sensing techniques.  SWOT will provide water-level altimetry and slope data for many major streams in the United States and around the world. But the USGS is also working to rapidly evolve particle image velocimetry and “reverse modeling” techniques that hold promise for accurate “reach-length” flow measurements and, depending on choice of airborne or fix-placed platform, sustained flow monitoring campaigns for streams of median width and complexity.  Such data may find applications in real-time monitoring and modeling of chemical spills, flood-induced erosion of levee systems, and habitat studies during seasonal spawning or migration.