The USGS streamflow gaging network provides information for calculating water balances of land surfaces and estimating the probability of extreme flows in rivers and streams. Eight variables related to water balances and extreme flows are used to evaluate the network: mean annual precipitation volume (PPT), median annual streamflow(Qmean), mean annual evapo-transpiration (ET), incremental median annual maximum streamflow (IncrQmax), incremental median annual minimum streamflow (IncrQmin), median annual fraction of time that streamflow is greater than zero (Hydroperiod), and mean annual withdrawals (Withdrawals). Each variable is calculated for incremental gaged areas (IGAs). Precipitation, evapo-transpiration, and withdrawals are calculated for 12-digit hydrologic unit code watersheds (HUC12). Incremental values of streamflow statistics are calculated as the difference between the statistics at the downstream (outflow) and upstream boundaries of an IGA.
Incremental streamflow (IncrQmean, IncrQmin, IncrQmax) values are only calculated for areas between sites with at least 5 years of daily streamflow values for water years 1981-2020 or for the drainage area of sites with no upstream sites. These sites comprise a censored network where IGAs have been merged if the gage at the downstream boundary is not in the censored network of sites. Any IGA with a downstream boundary gage that is censored is assigned a 'censored NETID' that identifies the first downstream site with data. The censored NETID can be used in a GIS to merge IGA polygons on the censored NETID and create IGAs for the censored network. For variables with censored networks, incremental streamflow statistics are calculated using available data and assigned to the IGA for the downstream boundary gage. The value represents combined incremental streamflow from all IGAs with that censored NETID. For example, the contributing area of an IGA in the censored network can be calculated as the sum of all of the IGAs that have a censored NETID that is equal to the censored network IGA's NETID.
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Three metrics are used to evaluate a monitoring network for each variable: coverage, resolution, and representation. The metrics are calculated from the cumulative distributions of the variables, shown below. Each metric varies from 0 to 1. Metrics are not displayed for multi-category variables (Climate Divisions, Ecoregions, Surficial Geology Types).
Coverage is the fraction of the total value of a variable that is found in areas that are monitored: 0 - none of the variable is in monitored areas; 1 - all of the variable is in monitored areas. Coverage can only be calculate for variables with known values outside of the monitoring network. .
Resolution indicates the deviation of the cumualtive distribution of IGA values from the mean IGA value: 0 - the value of a variable for monitored areas is concentrated entirely in one IGA, 1 - all IGAs have equal value (the mean IGA value of the variable). Resolution can only be calculated for spatially-conserved variables (count, length, area, volume, mass, or fluxes). The resolution of characteristics such as elevation, drainage area, temperature, or fraction of stream length or area with a designation or classification is based on either stream length or land surface area with the characteristic.
Representation indicates whether IGA values are evenly distributed among the decile intervals (0-10%, 10%-20%,...) of a benchmark distribution: 0 - all IGA values are outside the range of the benchmark distribution, 1 - 10% of IGAs are in each of the decile intervals of the benchmark distribution. Typically, HUC12s values are used for the benchmark distribution. If the HUC12 values are unknown, 10% intervals of the IGA range are used to calculate representation: ~0 - nearly all of the total value of a variable is in one interval of the range of values for IGAs, 1 - IGA values are linearly distributed (10% of IGAs are in each of intervals of the range of IGA values). Representation can be calculated for characteristics such as mean elevation or a percent of area in a land cover category, but they must be associated with a spatially-conserved variable such as stream length or land suraface area to create the distributions.
Network objectives can vary for different variables (for example, resolution of streamflow and representation of basin elevation) and should be considered when interpreting these metrics.
IGAs that are important for network coverage, resolution, or representations are designated as priorities to maintain monitoring sites. IGAs and HUC12 where additional gages would substantially improve network coverage, resolution, or representation are designated as priorities to add monitoring sites. IGAs are defined by upstream and downstream sites, so a site is a monitoring priority if it is at the upstream or downstream boundary of a priority IGA. The number of priorities IGAs is not a recommendation about the number of sites needed for monitoring. Indeed, a variable with few priority IGA simply indicates that there are a few areas of particularly high importantce for monitoring and that it may be possible to re-configure - but not eliminate - sites in non-priority IGAs without impacting network performance.
Priority designations are determined by applying consistent rules to the cumulative distributions of each variable. Generally priority areas have higher values of a variable but they may also be part of a variable's distribution that is not well represented by the monitoring network. The cumulative distributions shown for each variable include the specific criteria used for priority designations. Streamflow and material loads are not available for HUC12s, so HUC12s do not have priority designations for these variables. Ungaged areas (shown in the HUC12 layer as orange) can be considered as priorities to add gages for these variables.
Priority areas are not a network design. Network design including the total number of monitoring sites in the network and the specific locations for monitoring sites depends on the objectives and resources available for monitoring and considerations of access and safety. Priority designations are based on the USGS streamflow gaging network active in water year 2020 and could change as sites are added or removed from the network. Priority designations do not reflect legal requirements or local needs for streamflow information at specific locations. Any specific changes to a monitoring network should assess local information needs, access, and safety.
The map viewer has layers with monitoring sites, incremental gaged areas (IGAs), the priority designation of IGAs for individual variables, 12-digit hydrologic unit code watersheds (HUC12s), and the priority designations of HUC12s for individual variables.
Monitoring Sites: USGS gages active in water year 2020 with a pop-up link to the USGS Water Dashboard for each site
Incremental Gaged Areas: IGAs with the values of the variables for each IGA provided in a pop-up window. IGAs have a value of NA if the variable is not available.
IGA Priorities: priority designation of IGAs for each variable.
HUC12: HUC12 watersheds in the major river basin with the values of variables for each HUC12 provided in a pop-up window.
HUC12 Priorities: priority designation of HUC12s in the major river basin for each variable.
Boundaries of IGAs and HUC12s have been simplified for display in the map viewer.
IGAs often extend downstream of gages because they are delineated from National Hydrography Dataset catchments or Watershed Boundary Dataset HUC12s.
Pop-ups are only diplayed for the lowest active layer checked in the control box with data for the feature.
National maps can take a long time to render in the map viewer and only display ungaged HUC12s.
Ungaged areas (shown in orange on the HUC12 layer) are defined as HUC12s with less than half of their area upstream of a gage based on Watershed Boundary Dataset routing (https://www.usgs.gov/national-hydrography/watershed-boundary-dataset).
Data may be missing outside of CONUS where NHD catchments are not available.
Cumulative distributions are generated for each variable to evaluate the network and to identify priority areas for monitoring. Values of a variable for individual IGAs or HUC12s are shown on the horizontal axis. The vertical axis has cumulative values, area, or length depending on the type of variable. Priority areas for maintaining sites are plotted as open blue circles. Priority areas for adding sites are plotted as filled purple circles. The criteria for priority areas are listed at the bottom of each plot. The rules used to define the criteria depend on the type of variable. Missing plots indicate that the variable is not available. IGAs or HUC12 can be identified on the plots by hovering the cursor over a point. Axes can be adjusted with the cursor and reset using the menu at the top of the frame.
Cumulative distributions of incremental median annual minimum streamflow (IncrQmin), incremental median annual mean streamflow (IncrQmean), incremental median annual maximum streamflow (IncrQmax), and median annual fraction of a year with streamflow (Hydroperiod) for the gages at the downstream end of incremental gaged areas (IGAs). Incremental values are calculated as differences between the downstream gage and upstream gage(s) using median annual values for sites with 5 years of record from water year 1981 to 2020. IGAs have been merged where sites have less than 5 years of record (refer to Censored Networks above).
Cumulative distributions of precipitation volume (PPT), evapotranspiration (ET), and groundwater and surface-water withdrawals (Withdrawals) for incremental gaged area (IGAs) and 12-digit hydrologic unit codes (HUC12) watersheds.
Konrad, C.P., Anderson, S.W., Restivo, D.E., and David, J.E., 2022, Network Analysis of USGS Streamflow Gages, U.S. Geological Survey Data Release, accessed on August 26, 2022 at https://doi.org/10.5066/P9C8NYTO.
Wieczorek, M.E., Jackson, S.E., and Schwarz, G.E., 2018, Select Attributes for NHDPlus Version 2.1 Reach Catchments and Modified Network Routed Upstream Watersheds for the Conterminous United States (ver. 3.0, January 2021): U.S. Geological Survey data release, accessed on June 16, 2022 at https://doi.org/10.5066/F7765D7V.
U.S. Geological Survey, 2021, National Water Information System, accessed on November 17, 2021 at http://dx.doi.org/10.5066/F7P55KJN.