National Water Census
USGS has a long history of providing information on streamflow. Perhaps best known is the USGS national network of more than 8,000 streamgages. The USGS National Water Information System (NWIS) makes this valuable data available to the public, most of it in "near–real" time. The USGS National Water Census complements the streamgage network by estimating streamflow for ungaged locations throughout the country, by analyzing streamflow records, and by providing tools for analysis of streamgage data to end users.
However extensive, no streamgage network can provide direct observations of streamflow at every location of interest. Thus, managers need techniques that allow them to accurately estimate water availability – including metrics such as peak daily average flows or low flows – at specific ungaged locations. This is an important tool for decision-making, allowing managers to evaluate proposed water management scenarios in greater detail.
Therefore, one of the major components of the Water Census is a project to provide estimates of daily streamflow at subwatersheds nationally. A National Hydrologic Model has been built using the USGS Precipitation Runoff Modeling System (PRMS) . The National Water Census is building tools to allow easier access of this model and its results. Statistical models of streamflow have also been developed and are being used to improve calibration of the National Hydrologic Model.
One of the Water Census goals as called for in the SECURE Water Act is to analyze historic trends and provide annual updates of river basin flows. Information on trends are important for water management because changes in hydrology can impact water availability for public supply, industry, power generation, or agricultural use and can affect water quality and aquatic ecosystems. Characterizing trends in streamflow and developing a greater understanding of the causes of trends is thus critical to understanding future water availability.
Changes in surface–water hydrology can result from a wide variety of causes, including changes to water-management strategies, land–use changes, and climate variability or change. Climate variability and change is the least understood driver of streamflow change. To support research on trends over time, the USGS has conducted a number of studies to characterize trends in steamflow at gages that are relatively free of direct human influences. Most studies are based on gages included in the HydroClimatic Data Network (HCDN) , and the HCDN was recently updated to include 743 streamgages. Continued maintenance of a national network of such gages is necessary to detect and understand streamflow change.
To provide data from the existing streamgage network to decision-makers in a useful format, the USGS has conducted statistical analyses of streamgage records and developed hydrologic models for specific basins. For example, the StreamStats web application has been implemented for many States to provide estimates of these statistics. These estimates are derived using regression techniques that have been developed to perform well for streamflow statistics. The WaterWatch application provides nationwide estimates of runoff (flow per unit area) for HUC-8 watersheds in the continental U.S.