National Water Census
USGS has a long history of providing information on streamflow which can be useful for decisions about water management through a number of its existing programs. Perhaps best known is the USGS data on flow, which is collected through a national network of more than 7,000 streamgages as part of the USGS National Streamflow Information Program (NSIP). The USGS National Water Information System (NWIS) makes this valuable data available to the public, most of it in "near–real" time. The USGS Water Census will complement existing programs by enabling analysis of trends in surface water availability, providing tools for analysis of streamgage data to end users, and developing methods for estimating streamflow at ungaged locations.
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. This complements one of the goals of the USGS NSIP, which is to provide regional assessments of changes in streamflow. The USGS NSIP streamflow network provides a strong and essential foundation for analyzing trends, with its combination of near real-time information and historical datasets.
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 (Lins, 2012). 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.
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. There are many existing modeling techniques that could be employed to develop these streamflow estimates. To make the most efficient use of limited resources, a national plan is being developed by comparing the accuracy, ease of implementation, and value-added capabilities of several different streamflow estimation models.
Estimates will be provided for the most recent 30–year historical period. While estimated time series will have larger uncertainty than time series measured at a streamgage, these estimates can provide needed information for water managers to make decisions and for ecologists to evaluate ecological water requirements. These will be made available to the public through a point and click web application as part of the National Water Census Data Platform.
Lins, H.F., 2012, USGS Hydro-Climatic Data Network 2009 (HCDN–2009): U.S. Geological Survey Fact Sheet 2012–3047, 4 p., available only at http://pubs.usgs.gov/fs/2012/3047/.