National Water Census
Through the Water Census, USGS takes a programmatic approach to research on water availability and use, bringing together diverse avenues of hydrologic and ecological research. Work conducted under the Water Census shares the common theme of building national capacity to scientifically assess, account for, and analyze water availability. This includes new and improved methods of accounting for human factors and environmental/ecological criteria that affect water management decisions. Many of the projects will develop innovative techniques for quantifying components of the water cycle by investigating aspects of the physical, chemical and biological characteristics that are reliable indicators of water availability. Therefore, the science behind the Water Census approach can perhaps be best understood through the concept of a water budget. For more information about the basic science, visit the Water Budget page.
In addition, the Water Census provides estimates that help quantify water flows and storage in a way that will support analyses of water availability by local and regional agencies. By synthesizing and reporting scientific aspects of water availability and use at the regional and national scales, research projects are designed to compile information useful to States and others responsible for water management and natural resource issues. Consistent reporting standards are important for aggregating and analyzing water availability across jurisdictional boundaries. Therefore, the USGS works with Federal and non-Federal agencies, universities, and other organizations to ensure that the information can be aggregated with other types of water-availability and socioeconomic information. For more details about collaborative efforts, visit the Collaboration page.
Another major goal of the Water Census is to provide estimates of selected water-budget components at consistent spatial and temporal scales. This includes the challenge of measuring surface water. USGS already maintains a carefully calibrated streamgage network of thousands of stream gages and follows quality control practices of reviewing and validating data. However, the USGS streamgage network, though extensive, has a finite ability to measure surface water and there stakeholders are often interested in estimating streamflow rates at locations without gages. Therefore, the Water Census includes a research project to develop techniques for estimating streamflow at ungaged stations.
Some water-budget components have proven difficult to estimate accurately using existing measurement techniques. One of these is evapotranspiration, the release of water from plants into the atmosphere, which can pull moisture from the soil. Even less well understood is the rate at which snowpack is lost due to sublimation – the transition of ice directly into water vapor.
Groundwater also presents a particular challenge in providing a complete budget on a watershed basis. Aquifer systems are complex, three-dimensional geologic features. They can cover great distances, commonly do not conform to surface-water divides, and may obtain most of their recharge at locations far from where the groundwater is pumped from a well or flows to a surface-water body. Because of these and other complexities, groundwater systems are incorporated into the Water Census in two ways. A major element of the Water Census focuses on regional analyses of groundwater availability in 30-40 principal aquifers that collectively account for more than 90 percent of the Nation's total groundwater withdrawals. In addition, researchers are estimating groundwater recharge, storage, and discharge at the watershed scale to the extent possible using a combination of information from the large-scale studies, data from observation well networks, analysis of streamflow records, and other available information.
The USGS Water Census released its first 5-Year report in April 2013. To access the other datasets, reports, and other types of information products produced by the Water Census program, visit the Data and Products page.
The Water Census builds on USGS existing programs as much as possible to maximize the potential for innovation and eliminate duplication of efforts, most notably including:
The goal of the Water Census is to provide a nationally-consistent base layer of well-documented data covering the water budget components such as surface water, groundwater, ET, water quality, water use, and ecological water. These data represent a collection of values associated with points, lines, areas, and grids, and are derived from a range of activities as diverse as trapping of fish in individual stream reaches to the interpretation of satellite imagery. The long-term objective of the Water Census is to provide measured or estimated information for all water-budget components at the hydrologic unit code (HUC) 12-digit scale, a relatively small basin size averaging 34 square miles. At present many types of data can only be determined at coarser scales. Another long-term objective is to develop the capacity to capture seasonal variation by providing information for water-budget components on a monthly basis. Some components, such as water-use data, are generally only available annually, or at even less frequent interval.
Finally, uncertainty is always a consideration in hydrologic measurement and modeling that affects decision-makers. USGS strives whenever possible to quantify uncertainty for the end users of data on water availability. The Water Census will also allow USGS to continue to address uncertainty in the highest priority water data and information by improving spatial and temporal coverage for key hydrologic variables and improving estimation techniques through advanced incorporation of key data layers into statistical and physical models.