National Water Census
Environmental water studies refer to understanding the quantity, timing, and quality of water flows, as well as the water levels and storage required to sustain freshwater and estuarine ecosystems and the human livelihoods that depend on these ecosystems. The concept of ‘environmental flows’ in stream ecology are the basis of these studies, but they go beyond the understanding of surface flows and include the understanding of variations in water levels in other aquatic systems such as lakes, springs, wetlands, and aquifers.
Advances in ecological water studies rely on the development of techniques that can be used to link elements of hydrologic regimes to estimates or indicators of how a range of aquatic organisms living in the ecosystem will biologically respond. Developing accurate and reliable metrics requires studying biological responses – such as how different types of aquatic habitats respond to hydrologic changes and researching how aquatic organisms respond to key hydrologic variables – in order to identify indicator species which can be used to draw broader inferences about how the full community is likely to be affected. In a stream ecosystem, this includes aspects of the stream's hydrograph – for example, the seasonality, timing, and frequency of low and high flows, as well as the speed (or rate) at which hydrologic processes occur. It could also include water-quality characteristics, such as dissolved oxygen, temperature, or other factors that are known to influence the occurrence and distribution of aquatic organisms.
The USGS is developing innovative tools and resources that provide stakeholders and environmental water science practitioners with the hydrologic and ecological information necessary for comparing natural and altered hydrologic regimes and determining the effects of streamflow alteration and water withdrawals on aquatic ecosystems. The goal is to build stakeholders' capacity to use science to develop their own management metrics and guidelines. At the national scale, this includes developing stream simulation and modeling tools for building a national foundation of baseline streamflows that will ultimately produce hydrographs from which a suite of ecologically important hydrologic statistics for all ungaged stream locations in the U.S can be calculated. Baseline hydrographs provide a hydrologic reference point by which hydrologic alterations can be considered.
A national streamflow classification structure and set of flexible tools developed for the National Water Census will give stakeholders the capability to evaluate stream characteristics and ecological parameters at regional and local scales. This builds stakeholder capacity to use science to develop management metrics and guidelines. At the national scale, this includes stream simulation and modeling tools for building baseline streamflow estimates, which are derived from a suite of ecologically important hydrological statistics at un-gaged stream location in the United States. Baseline hydrographs provide a reference point by which hydrologic alterations can be considered.
In addition to the national-scale ecological water studies, the USGS is performing ecological water studies at the large-river basin scale in Focus Area Studies. These studies emphasize development of modeling tools that are highly transferable to other large-river basins. Several of these studies are incorporating components of a scientific framework for evaluating ecological water needs at multiple spatial scales known as the Ecological Limits of Hydrologic Alteration (ELOHA). ELOHA is a flexible framework for supporting water management decisions based on scientific information about ecological responses. The USGS role is to provide background scientific information that stakeholders can use to develop and test their own scenarios.