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National Water Census

USGS WaterSMART activities

USGS WaterSMART activities

Ecological Water


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Ecological water refers to understanding the quantity, timing, and quality of water flow and storage required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems. It is based on the concept of "environmental flows" used in stream ecology, but recognizes that the ecological effects of water extend beyond stream and river flows and include variation in water levels in other aquatic systems such as lakes, springs, wetlands, and aquifers. Ecological water science has advanced greatly over the past two decades due to a growing need to use this type of information to inform water management decisions, to prevent degradation of freshwater ecosystems, and support management decisions that require balancing human and ecological water needs.

Advances in the science of ecological water 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. Rather than requiring a complete inventory of how survival, reproduction, and mortality of all aquatic species are affected by a range of hydrologic processes, these techniques allow managers to draw broader inferences about the overall ecological response. 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 (for example, fish and invertebrates) in aquatic systems physiologically and behaviorally respond to key hydrologic variables in order to identify representative (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 occurance and distribution of aquatic organisms.

Baseline Hydrographs

As part of the Water Census, the USGS is developing novel tools to help stakeholders compare natural and altered hydrologic regimes and understand the effects of streamflow alteration and water withdrawals on aquatic ecosystems. The goal is to build stakeholder's capacity to use science to develop their own management metrics and guidelines. At the national scale, this includes developing and comparing modeling tools for a national foundation of baseline hydrographs of streamflows that will ultimately provide a hydrograph and a suite of ecologically important hydrologic statistics for all ungaged stream locations in the U.S. A baseline hydrograph commonly represents a statistical or modeled construct that strives to reflect the natural (also called "minimally impaired" or "unmanaged") fluctuations in the duration, magnitude, timing, frequency, and rate of change in streamflow. This information will be served directly to stakeholders via a web-based map interfaces called the Water Census National Data Platform, which will enable connections to other large ecological data compilations including those collected by other Federal, State and tribal organizations. Ultimately, estimates of daily streamflow and a suite of ecologically relevant hydrologic metrics will be integrated with biological observations in the platform through a common spatial framework.

Stream Classifications

In addition, a national streamflow classification structure and set of flexible tools are being developed that will allow stakeholders to evaluate a region of interest at the scale necessary for sound management. These tools are being designed to serve both a predetermined set of stream classes derived from a subset of existing baseline hydrographs and, alternatively, to provide the users with the option of deriving a set of stream classes based on user-specified input. These tools are meant to complement existing local and regional efforts to classify streams that stakeholders may be using for more detailed assessment and management purposes.

Ecological Effects of Hydrologic Alteration

In addition to the national scale efforts, USGS is investigating ecological water science at the large river basin scale (referred to as Focus Area Studies), with emphasis on developing modeling tools with a high degree of transferability to other 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) (Poff et al. 2010).

ELOHA is a flexible framework for supporting water management decisions based on scientific information about ecological responses. It involves building a hydrologic foundation of daily streamflow hydrographs representing a baseline condition that can be compared to a present-day management scenario. Then, river types are classified according to hydrologic and other characteristics and flow alteration from baseline conditions are assessed at every analysis point. Finally, flow-ecology relationships are developed to quantify biological responses to different degrees of hydrologic alteration, based on data and modeling. This can be used by decision-makers to assess the ecological outcomes of proposed management scenarios or policies. USGS role will be to provide background scientific information that stakeholders can use to develop and test their own scenarios.

In the Apalachicola-Chattahoochee-Flint Basin WaterSMART Focus Area, scientists are conducting field studies of fish species to calibrate simulation models. These models will couple changes in fish species occurrences to changes in streamflow patterns, enabling researchers to test how management scenarios are likely to affect the patterns of local extinction and colonization of fish species through time, at a variety of locations (known as "metapopulation dynamics"). The findings of this study are expected to be broadly transferable to other river basins and regions throughout the U.S. by expressing models in terms describing characteristics of those fishes or mussels that respond most strongly to different types of changes in streamflow (such as lower or more variable flow during warm or high water use months). Read more about the ACF Basin study>>>

Ecological water science activities in the Delaware River Basin WaterSMART Focus Area will broaden the capabilities of an integrated decision support system (known as DSS) by modeling the effects of alternative water management scenarios on habitat availability for key native species such as trout and American shad, and endangered species such as the dwarf wedgemussel. Field and laboratory experiments are being conducted to develop habitat suitability criteria that model how key aquatic species will physiologically respond to changes in temperature and hydrology. To accomplish this, modeled estimates of habitat characteristics are also being developed for a large portion of the Delaware River mainstem across a range of discharge levels using advanced aerial technologies such as bathymetric LiDAR imagery. Planned work with the decision support system will help identify data gaps, evaluate the feasibility of extending the modeled area further down the mainstem, and provide options for several alternative water management scenarios. Additionally, a concerted effort is underway to define relations between hydrologic alteration and aquatic community response in tributaries of the upper watershed. Streamflow models are being developed to estimate baseline and present day streamflow conditions at all ungaged watersheds throughout the upper basin and relations between changes in streamflow process and aquatic community response for a defined set of steam types will be developed.

The Upper Verde River is serving as the pilot basin for ecological water analysis in the Colorado River Basin WaterSMART Focus Area, and plans are to conduct similar studies in two or three additional sub-basins. A model is being developed to test the availability of habitat needed to support different types of aquatic species under different flow volumes and aquifer levels. The analysis includes determining the hydrologic record and how far the seasonal hydrology departs from the historic record. Studies are also examining the status of the native and nonnative species and a water budget, including groundwater, that will help identify stream reaches vulnerable to drying.


Poff, N.L., Richter, B.D., Arthington, A.H., Bunn, S.E., Naiman, R.J., Kendy, E., Acreman, M., Apse, C., Bledsoe, B.P., Freeman, M.C., Henrik-sen, J.A., Jacobson, R.B., Kennen, J.G., Meritt, D.M., O'Keeffe, J., Olden, J.D., Rogers, K.H., Tharme, R.E., and Warner, A.T., 2010, The Ecological Limits of Hydrologic Alteration (ELOHA): A new framework for developing regional environmental flow standards: Freshwater Biology: v. 55, p. 147-170 (Also available at

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