Functional Flows for Developing Ecological Flow Recommendations

by Sarah Yarnell, Alyssa Obester, Ted Grantham, Eric Stein, Belize Lane, Rob Lusardi, Julie Zimmerman, Jeanette Howard, Sam Sandoval-Solis, Rene Henery, and Erin Bray

To protect California’s native aquatic species, stream flows need to be managed to support important ecological processes and habitat needs.

In practice, such flows are difficult and controversial to define and implement. Water diversions, dams and other water infrastructure, land drainage, and changing climate conditions have altered the timing and availability of water, creating demands that impair restoration of the full natural flow regime. While restoring full natural flow regimes in California rivers may not be possible, preserving key aspects of the flow regime, or functional flow components, may provide a means to conserve the state’s freshwater ecosystems.

What are functional flows?

The functional flows approach provides a basis for estimating how much water is needed for the environment, where key components of the natural flow regime are targeted rather than the full natural flow regime. Desirable functional flow components have a disproportionately important role in supporting the physical and ecological processes that create and maintain habitat and trigger native species to reproduce, thrive, and migrate.

A natural versus functional flow regime. The functional flow regime preserves key aspects of the natural hydrograph. From Yarnell et al. 2015.

This process-based approach preserves the most important aspects of the variability of a natural flow regime to which native species have adapted.  It differs from environmental flow methods that focus on single species and their habitat requirements by instead capturing the needs of biological communities.

Most rivers in California have four functional flow components of the natural flow regime represent significant drivers of geomorphic and ecological processes:

  1. Wet-season initiation flows, or the first major storm event following the dry season. These flows represent the transition from dry to wet season, and serve important functions such as moving nutrients downstream and signaling species to migrate or spawn.
  2. Peak magnitude flows, which transport a significant portion of sediment load and maintain and restructure river corridors.
  3. Spring recession flows, which signify the transition from high to low flows and provide reproductive and migratory cues and redistribute sediment.
  4. Dry-season low flows, which favor native species adapted to withstand stressful periods.

Each of these four components are quantified by flow characteristics, including magnitude, timing, duration, frequency, and rate of change. Together, these aspects of flow comprise a functional flow regime; no single component or characteristic alone constitutes a functional flow regime. The relative importance of functional flow components may vary locally, while flow characteristics will likely vary based on water year type (dry, moderate, wet conditions). Year-to-year hydrologic variability provides periodic disturbances needed to support diverse aquatic and riparian communities. Additionally, characteristics should remain similar to natural values observed historically in order to protect native species and ecological processes.

Functional flow components are quantified using relevant flow characteristics linked to geomorphic and ecological processes.

While the functional flows approach can help estimate environmental water needs, it may not by itself provide a solution to ecosystem recovery. Additional physical habitat restoration may be required to realize the full benefits of a functional flow regime. For example, channel floodplain connections may be needed to support benefits of high flows that provide habitat diversity. Additional management measures may be needed to address a spectrum of water quality concerns in a watershed, including stream temperature, nutrients, sediment, and dissolved oxygen. These factors should be considered and explored further when utilizing a functional flows approach. Chapman et al. (2018) illustrate the success of this type of approach for Putah Creek, California.

How could this approach be applied in a management context?

A recent PPIC report recommended establishing an ecosystem water budget, which would allocate a block of water to the environment. Such budgets would bring the environment to the table as a partner in water management and allow for the trustee responsible for managing that water budget to participate in buying and selling water.

The functional flows approach provides a strategy for defining and allocating ecosystem water budgets. For example, by quantifying the functional flow components for a particular system, they can be aggregated to estimate an annual ecosystem water budget. Once budgets are established, they can also be used to guide when environmental water allocations are necessary to achieve targeted flow functions, for example by releasing water from dams at critical times to increase peak winter flows, or by curtailing water diversions to ensure that summer baseflows remain within the targeted range required for ecosystem health. An ecosystem water budget informed by a functional flows approach also offers flexibility during changing conditions (wet and dry years) by providing context to consider seasonal or interannual differences in allocations that can balance ecological and human use.

Hypothetical functional flow regimes in a California river in wet and dry years. Initial environmental water budgets for different water year types are computed as the annual volume of water required to meet functional flow requirements.

The functional flows approach offers a flexible means of informing flow recommendations that capture significant processes upon which native species are hypothesized to depend. This approach could be an effective way to manage an ecosystem water budget for California that preserves important components of the natural flow regime.

Sarah Yarnell is a senior researcher and Alyssa Obester is a researchers at the UC Davis Center for Watershed Sciences. Ted Grantham is faculty at the University of California, Berkeley, and an affiliate of the Center for Watershed Sciences. Eric Stein is a Principal Scientist at Southern California Coastal Water Research Project. Belize Lane is faculty at Utah State University. Rob Lusardi is also a researcher at the Center for Watershed Sciences and is the UC Davis-California Trout Coldwater Fish Scientist. Julie Zimmerman and Jeanette Howard are affiliated with the Nature Conservancy. Samuel Sandoval is an Associate Professor in the Dept. of Land, Air and Water Resources at UC Davis and an UC Agricultural and Natural Resources Cooperative Extension Specialist. Rene Henery is with Trout Unlimited. Erin Bray is faculty at California State University, Northridge. These individuals are continuing to work on implementing a functional flows approach across California via the Environmental Flows Workgroup, a sub-group of the California Water Quality Monitoring Council. Stay tuned for upcoming related blogs.

Further Reading

Chapman, E., E. Jacinto, and P. Moyle (2018). Habitat Restoration for Chinook Salmon in Putah Creek: A Success Story. California Water Blog.

Jeffres, C. (2011). Frolicking fat floodplain fish feeding furiously. California Water Blog.

Mount, J., Gray, B., Chappelle, C., Gartrell, G., Grantham, T., Moyle, P., Seavy, N., Szeptycki, L., and Thompson, B. (2017). Managing California’s Freshwater Ecosystems: Lessons from the 2012–16 Drought. Public Policy Institute of California.

Poff N.L., Allan J.D., Bain M.B., Karr J.R., Prestegaard K.L., Richter B.D., Sparks R.E., Stromberg J.C. (1997). The natural flow regime. BioScience 47: 769–784.

Yarnell, S.M., Stein, E.D., Lusardi, R.A., Zimmerman, J., Peek, R.A., Grantham, T., Lane, B.A., Howard, J., and Sandoval-Solis, S. (2018). An ecologically based approach for selecting flow metrics for environmental flow applications. In Review. Journal of Ecohydraulics.

Yarnell, S. M., Petts, G. E., Schmidt, J. C., Whipple, A. A., Beller, E. E., Dahm, C. N., … Viers, J. H. (2015). Functional Flows in Modified Riverscapes: Hydrographs, Habitats and Opportunities. BioScience, biv102.

Willis, A. (2018). The folly of unimpaired flows for water quality management. California Water Blog.

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4 Responses to Functional Flows for Developing Ecological Flow Recommendations

  1. Pingback: Water News for December 10, 2018

  2. J Rizzi says:

    SALT is the real issue not Flows! Shipping channel is dredged all the time keeping the channel open for ships, but it also causes the salt water intrusion and our fresh water flows out faster. Let us deal with the real problem SALT and not ASS U ME that water out flow is the only way to deal with our problems. Salinity barriers can hold back salt while let the shipping traffic pass and deal with our salt water intrusion problem.

    • BJ Miller says:

      This blog post is an important contribution to water management. The idea that management of flows should be based on their function is particularly important when water is scarce. So is the idea that ecosystem function should be the focus of management. This is an excellent piece of work.

      Here are a few additional considerations.

      Ecosystem function co-exists with mandates to protect specific species, especially natives. Ecosystem function cannot be in conflict with the Endangered Species Act, for example.

      Your statement that the four flow characteristics should be similar to natural values may not be true in all cases. Flow characteristics may have threshold values that differ considerably (up or down) from those that occurred naturally.

      Your caution that the functional flows approach may not by itself provide for ecosystem recovery is particularly appropriate in the San Francisco estuary, one of the most modified ecosystems in the world. It is possible that functional flows will have little effect if other factors limit ecosystem function. See, for example, Buchanan et al, “Survival of Juvenile Fall-Run Chinook Salmon through the San Joaquin River Delta, California, 2010-2015” No. Amer. Jour. Fisheries Management, March 2018, with particularly attention to the wet year of 2011.

      This argues for a more fundamental approach that incorporates the concept of functional flows as a key element. This fundamental approach first identifies factors limiting desired ecosystem function. This perhaps differs in emphasis from your approach, which seems to begin with functional flows as the basic consideration and considers other factors, channel floodplain connections and water quality concerns, for example, as possibly important.

      The limiting factors approach is especially important in highly altered ecosystems where water is scarce. It could be that benefits of functional flows cannot be realized, despite their high costs, because of nutrient contributions, changes in water clarity, growth of aquatic vegetation, proliferation of non-native predators, and/or toxic contaminants. This is not to say that functional flows should be abandoned, but, rather, that there should be no implicit assumption that such flows are the place to start.

  3. Pingback: A functional flows approach to managing environmental water – Grantham Lab

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