An Algorithm to Integrate Ecological Indicators with Streamflow Withdrawals

Historically, humans have controlled rivers for various applications such as transportation, hydro-power generation, flood control, and water supply, including agricultural irrigation. Though these activities proved to be economically beneficial, the altering of natural riverine systems resulted in negative affects on riparian ecosystems’ natural processes. The continued trend of river modification has resulted in damage to the biological integrity of many river ecosystems. Researchers have shown that natural streamflow variability over the full range of flows—from drought to flood flows—is critical to ecosystem dynamics. The variability of the flow regime, characterized by magnitude, frequency, duration, timing, and rate of change of hydrologic conditions, has been identified as integral in the sustaining of a natural ecosystem. Thus, the prevention of significant alteration to ecosystems needs to address a full regime of flow conditions. While progress is being made in assessing ecological integrity of riverine systems, a significant gap exists in the application of these advances into a watershed management setting. An annual based flow duration curve (AFDC) framework is the foundation of a methodology that links the ecological integrity of a riverine system with water resources management. The AFDC, as compared to the traditional period-of-record (POR) flow duration curve, has a robust statistical interpretation of streamflow, allowing for the determination of high and low flow AFDCs and their annual yield with a specified recurrence interval. The AFDC methodology integrates probability distribution functions (PDFs) to describe streamflow series. The methodology depends on sound ecological research that results in the identification of multiple “control points”, each vital in sustaining the integrity of a single species, biotic community, or an integral riverine process. A control point is characterized by that flow magnitude and corresponding frequency and duration which plays a critical role in the ecological processes of a riverine ecosystem. The use of multiple species or communities and their associated control points satisfies the need for maintaining the full range of natural habitat variation. Two specific measures for the framework implementation are required: 1) hydrologic flow measures and 2) allowable flow modifications. Control points are applied within the AFDC framework, allowing for the statistical representation of the magnitude, the duration, the portion of the flow regime, and the frequency of