Ecologically Sustainable Watershed Management using Annualized Flow Duration Curves

A challenge for policy makers is to formulate a withdrawal permitting strategy with a sound scientific basis that maintains ecosystem ecological integrity and resource sustainability. The proposed annualized flow duration curve (AFDC) framework can provide such a basis. The framework allows for aggregate current condition evaluation, critical ecological streamflow controls integration, firm yield determination, and withdrawal impact assessment. The fundamental premise of the AFDC approach is that maintenance of a stream's ecological integrity depends upon maintaining an appropriate flow regime (magnitude and frequency). The AFDC provides a graphical tool to illustrate the quantity and frequency of streamflow available in a river basin. The AFDC, as compared to the traditional period of record flow duration curve (FDC), has a robust statistical interpretation of streamflow that allows for the determination of high and low flow AFDCs with a specified recurrence interval T (T-year return period). Ecological data can be used to identify critical points on the AFDC that serve to guide the timing and magnitude of withdrawals such that critical flow levels are maintained. These points provide the basis necessary to determine maximum available withdrawal rates. An estimate of the available annual yield is determined from a quantitative analysis of this modified flow regime. The AFDC framework also facilitates the simulation of the modified streamflow regime based on historic time series data. This study illustrates the application of the AFDC methodology to minimum flows and levels establishment in the lower Suwannee River basin in Florida. The case study characterizes the median (2-year) AFDC and the family of AFDCs corresponding to a series of annual high and low flow return periods under existing conditions. Control points that correspond to the high, medium, and low flow regimes are used to demonstrate the establishment of the modified flow that meets these ecosystem requirements. The new streamflow regime is examined by inverting the AFDC.