• exceedance frequency;
  • instream flow;
  • municipal and irrigation use;
  • naturalized flow;
  • reservoir


The instream flow (IF) that is a required minimum flow amount for aquatic species and inhabitants has been one of key factors for dealing with the water allocation complexities since 1960s. In particular, the requirement of IF in river flow constraints the reservoir operation that focuses on water supply of municipal and irrigation use. This study focuses on evaluating the impact of IF on municipal and irrigation use from which the Waco reservoir is supplied. The exceedance frequencies of stream flow are performed based on three conditions: pre- or post-dam construction condition and reservoir storage reallocation condition. The monthly IF percentage expressed as a percentage of naturalized flow is computed using naturalized flow and IF available in Texas Water Availability Model (WAM) and applied to generate the monthly IF for 1939–1997 hydrologic-simulation periods. The effects of IF use are assessed on period and volume reliabilities for municipal and irrigation use. Sensitivity analysis was performed to provide the optimal amount and range of IF using two different kinds of constant and seasonal IF use percentage rate plans. These results in this study indicate that the presence of IF has not changed the exceedance frequencies of low flow but reduced exceedance frequencies of flood flows. Period and volume reliabilities for municipal and irrigation use have decreased by 2 or 4%. Copyright © 2010 John Wiley & Sons, Ltd.