Methodology for Developing Flood Rule Curves Conditioned on El Nino-Southern Oscillation Classification1

Regional climate varies on interannual and decadal time scales that in turn affect annual streamflows, flood risks, and reservoir storage deficits in mid-summer. However, these variable elements of the climate system are generally not included in water resources operating policies that attempt to preserve a balance between flood risk and other water resources system objectives. A methodology for incorporating El Nino-Southern Oscillation (ENSO) information in designing flood control curves is investigated. An optimization-simulation procedure is used to develop a set of ENSO-conditioned flood control rule curves that relate streamflow forecasts to flood control evacuation requirements. ENSO-conditioned simulated flood risk and storage deficits under current operating policy are used to calibrate a unique objective function for each ENSO classification. Using a case study for the Columbia River Basin, we demonstrate that ENSO-conditioned flood control curves constructed using the optimization-simulation procedure consistently reduce storage deficits at a number of interrelated projects without increasing flood risk. For the Columbia Basin, the overall improvements in reservoir operations are relatively modest, and (in isolation) might not motivate a restructuring of flood control operations. However, the technique is widely applicable to a wide range of water resources systems and/or different climate indices.