John Henz and William Badini HDR Engineering, Inc Denver Colorado
The Flathead Lake Drought Management Plan (DMP) was developed for use as a federally mandated input to assist in the operations of the Kerr Hydro-Electric Project(PPL-Montana) and the Hungry Horse Reservoir (Bureau of Reclamation) when conflicts occur between the minimum instream flow requirements of Article 56 and the lake levels set forth in Article 43 of the project license The DMP is implemented by use of hydro-climate based inputs.
Two prognostic indices and one diagnostic index were chosen for application in the DMP. The prognostic indices are the “August/September Multi-variant El Niño and Southern Oscillation (ENSO) Index” (referred to as the Aug/Sept MEI) and the “June/November Southern Oscillation Index” (referred to as the June/Nov SOI). These two indices provide the opportunity for a realistic prediction of drought conditions near the start of the water year. The diagnostic index is the October-December Montana Climate Division 1 (referred to as CD-1) precipitation which is available in January.
When the Aug/Sept MEI value is greater than 0.50 and the corresponding June/Nov SOI is less than 0.50, a dry water year outlook is issued that identifies the need to mandate the DMP in January. In January, if the CD-1 precipitation is less than 3.50 inches, then a critically dry year (October to March precipitation less than 70 percent of normal) would be expected and the DMP would be mandated or continued. The CD-1 precipitation index was refined by the Flathead Precipitation Runoff Index (FPRI) based on the statistical relationship between the observed April to September runoff and the observed October to April precipitation at eight precipitation stations in the watershed.
The combination of these two indices results in a greater than 75 percent correct forecast of a low runoff year from October to December based solely on the MEI value being greater than 0.50, indicating an El Niño climate factor. The application of the precipitation-based FPRI trigger in January, in concert with the MEI, results in an 86 percent correct forecast in January that improves to 96 percent correct in April. The technique was successfully applied in both the WY2003 and WY 2004.