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Journal of Hydrology, Volume 521, February 2015, Pages 360–373.
Zoran Micovic1, Melvin G. Schaefer2, George H. Taylor3.
- BC Hydro, 6911 Southpoint Drive (A02), Burnaby, BC V3N 4X8, Canada and
- MGS Engineering Consultants, 7326 Boston Harbor Road NE, Olympia, WA 98506, USA and
- Applied Climate Services, 5478 SW Philomath Blvd., Corvallis, OR 97333, USA
Abstract
An analysis of uncertainty associated with Probable Maximum Precipitation (PMP) estimates is presented. The focus of the study is firmly on PMP estimates derived through meteorological analyses and not on statistically derived PMPs. Theoretical PMP cannot be computed directly and operational PMP estimates are developed through a stepwise procedure using a significant degree of subjective professional judgment. This paper presents a methodology for portraying the uncertain nature of PMP estimation by analyzing individual steps within the PMP derivation procedure whereby for each parameter requiring judgment, a set of possible values is specified and accompanied by expected probabilities. The resulting range of possible PMP values can be compared with the previously derived operational single-value PMP, providing measures of the conservatism and variability of the original estimate. To our knowledge, this is the first uncertainty analysis conducted for a PMP derived through meteorological analyses. The methodology was tested on the La Joie Dam watershed in British Columbia. The results indicate that the commonly used single-value PMP estimate could be more than 40% higher when possible changes in various meteorological variables used to derive the PMP are considered. The findings of this study imply that PMP estimates should always be characterized as a range of values recognizing the significant uncertainties involved in PMP estimation. In fact, we do not know at this time whether precipitation is actually upper-bounded, and if precipitation is upper-bounded, how closely PMP estimates approach the theoretical limit.
Figure Legend
Figure shows estimates of the 24-hr Probable Maximum Precipitation for La Joie Watershed based on 2000 simulations for five sources of uncertainty (1. in-place moisture maximization; 2. maximum surface dewpoint; 3. storm horizontal transposition; 4. storm center location; 5. storm efficiency)
Figure credit: Journal of Hydrology (2015) / Vol. 521 / pp. 360–373.
