Incorporation of time-intensity and spatial variability into probable maximum precipitation (PMP) estimates
At present, considerations regarding the space-time variability of precipitation are absent from the procedures to establish design storms and PMP estimates, which are limited to depth-duration-area curves for a number of annual exceedence probabilities (return periods). In mountainous regions, where data is typically scarce and precipitation variability the most complex, these approximations lead to large forecast errors. Differences in mean annual precipitation of several hundred percent between windward slopes of orographic barriers and adjacent valleys or lee side slopes are usual. In fact, orographic precipitation associated with the forced ascent of moisture-rich air, followed by condensation and cloud formation and growth, is the major source of runoff in the western U.S. Eastward of the continental divide, orographic effects are not as important from the point of view of annual precipitation amounts, but they play an important role in defining spatial variability, and have been linked to catastrophic intensification of warm season storms such as the Virginia storm of 1969, and the Big Thompson storm in 1976. A methodology to incorporate time-intensity and areal precipitation patterns into the estimation of Probable Maximum Precipitation (PMP) is presented in this paper.
