Duration of Probable Maximum Precipitation on Lake Catchments: Alternative Analysis

An analysis of records from a New Zealand lake illustrates two methods for estimating the duration of wet weather that will cause the probable maximum lake level. Method 1 shows that, in 68 years, the 17 top-ranked annual maximum levels coincide well with the 17 top-ranked annual maximum 14-day inflows, and not so well with shorter or longer maxima. Method 2 shows that the solution of the differential equation relating lake level, discharge, and inflow has a maximum level for a 14-day duration inflow and depends on the lake surface area, the rate of increase of lake discharge as the level rises, and the rate of decrease of average flood inflow as the duration of the average increases. During these floods, the wind is always from the northwest, rain is enhanced by an orographic barrier, and there are distinct 3-day episodes of unusually intense rain that coincide with slowly moving fronts that increase the orographic effect. Probable maximum precipitation estimates up to now have applied moisture maximization to just one of these episodes. However, the flood rises continue for many more days, with only minor recessions, and it is suggested that a sequence of two episodes over 14 days should be maximized when estimating the probable maximum precipitation.