Antecedent Precipitation Analysis for Radar-Based Design Storm Development in Phoenix, AZ

As part of the development of radar-based design storms in the Phoenix, AZ, area used for water infrastructure design, an analysis of expected antecedent moisture conditions was needed to support hydrologic-hydraulic modeling. Several hundred precipitation gauges found in Phoenix from 1994 to 2012 were identified and evaluated to find the ideal antecedent precipitation for a design storm. When using a design storm of a given frequency for infrastructure planning, some assumptions must be made regarding the moisture conditions likely present at the beginning of the storm. If a relatively rare moisture condition is assumed, the combined probabilities (i.e., event probability and antecedent moisture probability) can be quite different than the intended design frequency. The task reported herein used antecedent precipitation in the Phoenix area as a surrogate for antecedent moisture conditions. The study evaluated the likelihood of antecedent precipitation affecting the design storm response and found that dry antecedent moisture conditions can safely be assumed and are recommended for a design storm.