Areal Rainfall Reduction Factors for Areas of the US Southwest Dominated by Thunderstorms During the North American Monsoon

ABSTRACT Areal reduction factors (ARFs), also referred to as depth‐area ratios, are applied to point‐scale observations of precipitation to estimate the average accumulation across an area. They are commonly used by engineers to design storm drainage, transportation infrastructure, and conservation practices. In the Desert Southwest, determination of ARFs is challenged by the summer North American Monsoon, which spawns air‐mass thunderstorms that feature short, intense, localized rainfall events. The Walnut Gulch Experimental Watershed (WGEW) in SE Arizona has an extensive, high‐resolution rain gauge network operating since 1953. In 1980, ARFs were computed using observations from WGEW for the period 1957–1976 and were compared to curves presented in NOAA Atlas 2. This study updated the WGEW ARFs for a longer period of record (1957–2018) and compared them to the 1980 ARFs. Additionally, spatial variations in ARFs and extreme rainfall across WGEW were examined. Results indicate the updated ARFs are larger (therefore resulting in less of a reduction) for design storm durations of 60, 120, and 360 min with return periods of 10 and 100 years. For these cases, it is implied that infrastructure designs using the 1980 ARFs are now under‐designed. Spatial variations between ARFs computed across the WGEW were relatively small, in the range of 1%–6%.