Areal rainfall reduction factors for areas of the US Southwest dominated by thunderstorms during the North American Monsoon

Authors: STRONGMAN, K.J. - University Of Arizona; Unkrich, Carl; Goodrich, David; FULLHART, ANDREW - University Of Arizona

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2026
Publication Date: 5/12/2026
Citation: Strongman, K., Unkrich, C.L., Goodrich, D.C., Fullhart, A.T. 2026. Areal rainfall reduction factors for areas of the US Southwest dominated by thunderstorms during the North American Monsoon. Journal of the American Water Resources Association. 62(3). Article e70120. https://doi.org/10.1111/1752-1688.70120.
DOI: https://doi.org/10.1111/1752-1688.70120

Interpretive Summary: The North American Monsoon spawns summer storms in the Desert Southwest that feature short, intense, spatially-compact rainfall events. This results in a unique challenge for engineers designing storm drainage and transportation infrastructure to come up with a “design storm” to use in growing population centers in the Southwest. Design storms are based on an estimate of rainfall intensity at a rain gauge location for a given storm duration and the average number years between a large storm (e.g. 1 hour, 50 return period). The design storm concept also requires a “Area Reduction Factor” (ARF). This accounts for the fact a rainfall intensity and total storm rainfall depth at a gauge generally does not occur over a larger watershed or planning area. The Agricultural Research Service, Southwest Watershed Research Center operates the Walnut Gulch Experimental Watershed (WGEW) in SE Arizona has an extensive rain gauge network (85 rain gauges in the 58 square mile WGEW) operating since ~1960. It is an ideal network to estimate the information for design storms. The National Weather Service (NWS) has used much less dense rain networks to estimate design storms. We found that the ARF estimates from the WGEW decrease much more rapidly than the NWS estimates. This suggests that storm water engineers and conservation planners using NWS area reduction factors may have over-designed storm infrastructure in the Desert Southwest. This would result in higher infrastructure building costs.

Technical 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%.