Reanalysis of the depth-area ratios in the Walnut Gulch Experiemental Watershed

Kevin J. Strongman, David C. Goodrich1, Carl L. Unkrich1
1Department of Hydrology and Atmospheric Sciences, University of Arizona

The North American Monsoon affects Desert Southwest storms that feature short, spatially-compact, intense rainfall events. This results in a unique challenge for engineers in designing storm drainage and transportation infrastructure for growing population centers. The USDA-ARS-SWRC1 Walnut Gulch Experimental Watershed (WGEW) in SE Arizona has an extensive, high-resolution rain gauge network operating since ~1960. In 1980, rainfall observations from 40 gauges from 1957-1976 were used to create depth-area ratio curves for comparison to NOAA Atlas 2 curves that are commonly use to design stormwater infrastructure.

This study used the annual maximum rainfall intensities for 85 gauges in the WGEW through 2018. With nearly four decades of additional observations and 45 more gauges, we found the new depth-area ratios have increased over the 1980 analysis for the 60-, 120-, and 360-minute durations, but they are still well below the NOAA Atlas 2 ratios. This suggests that storm water engineers using NOAA Atlas 2 may have been over-designing storm infrastructure within the Desert Southwest, leading to higher building costs. Depth-area ratios will need to be further refined to account for multiple storms that behave differently from one another (e.g. storm duration and motion) compared to more heterogeneous storms with similar intensity and areal coverage. Storms that impact certain groups of rain gauges, or individual gauges that are not in close proximity to each other will also be examined.

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