Review of the Gladstone, Colorado, Rainfall Observation, October 5, 1911, and its Impact on Site-Specific Probable Maximum Precipitation (PMP) Values

The October 4-6, 1911, rainstorm over the southwestern US produced large rainfall amounts and significant flooding. The eight inch daily rainfall total reported on October 5 is an extreme amount for the Gladstone elevation of 10,400 feet. The reliability of the reported rainfall observation has been questioned by many investigators but, lacking definitive evidence that the observation was unreliable, it remained in the data base. Applied Weather Associates (AWA) conducted several site-specific Probable Maximum Precipitation (PMP) studies for basins in western Colorado where the Gladstone observation had a dominate impact. However, lacking evidence that the Gladstone observation was erroneous, it was used in the studies. AWA became aware of a thesis produced at Colorado State University (CSU) that investigated paleoflood flood evidence in the Cement Creek drainage basin where Gladstone is located. The thesis concluded that a relatively large but not catastrophic flood had occurred in the drainage basin. A hydrologic run-off modeling effort was initiated to simulate the flood that would have been produced in the Cement Creek basin had the reported daily rainfall of eight inches occurred. The results of the modeling effort and comparison with the computed flows based on the paleoflood evidence showed a factor of 16 difference. Considering potential errors associated with both the paleoflood flow estimations and the run-off modeling, an order of magnitude difference remained. Based on this analysis and comparison, it was recommended that the Gladstone observation be considered erroneous and a recommendation was made that the observation not be used in future hydrologic studies. The Colorado State Engineer Office has concurred with the recommendation. The impact of removing the Gladstone observation from the rainfall data base is approximately a 35% decrease in site-specific PMP values for western Colorado. This impact would also apply to regions of southeastern Utah, northern New Mexico and northern Arizona. 9 pp., 13 references.