Flagstaff area stream flow and rainfall - 2008-2019

Flagstaff, Arizona has unique surface water hydrology due to climate, geology, and vegetation. The area experiences extremely low rainfall-runoff in natural undisturbed areas. This “complacent” watershed condition leads to dramatic shifts in flow and flooding when disturbances such as urbanization, wildfire, or even forest thinning are introduced to the landscape. Using 57 stream and rain gauges this report provides preliminary data to inform managers, engineers, and scientists on both the complacent and “violent” watershed characteristics of the Flagstaff area. This is the first regional surface water hydrology report since the 1988 US Geological Survey report on flood frequency in the Flagstaff area. Preliminary results indicate that previous flood frequency analyses provide a much higher predicted flood flow than empirical gauge results have observed. In some sites the over-prediction of regional regressions is over twice observed values. The hope is that this preliminary report will provide a “stepping stone” towards a greater understanding of the hydrologic drivers and stream character of the area. More data, over a longer time period, is required for making defensible predictions of rainfall-runoff, flood frequency, and flood mitigation design in the Flagstaff area.