Surface Water Hydrology and Flood Recurrence in the Flagstaff, Arizona Area, 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.

亚利桑那州弗拉格斯塔夫地区因其气候、地质和植被的独特性而呈现出独特的地表水水文特征。该区域在自然未受干扰的领域中经历了极为低量的降雨径流。这种‘平和’的水系状况，在诸如城市化、野火甚至森林疏伐等干扰因素引入景观时，会导致流量和洪水情况的显著变化。本报告利用57个河流和雨量计，提供了初步数据，旨在为管理者、工程师和科学家们提供关于弗拉格斯塔夫地区‘平和’与‘剧烈’水系特性的信息。这是自1988年美国地质调查局关于弗拉格斯塔夫地区洪水频率报告以来，该地区首次发布的区域性地表水水文报告。初步结果表明，以往关于洪水频率的分析预测的洪水流量远高于实际观测的实测值。在某些地点，区域回归模型的过度预测超过了观测值的两倍。本报告寄望于成为深入理解该地区水文驱动因素和河流特性的‘垫脚石’。为了对弗拉格斯塔夫地区的降雨径流、洪水频率和洪水防治设计做出具有说服力的预测，需要更多的数据，以及更长时间跨度的研究。