Hydrologic analysis of the Fort Collins, Colorado, flash flood of 1997

On 28 July 1997, an unusually moist air mass, driven westward towards the f oothills of the Rocky Mountains near Fort Collins, Colorado, produced torre ntial rainfall. The nearly saturated atmospheric column in conjunction with light upper-level winds resulted in a "warm rain" process convective storm with little net, motion. On the evening of 28 July over 200 mm of rain fel l in western Fort Collins. This extreme rainstorm was observed by three S-b and weather radars, including two National Weather Service WSR-88D radars a nd the dual-polarization CSU-CHILL radar. Fourteen recording rain gages in and near the affected area recorded the event. The US Geological Survey, Co lorado District, performed indirect peak discharge measurements. In our ana lysis, the two-dimensional, physically-based hydrologic model CASC2D is app lied to examine the influence of rainfall and land surface data uncertainty on runoff predictions in the 25 km(2) Spring Creek watershed. Soil saturat ed hydraulic conductivity values are calibrated in simulations of the rise in nearby Horsetooth Reservoir. Results of simulations driven by polarimetr ic and single-polarization radar-rainfall estimates and recording rain gage data show that for this extreme event in an urbanized watershed, rainfall estimation errors give rise to the most significant errors in runoff predic tions. Hydrologic simulations with various levels of land-surface detail re veal that uncertainty in watershed characteristics has a considerably small er effect on runoff predictions than uncertainty in the space/time distribu tion of rainfall. The soil saturated hydraulic conductivity, fraction of im pervious area, and the retention depth are the most sensitive land-surface parameters. (C) 2000 Elsevier Science B.V. All rights reserved.