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

On 28 July 1997, an unusually moist air mass, driven westward towards the foothills of the Rocky Mountains near Fort Collins, Colorado, produced torrential 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 fell in western Fort Collins. This extreme rainstorm was observed by three S-band weather radars, including two National Weather Service WSR-88D radars and the dual-polarization CSU-CHILL radar. Fourteen recording rain gages in and near the affected area recorded the event. The US Geological Survey, Colorado District, performed indirect peak discharge measurements. In our analysis, the two-dimensional, physically-based hydrologic model CASC2D is applied to examine the influence of rainfall and land surface data uncertainty on runoff predictions in the 25 km2 Spring Creek watershed. Soil saturated hydraulic conductivity values are calibrated in simulations of the rise in nearby Horsetooth Reservoir. Results of simulations driven by polarimetric 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 predictions. Hydrologic simulations with various levels of land-surface detail reveal that uncertainty in watershed characteristics has a considerably smaller effect on runoff predictions than uncertainty in the space/time distribution of rainfall. The soil saturated hydraulic conductivity, fraction of impervious area, and the retention depth are the most sensitive land-surface parameters.