In May 2000, the Cerro Grande fire devastated lands in and around Los Alamos National Laboratory (LANL) in northern New Mexico and dramatically increased stormwater runoff from the impacted areas. The purpose of this paper is to demonstrate the recovery of burned lands and the corresponding decrease in flood flows and floodplains over a 7-year period in Pajarito Canyon near Los Alamos. A secondary purpose was to evaluate the applicability of the U.S. Army Corps of Engineers hydrologic modeling tool, Hydrologic Engineering Center—Hydrologic Modeling System (HEC-HMS) in Pajarito Canyon by comparing computed runoff to stream gage runoff in Pajarito Canyon. An initial study completed by Wright Water Engineers for LANL in 2001 predicted increased flood flows and floodplains under post-Cerro Grande fire conditions on LANL lands and surrounding property. Rainfall runoff calculations were made using the Natural Resources Conservation Services curve number (CN) method. For calibration, a hydrograph output from HEC-HMS was compared to hydrographs from two gaging stations in Pajarito Canyon. The model predicted an average of 42 percent of the peak discharge in upper Pajarito Canyon and 24 percent of the total volume discharge for three calibration simulations. For the same calibrations it predicted 37 percent of the peak discharge and 18 percent of the volume discharge in Twomile Canyon. While underestimated, these values may have been skewed by precipitation measurements. To demonstrate the watershed's recovery through CN modification, a design storm with an average recurrence interval (ARI) of 100-year was used to model the watershed's response for conditions representative of both year 2000 and 2007. The model consistently under-predicted the hydrograph peak and discharge volume for the calibration simulations and given the magnitude of the underestimation it was expected that the design storm predictions were underestimated as well. Through evaluation of the variability of the model's predictions and the intense data requirements associated with HECHMS it was determined that HEC-HMS has limited applicability within Pajarito Canyon.
Cerro Grande fire, Los Alamos National Laboratory (LANL), stormwater runoff, Pajarito Canyon, Twomile Canyon, watershed recovery
Meadows, Jake. "Comparison of Predicted and Observed Flood Flows in Pajarito Canyon Following the 2000 Cerro Grande Fire." (2008). http://digitalrepository.unm.edu/wr_sp/96