Mapping Post Wildfire Curve Number Using Geographic Information System (GIS)

Abstract

This study aimed to develop a method for accurately updating the Curve Number (CN) after fire and assessing the impact of wildfire on the hydrological processes of a watershed in southern Arizona, USA. The proposed method used geographic information system (GIS), DEM map, vegetation map, soil map, lookup table, and a percent canopy reduction map to update the CNs for each 30-meter pixel in the study area. Two models were developed based on the USDA Technical Release-55 (TR-55) and PC-Hydro (GCNM-PC) manual to create the gridded CN map with the resolution of 30 meters. The JE Fuller’s HEC-HMS model used by Pima County was also used for comparison of the results. Also, USGS regional regression model and field data were used as the reference to select the best model for updating CNs for the post-fire conditions. The results indicated that the pre-fire runoff calculated using the CN of GCNM-PC had smaller errors than that from the JE Fuller model and TR-55, demonstrating the effectiveness of the proposed method. The GCNM-PC model exhibited a stronger correlation with the USGS regional regression data for pre-fire condition compared to the JE Fuller and TR-55 models. Thus, the GCNM-PC model can be considered as a more reliable and accurate model for updating the CN map.The analysis of CN changes in different areas of the study site revealed that areas with high burn severity experienced the most significant increase in CN values, while areas with low burn severity had the least increase in CN values. The peak discharge at the watershed outlet increased following the fire, with 21 of the 26 sub-basins’ outlets experiencing an increase in discharge up to 22%, highlighting the importance of considering the effects of wildfire on hydrological processes in watershed management and planning.