Effects of wildfires on peak discharges in watersheds

Wildfires can alter the hydrological processes in watersheds resulting in increases in peak discharge - one of the most important hydrological variables used in water resources applications. It thus follows that the standard methods used to calculate rainfall runoff should be modified in order to model the potential changes in watershed response under post-fire conditions. However, no reliable methodology for quantitatively assessing the effects of wildfires on hydrological parameters, such as curve numbers or runoff coefficients, has been identified to date. The approaches currently used are usually site-specific, mainly based on personal experience or very simple empirical strategies and then affected by a degree of uncertainty. This paper addresses issues regarding the estimation of the Soil Conservation Service Curve Number (SCS-CN or CN) and considers the case study of San Giuliano, L’Aquila (Italy), a small urban basin recently affected by a wildfire that resulted in a significant reduction in forest cover. The effects of the fire on runoff are modelled by adjusting CNs according to existing approaches from the literature in order to perform a sensitivity analysis for post-fire conditions; this allows us to examine the effects of the variability in model input parameters (estimates of post-fire CNs) upon expected peak discharges related to different return period storms. The fire effect ratio, which can be seen as a global parameter for describing alterations in the watershed response due to fire, is calculated by dividing post-fire peak discharge by pre-fire peak discharge. For the present case study, this ratio ranged between 1.1 and 2.3, indicating the urgent need for quantitative research on the effects of wildfires on the hydrological variables affecting runoff calculations.