Event Based Post-Fire Hydrological Modeling of the Upper Arroyo Seco Watershed in Southern California

Abstract

Understanding, development and integration of pre-fire and post-fire watershed hydrological processes into a watershed hydrological model in a wild-fire repeating region similar to parts of California is critical for emergency assessments. 95% of the upper Arroyo Seco watershed located in Los Angeles County in southern California was burned by the Station fire that occurred in August 2009, significantly increasing the watershed observed runoff. This watershed was employed to develop the January 2008 rainfall runoff model as a pre-fire event-based watershed hydrological model. This pre-fire watershed model was subsequently employed in the rainfall events of 18 January 2010 and 27 February 2010, a few months after the fire event of August 2009. The pre-fire watershed model when employed in the post-fire rainfall events without considering the fire effects vastly underestimated the simulated discharge. For this reason, in this study of the post-fire catchment runoff modeling the following points are taken into consideration: (a) a realistic distributed initial soil moisture condition; (b) a formulation that includes a reduction factor and a burn severity factor, as multiplying factors to soil hydraulic conductivity in the soil characteristic curve; and (c) runoff routing parameterization under burned conditions. Developing the post-fire Arroyo Seco watershed model by using the above-mentioned points enhanced the Nash–Sutcliffe Efficiency from −24% to 82% for the 18 January 2010 rainfall event and from −47% to 96% for the 27 February 2010 rainfall event.

Modeling Post‐Wildfire Hydrologic Response: Review and Future Directions for Applications of Physically Based Distributed Simulation

“…The models reviewed here have made tremendous progress on accurate simulation of post‐wildfire hydrologic response. To achieve continue advancement of the accuracy of post‐wildfire hydrologic modeling, it may be helpful to develop new models from the ground up to simulate processes where post‐wildfire effects cause substantial deviations from expected behavior, such as infiltration (Ebel & Moody, 2013; Imeson et al., 1992; Moody et al., 2009; Shillito et al., 2020), rather than trying to alter existing infiltration approaches through parameter adjustment (Ebel & Moody, 2020; Pradhan & Floyd, 2021; Rengers et al., 2019). In parallel with building new models and finding better ways to use existing models, much may be gained through developing a modular, community modeling approach (Weiler & Beven, 2015; Wood et al., 2011) for post‐wildfire hydrologic simulation.…”

Examining the effect of moisture thresholds on post-fire water-repellent soil: a large-scale modelling approach applied to the Upper Arroyo Seco watershed, California, USA

“…Upper Arroyo Seco pre-fire hydrological model Pradhan and Floyd (2021) developed the Upper Arroyo Seco watershed GSSHA hydrological model, incorporating processes such as infiltration (Green and Ampt 1911), soil moisture accounting (Pradhan et al 2020), two-dimensional hillslope/ overland diffusive wave routing and one-dimensional diffusive wave channel routing. To numerically define these processes and identify the parameter values, the post-fire infiltration and routing process used the burn severity map overlaid on the land-use and soil maps shown in Figs 3b and 4, respectively.…”

“…Upper Arroyo Seco post-fire hydrological model Pradhan and Floyd (2021) developed a formulation for postfire conditions by incorporating multiplying factors explicitly linked to the wildfire burn severity conditions. These multiplying factors reduce the hydraulic conductivity from the unburned soil condition and are applied in the infiltration process unsaturated soil characteristic curve.…”

“…A distributed initial soil moisture level, based on satellite imagery and soil properties formulation, was used. In the post-fire routing process, the hydraulic Manning roughness at a location in the watershed varies according to the burn severity condition (Pradhan and Floyd 2021). Further details regarding the initial soil moisture content, wildfire-induced soil hydraulic factors and soil moisture threshold formulation are discussed in the following sections.…”

“…This study identifies the soil moisture threshold value for the transition from hydrophobic to hydrophilic conditions at the watershed scale through the integration of distributed initial soil moisture conditions and a distributed post-fire hydrological process (Pradhan and Floyd 2021) with the physics-based Gridded Surface Sub-surface Hydrological Analysis (GSSHA) model (Downer and Ogden 2004;Pradhan et al 2020). Pradhan and Floyd (2021) developed the post-fire hydrological model of the Arroyo Seco watershed covering 41.7 km 2 in Los Angeles County, CA, USA. This model introduced reduction and burn severity factors as multipliers for soil hydraulic conductivity in the soil characteristic curve of the infiltration/runoff-generation process.…”