20 Aug 2025

Examining the Impact of the 2007 Zaca Fire on the Long-Term Hydrological Recovery of the Santa Cruz Creek Watershed in Southern California

Abstract: This study focuses on the Santa Cruz Creek watershed in Southern California, an area severely impacted by the 2007 Zaca Fire. The region is representative of wildfire-prone Mediterranean-climate catchments. We assess long-term post-fire hydrological recovery using a novel dual approach: (1) simulating 16 storm events over a 23-year period to evaluate pre-fire, post-fire, and recovery conditions, and (2) directly comparing two similar storm events—one pre-fire and one during recovery—to isolate changes in watershed response. Hydrological modeling employed HEC-HMS with the Deficit and Constant Loss Method, the ModClark Transform Model, and the Linear Reservoir Baseflow Model. Remote sensing data, including Enhanced Vegetation Index and SERVES Soil Moisture, enhanced modeling and analysis. Vegetation cover, soil moisture, and several watershed parameters show substantial recovery after five years. EVI reached 84 % of pre-fire values, while initial soil moisture deficit, time of concentration, and storage coefficient each recovered to roughly 70 %. Fast baseflow exceeded pre-fire levels at 143 %, but slow baseflow declined to 20 %. Peak discharge and direct runoff volume declined from post-fire highs of 173 % and 136 % to 125 % and 84 % of pre-fire levels, respectively. Although vegetative conditions stabilize, watershed hydrology remains altered.

13 Jun 2025

User Guidelines on Catchment Post-Wildfire Hydrological Modeling

Abstract: Wildfires significantly alter watershed hydrology by increasing runoff due to reduced infiltration from soil-water repellency. To predict long-term wildfire impacts, a coupled framework was developed to simulate postfire changes in soil hydraulic properties, infiltration, and hydrological response. This framework integrates Wildfire-Induced Soil Hydraulic (WISH) Factors with a Soil-Moisture Threshold (SMT) formulation in the Green and Ampt infiltration model, representing reduced infiltration due to water repellency. Postfire inputs, including burn severity, soil type, and land use, are formatted for the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model to ensure realistic hydrological simulations. The approach was applied to the 41.7 km² Upper Arroyo Seco watershed in northeast Los Angeles County, where 95% of the area was burned during the August 2009 Station Fire. Hydrological simulations effectively captured increased water repellency and excess runoff following postfire rainfall, demonstrating the model’s ability to represent wildfire-induced watershed changes and improve postfire hydrological assessments.