Soil Processes After Fire in an Urban Watershed

Abstract:

This research examines the post-fire response in an urban watershed with dense invasive plant species to better understand soil processes during the storm season. Giant reed is the most common invasive plant found in riparian, floodplains, or coastal areas of southern California (United States) and can alter fire and hydrological regimes. A human-ignited fire (June 2018) burned a portion of Alvarado Creek in San Diego, California that consists of developed areas, chaparral vegetated hillslopes, and non-native plants in the riparian areas. Soil samples were collected during the 2018-2019 storm season. Samples consisted of a top (0-15 cm) and bottom (15-30 cm) layers for upland and riparian areas surrounding the creek. The soils were characterized by a 1) loss-on-ignition (LOI) test to measure the changes in organic content; 2) sieve analysis to estimate the particle size distributions; and 3) a fall cone test to determine the undrained shear strength and liquid limit to investigate changes of the water content at which the soil transitioned from a semi-solid to water state. Average values of organic content showed a 2.45% loss of organic matter after the first post-fire storm in the upland top layer and 1.47% in the riparian top layer. The average values of organic content for the bottom layer in the upland and riparian layer after the first storm event were 0.76% and 1.60%, respectively. Preliminary results noted that sediments less than 0.180 mm were more susceptible to erosion in the riparian area for the top layer and experienced a maximum decrease of 12.99%. After the first storm of the season, particles between 4.75mm to 0.85mm in the top layer increased by 15.17% and by the end of the storm season returned to baseline conditions, suggesting high spatial variability in the upland compared to the riparian areas. The fall cone test showed that the water content in the upland soil increased by 10% at the liquid limit, which suggests particle transport occurred due to the decrease in soil shear strength from 1.9 kPa to 2.8kPa. The study results demonstrate that the physical characteristics of the soil horizon were primarily influenced by Arundo Donax in the riparian area. This research will help to guide strategies for regional implementation in San Diego and other urban waterways undergoing similar transformations.