Modelling overland flow on burned hillslopes using the KINEROS2 model

Abstract

A process-based study was conducted to investigate soil infiltration and overland flow dynamics in response to post-fire soil-surface characteristics on a dry burned hillslope one year following wildfire. The study consists of analysing data from paired rainfall-runoff plots, field and laboratory measurements, and plot-scale simulations of overland flow using the KINEROS2 rainfall-runoff model. Post-fire soil-surface characteristics and rainfall data were collected from paired rainfall-runoff plots located in dry eucalyptus forest, southeast Australia, severely burned by wildfire in January 2013. Field and laboratory measurements were undertaken on six separate occasions during the study period and confirmed the existence of strong hydrophobicity within 4 cm soil depth, restricting vertical infiltration and increasing the rate of runoff, although with a declining trend with time since fire. The strength of hydrophobicity steadily weakened from depth 4 cm downward, becoming non-repellent below 8 cm. Event-based simulations of soil infiltration and surface runoff were implemented using the KINEROS2 model for fourteen rainfall events. Spatial factors representing post-fire soil-surface conditions were calibrated with highly acceptable model efficiencies for the majority of the events. Simulated hydrographs were validated against observed data from runoff plots. The key properties impacting dynamics of simulated hydrographs were identified as soil hydraulic conductivity (Ksat), net capillary drive (G) and surface roughness respectively. The existence and breakage of fire-induced water repellency was simulated by appointing variable values for effective hydraulic conductivity. Parameterized values of soil saturated hydraulic conductivity (Ksat) showed that the existence and breakage of fire-induced water repellency during the first year following fire did not follow seasonal oscillations of natural water repellency. The calibrated effective hydraulic conductivity was in the range of 2-4 mm/h during the first winter following fire, reflecting strong water repellency. Fire-induced water repellency broke down during the early autumn next year and soil hydraulic conductivity increased suddenly to (Ksat>10 m/h). A constant value of 2 mm was parametrized with the factor of 0.1 for the net capillary drive (G) parameter. The capillary depression could be result of large contact angle (>90o) between liquid-solid interface in water repellent soil. Hydraulic roughness was represented by Manning's n coefficients and simulated hydrographs with higher roughness values obtained lower errors in peak and mean values. The theoretical primary roughness factor that only included bed size particles (0.085 sm-1/3) was re-calibrated by the factor of seven after calibration (0.64 sm-1/3). Similar trend of higher values for Manning's n was also reported by Chen et al. (2013) who used Kineros2 for modelling rainfall-runoff in fire affected watersheds. Dynamics of simulated hydrographs were found less sensitive to variations of the second group of soil hydraulic properties (porosity, pore index and initial water content), <5% variations in model efficiencies and error indicators. The soil infiltration model in KINEROS2 was well-adjusted to fire-induced hydrophobic soil by adapting infiltration concepts and domains to measurements and observation of post-fire soil-surface conditions in the initial model setup and parametrization (ME > 50% and R2 > 0.5). The model successfully simulated dominant factors in controlling vertical preferential water movement and their trend change during post-fire recovery period.