Integrating Remote and In-Situ Data to Assess the Hydrological Response of a Post-Fire Watershed

Luca Folador,Alessio Cislaghi,Giorgio Vacchiano,Daniele Masseroni

doi:10.3390/hydrology8040169

Abstract

Forest fire is a common concern in Mediterranean watersheds. Fire-induced canopy mortality may cause the degradation of chemical–physical properties in the soil and influence hydrological processes within and across watersheds. However, the prediction of the pedological and hydrological effect of forest fires with heterogenous severities across entire watersheds remains a difficult task. A large forest fire occurred in 2017 in northern Italy providing the opportunity to test an integrated approach that exploits remote and in-situ data for assessing the impact of forest fires on the hydrological response of semi-natural watersheds. The approach is based on a combination of remotely-sensed information on burned areas and in-situ measurements of soil infiltration in burned areas. Such collected data were used to adapt a rainfall–runoff model over an experimental watershed to produce a comparative evaluation of flood peak and volume of runoff in pre- and post-fire conditions. The model is based on a semi-distributed approach that exploits the Soil Conservation Service Curve Number (SCS-CN) and lag-time methods for the estimation of hydrological losses and runoff propagation, respectively, across the watershed. The effects of fire on hydrological losses were modeled by adjusting the CN values for different fire severities. Direct infiltration measurements were carried out to better understand the effect of fire on soil infiltration capacity. We simulated the hydrological response of the burned watershed following one of the most severe storm events that had hit the area in the last few years. Fire had serious repercussions in regard to the hydrological response, increasing the flood peak and the runoff volume up to 125% and 75%, respectively. Soil infiltration capacity was seriously compromised by fire as well, reducing unsaturated hydraulic conductivity up to 75% compared with pre-fire conditions. These findings can provide insights into the impact of forest fires on the hydrological response of a whole watershed and improve the assessment of surface runoff alterations suffered by a watershed in post-fire conditions.

Highlights

Fires severely alter the hydrological response of watersheds to rainfall [1,2]
An integrated approach that exploits a combination of remote and in-situ data for a comprehensive evaluation of the impact of forest fire on hydrological response of natural watersheds was tested on a real case study located in northern Italy
The Normalized Difference Vegetation Index (NDVI) obtained by Sentinel-2 images was used to identify burned areas in the watershed and their level of fire severity, whereas direct infiltration measurements performed in those areas through a Mini Disk Infiltrometer were used to better understand the infiltration capacity of the soils in post-fire conditions

Summary

Introduction

Fires severely alter the hydrological response of watersheds to rainfall [1,2]. Fireinduced loss of canopy cover, litter consumption, and the formation of water repellent layers on the soil surface reduce canopy interception and soil infiltration, increasing flood peak and volume runoff [3,4,5,6].Fire acts as a generalist herbivore that removes plant material above the ground surface. Fireinduced loss of canopy cover, litter consumption, and the formation of water repellent layers on the soil surface reduce canopy interception and soil infiltration, increasing flood peak and volume runoff [3,4,5,6]. Depending on fire behavior and species-specific resistance, trees can suffer death or defoliation, survive, or re-sprout following fire. This natural process leads to a reduction in evapotranspiration fluxes, leaf rainfall interception and tree suction capacity [7]. The burning process degrades soil structure and porosity, causing considerable nutrient loss through volatilization, leaching and erosion, and alters the quantity and specific composition of microbial and soil-dwelling invertebrate communities [9]

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