Modeling and evaluation of the effect of afforestation on the runoff generation within the Glinščica catchment (Slovenia)

Modeling and evaluation of the effect of afforestation on the runoff generation within the Glinščica catchment (Slovenia)

Gregor Johnen¹, Klaudija Sapač², Simon Rusjan², Vesna Zupanc³, Andrej Vidmar², Nejc Bezak²

¹ Radboud University Nijmegen, Faculty of Science

² University of Ljubljana, Faculty of Civil and Geodetic Engineering

³ University of Ljubljana, Biotechnical Faculty

 

Abstract:

Increases in the frequency of flood events are one of the major risk factors induced by climate change that lead to a higher vulnerability of affected communities. Natural water retention measures such as afforestation on hillslopes and floodplains are increasingly discussed as cost-effective alternatives to hard engineering structures for providing flood regulation, particularly when the evaluation also considers beneficial ecosystem services other than flood regulation. The present study provides combined modelling approach and a cost-benefit analysis (CBA) of the impacts of afforestation on peak river flows and on selected ecosystem services within the Glinščica river catchment in Slovenia. In order to investigate the effects, the hydrological model HEC-HMS, the hydraulic model HEC-RAS and the flood damage model KRPAN, that was developed specifically for Slovenia, are used. It was found that increasing the amount of tree cover results in a flood peak reduction ranging from 9-13 %. Flood extensions were significantly lower for most scenarios leading to reduced economic losses. However, a 100-years CBA only showed positive net present values (NPV) for one of the considered scenarios and the benefits were dominated by the flood regulation benefits, which were higher than for example biodiversity or recreational benefits. Based on our findings we conclude that afforestation as a sole natural water retention measure (NWRM) provides a positive NPV only in some cases (i.e. scenarios) and if additional ecosystem co-benefits are considered.