Modelling and Evaluation of the Effect of Afforestation on the Runoff Generation Within the Glinščica River Catchment (Central Slovenia)

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 protection, particularly when the evaluation also considers beneficial ecosystem services other than flood protection. The present study provides a combined modelling approach and a cost–benefit analysis (CBA) of the effects of afforestation on peak river flows and on selected ecosystem services within the Glinščica River catchment in Slovenia. In order to investigate these effects, the hydrological model HEC-HMS, the hydraulic model HEC-RAS, and the flood damage model KRPAN, which were developed specifically for Slovenia, are used. Three scenarios were evaluated where the main difference was the extent of afforestation. It was found that increasing the amount of tree cover (i.e., ≈15–60%) results in a flood peak reduction ranging from 9 to 13%. Flood extensions were significantly lower for most scenarios leading to reduced economic losses. However, a 100-year CBA only showed positive net present values (NPV) for one of the considered scenarios, where the afforestation was considered only in floodplain areas and the prevailing benefits were those of flood protection measures, which were higher than, for example, biodiversity or recreational benefits. Additional ecosystem co-benefits that are not directly linked with flood protection are considered in case of all three scenarios.