Water retention characteristics of mineral forest soils in Finland: impacts for modeling soil moisture

Soil hydraulic properties are central for soil quality and affect forest productivity and impacts of climate-change on forests. The water retention characteristics (WRC) of mineral forest soils in Finland are not well known and practical tools to predict them are lacking. We statistically analyzed mineral forest soils WRC from over 130 sites in Finland, focusing on the humus layer and main root zone (0-19 cm depth). We showed that mineral forest soils can be grouped to five WRC classes that are well predictable from soil bulk density, organic matter content and clay fraction. However, majority of the forest soils are hydrologically rather similar. We found that neither topsoil maps nor any combination of open geospatial data was able to predict WRC. Thus, in absence of site-specific soil data, parameterizing WRC as a function of forest site type was proposed. We demonstrated the approach in soil moisture modeling at a small forest headwater catchment and showed soil moisture response to weather conditions is jointly affected by WRC, stand attributes and topography. We showed drought risks are highest on dense mature forests at nutrient poor sites, and lower in young stands on peatlands and lowland herb-rich sites with groundwater influence. The results improve hydrological predictions for Finnish forests, and the open dataset can contribute to larger synthesis and development of boreal forest soil pedo-transfer functions.