Modeling boreal forest evapotranspiration and water balance at stand and catchment scales: a spatial approach

Abstract

Vegetation is known to have strong influence onevapotranspiration (ET), a major component of terrestrialwater balance. Yet hydrological models often describe ETby methods unable to include the variability of vegetationcharacteristics in their predictions. To take advantage of theincreasing availability of high-resolution open GIS data onland use, vegetation and soil characteristics in the borealzone, a modular, spatially distributed model for predictingET and other hydrological processes from grid cell to catch-ment level is presented and validated. An improved approachto upscale stomatal conductance to canopy scale using in-formation on plant type (conifer/deciduous) and stand leaf-area index (LAI) is proposed by coupling a common leaf-scale stomatal conductance model with a simple canopy radi-ation transfer scheme. Further, a generic parametrization forvegetation-related hydrological processes for Nordic borealforests is derived based on literature and data from a borealFluxNet site. With the generic parametrization, the modelwas shown to reproduce daily ET measured using an eddy-covariance technique well at 10 conifer-dominated Nordicforests whose LAI ranged from 0.2 to 6.8 m2m−2. Topogra-phy, soil and vegetation properties at 21 small boreal headwa-ter catchments in Finland were derived from open GIS dataat 16 m×16 m grid size to upscale water balance from standto catchment level. The predictions of annual ET and specificdischarge were successful in all catchments, located from 60to 68◦N, and daily discharge was also reasonably well pre-dicted by calibrating only one parameter against dischargemeasurements. The role of vegetation heterogeneity in soilmoisture and partitioning of ET was demonstrated. The pro-posed framework can support, for example, forest traffica-bility forecasting and predicting impacts of climate changeand forest management on stand and catchment water bal-ance. With appropriate parametrization it can be generalizedoutside the boreal coniferous forests.