Modelling the effects of forest use change on brownification of Finnish rivers under atmospheric pressure

Browning of surface waters due to increased terrestrial loading of dissolved organic matter (DOM) is observed across the Northern Hemisphere. The effects influence several ecosystem services from freshwater productivity to water purification. Brownification is often explained by changes in large-scale anthropogenic pressures and ecosystem functioning (acidification, climate change, and land cover changes). Considering the effects of global pressures, this study examined the effect of forest use changes on water browning in Finland. Our goal was to find the ecosystems and geographic areas that are most sensitive to environmental pressures that increase the loading of DOM. We were also looking for land use strategies that decrease browning. We combined mathematical watershed modelling (Simply-C) to climate change, atmospheric deposition, and forest use change scenarios until 2059. The study area covered 20 watersheds from south to north of Finland. Measurements of river discharge (daily values) and water quality (grab samples: on average 33 per year) were collected in national monitoring programs at the outlet of the river. The model was calibrated and validated against observed discharge and TOC concentrations in 1985-2020. Soil data originated from the Finnish Soil Database and land cover was obtained from the national CORINE Land Cover dataset (years 2000, 2006, 2012 and 2018). The dataset provides information on Finnish land use and land cover, and its changes, including forest cut and forest gain area. There is also more detailed information of forest structure available in the Multi-Source National Forest Inventory (MS-NFI) data. Long time series and wide spatial coverage included different geographical areas and hydrological years. For scenarios we used daily data from five global climate models (CMIP5) under representative concentration pathway (RCP) scenarios RCP4.5 and RCP8.5. For atmospheric SO 4 deposition we used the chemical transport model results that are based on the EMEP MSC-W model. Changes included also scenarios of forest harvest and protection of forest, that were derived from the European Union’s regulation. According to the simulations, brownification continues in northern Finland. In southern Finland global influence (atmospheric deposition, climate change) seems to weaken, giving more space for local forest use change influencing brownification. Forest use change was more influential in river basins dominated by organic than mineral soils. Extending forest protection decreased brownification especially in areas where the influence of atmospheric pressure is decreasing. When forest protection is planned to provide a carbon storage and sequestration potential and to favor biodiversity, it has favorable effect on surface water quality as well.