Land cover controls on river discharge in Sweden

Abstract As humans alter landscape, vegetation, climate and atmospheric composition, changes in the terrestrial water balance and fresh water resources are likely to occur. Understanding how climate, vegetation, humans and hydrology interact is key for accurate projections of future fresh water resources. In this study we focus on forest dominated Sweden where significant changes in climate and increasing human activity have co-occurred during the past 50 years. For 280 catchments in Sweden, we related runoff coefficients and change trends thereof to land-surface characteristics. With these relationships we created average and change trend maps for runoff and evapotranspiration across Sweden. All this information is summarized by plotting water use efficiency (actual evapotranspiration (ET)/precipitation) against energy use efficiency (actual ET/potential ET ) in a Budyko-type framework for areas with unique land cover across Sweden. This plot clearly shows that wetlands tend to have lower water and energy use efficiencies compared to “open waters”, forests and agriculture, and that agriculture and forests have comparable water and energy use efficiencies closest to those of “open waters”. These results demonstrate how a change in land cover driven by climate change or by humans is likely to alter land-cover-atmosphere interactions, thereby changing both the water and energy balance of catchments. Looking at runoff coefficient change trends during the last 50 years we see that forests tended to become more efficient in using water and energy (i.e. the fractions of water and energy converted into river runoff and heat decreased). As this behavior coincides with an increase in precipitation it signals an acceleration of the hydrological cycle of Swedish forests. In this presentation we will discuss these findings focusing on the impact of forests on river discharges and the implications for future water cycles.