(1) Quantity: Hydrological Regimes
Swedish hydrological regimes are generally characterized by rather low winter streamflow with a dominating snowmelt-driven spring flood peak (mainly in central and northern Sweden), followed by low summer flows and/or a somewhat lower precipitation-induced flood peak in the fall (mainly in southern Sweden). In a future climate, however, streamflow is projected to change to a more even regime with dominating large winter streamflow and no spring flood peak at all–. Annual water availability in general is expected to increase as a result of increasing precipitation. There are, however, large seasonal variations: especially during summer months, water availability is likely to decline as a results of increasing evaporation rates in large parts of the country. In southern Sweden, water shortages during summer increasingly affect the drinking water supply, both in terms of quality and quantity.
Figure: Projected changes in hydrologic regimes representative for (a) northern Sweden and (b) southern Sweden.
(2) Extreme Events: Floods & Droughts
Hydrological extreme events, which are defined by the departure of surface and subsurface water supplies from average conditions at various points in time, can cause severe habitat damage, problems for agriculture, forestry, industry, building infrastructure, energy production and water supply. In Sweden, past changes in climate and land cover have had a major impact on streamflow patterns. In a changing climate, shifts in meteorological conditions are expected to even further perturb regional hydrology, and thereby also the occurrence, frequency and duration of both floods and droughts.
Climate models project that extreme floods are expected to occur less often in northern inland Sweden and the northern coastal areas, while most the rest of the country is likely to suffer from more common extreme floods in a future climate. Concurrently, more days with low river flows (i.e., hydrological droughts) are expected in southern Sweden and large parts of central Sweden.
Although Sweden has historically been a region abound with water, it is not exempt of droughts: the 2003 summer drought severely impacted the European continent, including Scandinavia. There was, however, large spatial variability in hydro-climatic patterns across the country, which indicates the complex interplay of meteorological and topographic features and the resulting hydrological impacts at the catchment scale. Events such as the European-wide 2003 drought could become more frequent in coming decades, and, thus, the early recognition of critical drought conditions is essential for drought risk management with large economic and social benefits. Yet, most available hydrological climate change impact studies concerning Sweden neglect hydrological droughts. To make matters even more concerning, interviews among Swedish municipalities and drinking water producers revealed that only 12% specifically considered potential effects of droughts on drinking water in their risk assessment. Thus, there is now an urgent need to estimate water availability in a changing climate and a developing society.
(3) Quality: Nutrient Loads
Multiple ongoing global changes have reshaped the pools and fluxes of biogeochemical elements in terrestrial and aquatic ecosystems. Of these, dramatic increases in the loading of bioreactive nitrogen (N) and phosphorus (P) to terrestrial ecosystems during the 20th century have drawn particular attention and are linked to multiple environmental problems, ranging from declines in species diversity to stratospheric ozone loss. Large quantities of anthropogenically mobilized N and P are also flushed from land to water, contributing to freshwater and marine eutrophication-, and connecting mounting water quality concerns to hydrological patterns that are themselves sensitive to climate drivers. Concurrent to these global changes, warming temperatures, longer growing seasons, and rising atmospheric CO2 concentrations may lead to increased plant growth, greater nutrient uptake and accumulation in terrestrial ecosystems, and reduced nutrient losses to surface waters in some cases. A continued intensification of the forest industry, in particular extensions of managed forest land and increasing use of fertilization, may increase the risk of nutrients leaching from watersheds.
The main controllers on nutrients in Swedish boreal rivers are seasonality, temperature and streamflow-. Consequently, nutrient loads are likely to increase as a result of increasing water availability (i.e., more streamflow) especially during winter, which supersedes the loss of spring flood. However, the combined influence of streamflow shifts and changing forestry practices on nutrient loadings to the Baltic Sea are still uncertain and emphasize the need for further research.
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