Do we see trends in the observed timeseries?
The higher temperatures that have been observed in Sweden over the last decades have had a distinct impact on hydrology (Figure 1). Higher winter temperatures have resulted in more runoff during winter and a decreased spring flood due to a thinner snow cover.
However, if you are studying a longer time period, it is difficult to see any clear trend in water runoff, although there are clear trends in precipitation and temperature (Figure 2). Thus it is difficult to establish how climate has affected the water balance historically and even harder to make judgments for the future.
Modelling future hydrology
To simulate future hydrology, hydrological models (e.g. HYPE or HBV) are used with input data (mainly temperature and precipitation) from future projections calculated by global (GCM) and regional (RCM) climate models (Figure 3).
The various climate models, however, give different signals and there are problems in introducing the results directly into the hydrological models. Efficiency studies are often related to the current climate and this can be a problem because climate models represent a statistical climate and not the observed. Within the SMHI hydrological research group work is ongoing to improve the link between climate and hydrological models in collaboration with the .
It is also unclear whether the various assumptions of the hydrological models really applies to a future climate and how other factors, such as land use or water management, may also change concurrently impacting on the future hydrological conditions. These are the questions that the hydrological scientific community are seeking to address.
Effects of large scale patterns
Climate models' results are used by hydrologists for calculations of the water cycle in model systems that cover large geographic areas. This allows the effects on water assets to be mapped as well as the large-scale patterns of variation.
Many different variables are analyzed including average runoff, extremely high or low flows, groundwater variation, snow conditions and soil moisture. Changes over large regions are analyzed with relatively high resolution. Calculations are available in web products for Sweden and Europe, as well as from a number of different .
Impacts on water quality
Water Status is dependent on the emission of pollutants but also on biogeochemical processes, water quantity and flow paths. The latter are affected by temperature and precipitation patterns, which may alter in a future climate.
With the aid of computational models how water quality and transport of contaminants may change in the future and which uncertainties there are in these calculations is studied.
In connection with eutrophication problems the effeciency of various measures to reduce leaching of nitrogen and phosphorus are also studied in the present and future climate; for example the HELCOM Baltic Sea Action Program (BSAP) been evaluated for a number of climate projections within the project ECOSUPPORT.
Impacts on urban storm water drainage
Today’s RCMs generally have a spatial resolution of ~25×25 km, which is too coarse for certain hydrological applications. For example in cities where the catchments are small (~1 km²) and flooding is often caused by local intense rainfall such as thunder showers. In this case a high temporal resolution is also required to capture the rapid runoff processes.
In different research projects methodologies are developed to statistically downscale the climate model output to the local scale required. In collaboration with cities and municipalities (e.g. Stockholm, Göteborg and Arvika), the methodology is tested in case studies to estimate future climate impacts (Figure 4) and to evaluate the effect of adaptive measures.
Together with the Rossby Centre test runs of climate models with a very high spatial resolution, down to ~5×5 km, are evaluated. This higher degree of detail allows for improved descriptions of small-scale processes that are expected to improve the simulations of local climate.
More or less forest fires?
The risk of forest fire in a region is closely coupled to hydrological processes and often hydrological models are used as a tool in risk assessment. However, other meteorological variables, such as wind speed and humidity, not generally used in hydrological modelling are also required. With the objective of climate change impact assessment, methods to convert climate model ouput to tailored input for forest fire risk risk models have been developed.
SMHI participates in a number of international projects, which contribute to hydrological research and development coupled to the climate. Within EU-projects, different types of climate change impacts are investigated in different regions; for example in the Baltic Sea region or in case studies from all over Europe.
So called “climate services” are being developed, aimed at improving the accessibility and applicability of climate model results for local or regional impact studies.
Cooperation focusing on other continents; such as Africa, Arctic, Asia and South America; provide useful knowledge on global development and hydrological modelling in different climate regions that highly beneficial for future impact studies.
- Andersson, L., Wilk, J., Graham, P., Warburton, M., 2009. Local assessment of vulnerability to climate change impacts on water resources in the Upper Thukela River Basin, South Africa – Recommendations for adaptation. SMHI Rep. Climatology No 1, September 2009. SMHI, Norrköping, Sweden. ISSN 1654-2258, 47 pp.
- Andersson, L., Samuelsson, P. and Kjellström, E., 2011. Assessment of climate change impact on water resources in the Pungwe river basin. Tellus, 63A(1), 138-157. DOI: 10.1111/j.1600-0870.2010.00480.x
- Andréasson, J., Bergström, S., Carlsson, B., Graham, P.L., Lindström, G. (2004) Hydrological change - climate change impact simulations for Sweden. Ambio, 33:4-5.
- Andréasson, J., Hellström, S-S., Rosberg, J. och Bergström, S. (2007). Översiktlig kartpresentation av klimatförändringars påverkan på Sveriges vattentillgång – Underlag till Klimat- och sårbarhetsutredningen. SMHI Hydrologi, nr 106, Norrköping.
- Graham, L.P., Andréasson, J. and Carlsson, B., 2007. Assessing climate change impacts on hydrology from an ensemble of regional climate models, model scales and linking methods - a case study on the Lule River Basin . Climatic Change 81, 293-307.
- Graham, L. P., Olsson, J., Kjellström, E., Rosberg, J., Hellström, S.-S. & Berndtsson, R. 2009: Simulating river flow to the Baltic Sea from climate simulations over the past millennium. Boreal Env. Res. 14: 173–182.
- Hellström, S. & Lindström, G. (2008) Regional analys av klimat, vattentillgång och höga flöden. SMHI Rapport Hydrologi nr 110.
- Olsson, J., Berggren, K., Olofsson, M., Viklander, M. (2009) Applying climate model precipitation scenarios for urban hydrological assessment: A case study in Kalmar City, Sweden. Atmospheric Research 92:364-375.
- Olsson, J., Dahné, J., German, J., Westergren, B., von Scherling, M., Kjellson, L., Ohls, F., and A. Olsson (2010) En studie av framtida belastning på Stockholms huvudavloppsnät, SMHI Reports Climatology No 3, SMHI, SE-601 76 Norrköping, 42 pp.
- Olsson, J., Yang, W., Graham, L.P., Rosberg, J., and J. Andréasson (2011) Using an ensemble of climate projections for simulating recent and near-future hydrological change to Lake Vänern in Sweden, Tellus, 63A, 126–137.
- Wetterhall, F, Graham, P, Andréasson, J, Rosberg, J and Yang, W., (2011), Using ensemble climate projections to assess probabilistic hydrological change in the Nordic region, Nat. Hazards Earth Syst. Sci., 11, 2295–2306.
- Yang, W., Andréasson, J., Graham, L.P., Olsson, J., Rosberg, J., and F. Wetterhall (2010) Improved use of RCM simulations in hydrological climate change impact studies, Hydrological Research, accepted.