In order to assess climate change impacts on river runoff patterns at the end of this century, the hydrological model METQ2007BDOPT was applied to 8 river basins and sub-basins in Latvia, which is a part of the southeast Baltic Sea basin. The climate data we used originate from the PRUDENCE project and were prepared in a separate study. Changes in hydro-climate were analysed using one control run ) and 2 IPCC scenario runs (A2 and B2; 2071-2100). For the A2 scenario, both annual and seasonal analysis predicted the major significant changes in most cases. For both scenarios, an increase of the mean annual climate data (air temperature, precipitation and evapotranspiration) is forecast, whereas the mean annual river runoff is predicted to decrease. The seasonal runoff pattern is expected to change towards significantly higher runoff during winter, following a decrease in spring and autumn. Maximum river discharge will occur in winter instead of spring. No considerable change in streamflow is predicted for summer. Future climate will change, leading to modifications in the river runoff regime and the shape of the hydrograph, which will be similar to that of the present Western European rivers, i.e. 2 principal periods instead of 4: one with high flow, mostly falling in the cold period of the year; and one with low flow, mostly in the warm period.

Changes in the hydrological regime of the lakes of Latvia depend on natural and anthropogenic causes. This publication summarises the results of the research on the long-term changes in the water level, thermal and ice regimes in the seven largest lakes of Latvia: Liepājas, Usma, Ķīšezers, Burtnieks, Rāzna, Sventes and Lielais Ludzas, and their regional specifics from 1926 to 2002. For most of the studied lakes, the water level has been regulated, except for the lakes Liepājas, Burtnieks and Ķīšezers. Global climate warming has caused considerable changes in the hydrological regime of the lakes during the last decades and the surface water temperature has increased. At the same time, the number of days with ice cover and the thickness of ice have decreased. A positive trend in the freezing date and a statistically significant negative trend for the ice break-up date was found for all lakes. The lakes Liepājas and Usma are located in the western part and Lake Kīšezers in the central part, therefore their hydrological regime, in particular, the thermal and ice regime, differs from the lakes Burtnieks, Rāznas, Sventes and Lielais Ludzas which are located in the northern and south-eastern part of Latvia.

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The study deals with turn-of-the-century changes in the total annual river runoff distribution and high and iow flows in Latvia, covering river basins within four hydrological districts which vary according to size and physiographical conditions. Mathematical statistical methods were applied in the analysis of river discharge data series for two study periods of 1951-2009 and 1881-2009. The present results confirm the basic statement concerning the Baitic countries that major significant changes in river runoff during the iast two decades have occurred between spring (decrease) and winter (increase) seasons. Mostly insignificant changes in summer runoff and significant/insignificant changes in autumn runoff were found. Analysis shows that a statistically significant trend of increase in low flow for the cold period and a significant trend of decrease in the high discharge and coefficient d of uneven runoff distribution were detected. Changes in river hydrological regime are mainly caused by changes in large-scale atmospheric circulation processes following climate warming, which has taken place. Latvian river hydrography has therefore changed and become more similar to Western European rivers.

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