This paper presents the results of the study of long-term changes of Daugava River ice phenology, i.e. the freeze-up date, the break-up date, and the duration of ice cover from 1919/1920 to 2011/2012, under the impact of the cascade of hydro power plants. The long-term changes of ice phenology were determined by global climate warming at the turn of the 20th and the 21st centuries and anthropogenic activities after the year 1939. The Mann-Kendall test showed that the ice freeze-up date has a positive trend, while the ice break-up date and the duration of ice cover had negative trends. The changes were statistically significant. Data series covering twenty years before and after construction of the hydro power plants were used for assessing the impact of each hydro power plant on changes of Daugava River ice phenology parameters. The study results showed that the duration of ice cover was significantly longer in water reservoirs, i.e. the freeze-up date was earlier and the break-up date was later. Downstream of dams duration of ice cover was shorter with later freeze-up dates and earlier break-up dates. The impact of hydro power plants on ice phenology parameters gradually decreased with distance down from the dams.
Changes of total annual runoff distribution, high and low discharges in Latvian rivers
The paper examines climate change impacts on the hydrological regime of nineteen different river basins in Latvia. Hydrological data series for the period of 1951-2006 were analysed for river basins of four hydrological districts: Western, Central, Northern and Eastern. Climate change has influenced the temporal and spatial distribution of total annual river runoff and high and low flows in Latvia at the turn of century. The results confirm the hypothesis that the main tendency in the run-off change is a decrease in spring floods and increase in winter. Generally, statistically insignificant long-term trends were observed for summer and autumn.
The study describes the trends of monthly mean water temperature (from May to October) and the annual maximum water temperature of the rivers in Latvia during the time period from 1945 to 2000. The results demonstrated that the mean water temperatures during the monitoring period from May to October were higher in the largest rivers (from 13.6 °C to 16.1 °C) compared to those in the smallest rivers (from 11.5 °C to 15.7 °C). Similar patterns were seen for the maximum water temperature: in large rivers from 22.9 °C to 25.7 °C, and in small rivers from 20.8 °C to 25.8 °C. Generally, lower water temperatures occurred in rivers with a high groundwater inflow rate, for example, in rivers of the Gauja basin, in particular, in the Amata River. Mann-Kendall test results demonstrated that during the monitoring period from May to October, mean water temperatures had a positive trend. However, the annual maximum temperature had a negative trend.
Application of Conceptual Rainfall-Runoff Model METQ for Simulation of Daily Runoff and Water Level: The case of the Lake Burtnieks Watershed
In this study a conceptual rainfall-runoff METQ model—the latest version METQ2007BDOPT—was applied to simulate the daily runoff and water level of the Lake Burtnieks watershed from 1990 to 1999. The model structure and parameters were basically the same as in the METQ98, with some additional improvements and semi-automatical calibration performance. Model calibration was done for four rivers and one lake gauging station. The results of calibration showed a good correlation between the measured and simulated daily discharges. The Nash-Sutcliffe efficiency R2 varied from 0.90 to 0.58 and correlation coefficient r from 0.95 to 0.83. The highest values of R2 = 0.90 and r = 0.95 were obtained for the River Salaca and the lowest R2 = 0.53 and r = 0.83 for Lake Burtnieks. We observed some relationships between the model parameter values and physiographic characteristic of the sub-catchments.
Elga Apsīte, Mārtiņš Kriķītis, Inese Latkovska and Andrejs Zubaničs
Changes in the hydrological regime of the lakes of Latvia have been caused by several natural and human factors. This publication summarises the results of research on the long-term and seasonal changes in the water level, and thermal and ice regimes of the three biggest lakes of Latvia (Usma, Burtnieks, and Râzna) and their regional features in the period from 1926 to 2002. The levels of the lakes Usma and Râzna have been controlled, but it can be considered that changes of the water level in Lake Burtnieks have been due to the impact of natural factors during the period from 1947 to 2002. Global climate warming has caused considerable changes in the hydrological regime of the lakes during the last decades, as the water level and temperature have increased and the number of days with ice cover and the thickness of ice have decreased. A positive trend in the freezing data and statistically reliable negative trend for the ice break-up date were observed for all the lakes. Lake Usma is located in the western part of Latvia, therefore, its hydrological regime, in particular, the thermal and ice regime, differs from those of lakes Burtnieks and Râzna which are located in the northern and eastern part of Latvia, respectively.