Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 116 -
Ice Regime of Rivers in Latvia in Relation to Climatic Variability and the
North Atlantic Oscillation
Māris Kļaviņš 1 , Tom Frisk 2 , Agrita Briede 1 , Valery Rodinov 1 , Ilga Kokorīte 1
1 University of Latvia, Department of Environmental Sciences, Raiņa bulv. 19, LV 1586, Rīga, Latvia
2 Pirkanmaa Regional Environmental Centre, Tampere, Finland
1. Introduction
Records of the time of ice freeze-up and break-up in
rivers allow the assessment of long-term and seasonal
variability of climate, especially the relation to climate
change. There are three major reasons why ice regime
studies are important: a) as calendar dates of freezing and
thawing of lakes and rivers have been recorded in many
rivers (also in Latvia) well before scientific observations
began, thus the data cover a longer period than for other
hydrological factors; b) the ice regime of waters models
the hydrological regime during the period of maximum
discharge of accumulated atmospheric precipitation; c)
ice conditions are sensitive and reliable indicators of
climate.
The river discharge and ice regimes during winter have
been observed to be related with the North Atlantic
oscillation (NAO) pattern (Osborn et al. 1999) of largescale
anomalies in the North Atlantic atmospheric
circulation. Also Southern Oscillation has been proven to
be able to influence the ice regime of lakes and rivers in
the Northern Hemisphere (Robertson et al. 2000). Breakup
dates for the last two centuries for rivers in the
Northern Hemisphere provide consistent evidence of later
freezing and earlier break-up (Magnuson et al. 2000).
The ice regime of the Baltic Sea has been previously
analyzed using a historical time series of ice break-up at
the port of Riga (Jevrejeva 2001) to reconstruct the
climate (Tarand and Nordli 2001).
The aim of this study is to assess long-term changes of
the ice regime of rivers in Latvia, in relation to long-term
climate change (temperature and precipitation), river
discharge, and NAO patterns.
2. Materials and methods
P. Stakle (1931) published the first time series of ice
break-up data for the River Daugava. Data on river
discharge and the ice regime were obtained from the
Latvian Hydrometeorological Agency, and temperature
records for the period 1795 to 2002 from the
Meteorological Station Riga–University. The standard
homogeneity test was applied on the data set before
analyses.
The multivariate Mann-Kendall test for monotone trends
in time series of data grouped by sites, plots and seasons
was chosen for determination of trends, as it is a
relatively robust method concerning missing data and it
lacks strict requirements regarding data
heteroscedasticity. In the present study the seasonal NAO
index from year 1709 was used. Classification of the
index data is based on the definition of 3 categories: high
(NAO > 1) – strong westerly, normal (NAO ~1) and low
(NAO < -1) – weak westerly.
3. Results and discussion
The climate, hydrological processes and ice regime of
inland waters of Latvia are determined by its physico-
geographic location: flat surface topography, dominance
of Quaternary glacial and ancient sea sediments, and
dominance of humic podsol soils. Long-term data on
river discharge do not show significant trends, for
example for the Daugava, Venta, Salaca and Dubna
rivers, but rather can be described as periodic oscillation
around a mean value. However, in the shorter term the
changes can be considered as significant. There has been
an increase of runoff during the last twenty years (1982-
2002), compared with the previous period: the water
discharge of rivers in eastern Latvia has increased by ~
10 %, and in the western part (Venta and Tebra rivers) by
about 40 %. Regardless of the long-term patterns of
water discharge, the present water flow regime may be
regarded as comparatively increased when compared
with centennial mean values for rivers of Latvia. The
long-term trends of annual river discharge are not
significant. The trends for particular seasons, which are
periodically changing, appear to show different patterns.
The river discharge (for example, Daugava, Venta, and
Lielupe rivers) in winter (December – February) shows a
significant increasing trend, but not in other seasons. A
particularly significant increase of river discharge in
winter can be observed during the last two decades.
The seasonal air temperatures, according to records from
the Meteorological Station Riga-University, have
changed substantially during the last 200 years (1795 -
2002). The air temperatures in winter have increased by
1.9 ºC, in spring by 1.3 ºC and in autumn by 0.7 ºC. The
mean annual temperature has increased by 1.0 ºC. In
comparison with the long-term mean (1961-1990), the
lowest mean temperature occurred during the period from
1830 to 1930 for the annual and seasonal temperatures
(autumn, spring and summer). Winter season
temperatures have been increasing gradually since the
19th century and during the 1830-1930 period the longterm
minimum was not reached. Notable increases of
winter and spring air temperatures have been observed
since the 1970ies.
There exist direct links between temperature, ice regime
on rivers and their discharge pattern. The time series of
date of ice break-up for Daugava River at Daugavpils
indicates a mean date of April 3. In comparison with the
other studied rivers, the break-up time can differ by more
than one month, depending on the distance from the
Baltic Sea and Gulf of Rīga and river catchment
characteristics.
A decreasing linear trend indicates shifting of the date of
ice break-up to earlier dates. The calculated regression
equation estimated that the time of ice cover during the
20th century (observation periods of 77-60 years
depending on river station) was shifting to an earlier time
by 2.8 to 5.1 days every 10 years.
In general, the shift in the river break-up towards earlier
dates, indicating an earlier start of river flooding, can
explain the increase of winter runoff of rivers in Latvia,
seen to be associated with climatic variability and also