THE REASONS FOR THE CHANGES IN THE WATER REGIME OF THE BIALOWIEZA FOREST

Tomasz Okruszko,
Department of River Engineering
and Environmental Rehabilitation,
Warsaw Agricultural
University,
Warsaw,
Poland

THE WATER BALANCE IN THE BIALOWIEZA REGION

There is some public concern regarding the sustainability of the biodiversity of the Bialowieza National Park due to the deteriorating conditions of surface waters. The drying out of these valleys implies that water-fed plant communities are going to be replaced by flora more adapted to drier places. In general, the large irrigation schemes on the both sides of the state border, coupled with the regulation of the streams in the Bialowieza Forest and it's vicinity, are blamed for this situation. In the author's opinion, however, the situation is somewhat more complicated.

In order to look in more depth at this problem, it is useful to consider the simplified water balance equation of a water catchment system:

P = E + Q ± DH

Where:
P — precipitation [mm];
E — evaporation/transpiration [mm];
Q — discharge from the catchment [mm];
DH — change in retention [mm].

Using this equation, the effect of changes in particular elements of the water situation in the river valley can be assessed.

LESS SNOW COVERAGE IN WINTER

The average precipitation in the Podlasie region is 640 mm/y, with a range from 430 mm to 940 mm in any particular year. During the last one hundred years of meteorological observations, there have been no major changes in the quantity of precipitation observed. In other words, on average, the same amount of water in the form of rain and snow falls each year on Bialowieza Forest. However, there have been changes in the form of this precipitation and its temporal distribution. Up to the 1950s, the average period of snow cover in the region was close to 100 days per year. In the 1990s, snow cover lasted no more than 75 days and, moreover, there are now some periods during the winter when the snow melts, which was previously very rare.

EFFECTS ON RIVER SYSTEMS

As a result of the lack of snow retention, the spring floods are less during the snow thaw period. The valley is no longer fed by river water and the floodplain does not receive enough water to sustain it through the dry summer conditions. This means that the effect of the so-called "flood impulse" in the development of valley ecosystems has diminished.

Natural alder communities need a high water level
Figure 22. Natural alder communities
need a high water level (CO)
The process of diminishing floods, during the snow thaw, has been studied in recent years for the Upper Narev River. This study shows that the frequency of floods in the Suraz gauge station has rapidly decreased since the mid-1980s. It should also be stressed that these studies were conducted prior to the Siemianowka water reservoir coming into operation, which in the last few years has further decreased peak flows. To the author's knowledge there are no similar studies for the Narevka and Lesna Rivers. However, since they share similar physical and hydro-meteorological conditions to the Upper Narev River, it seems reasonable to assume that the effects established in the study mentioned above can be extended to the whole Bialowieza territory.

The Bialowieza Forest is criss-crossed by many natural brooks and rivers
Figure 23. The Bialowieza Forest is
criss-crossed by many natural
brooks and rivers (CO)
River regulation has led to an increase in the speed of water outflow from the water catchment area. With the same average discharge from the catchment area in the recent years, this means that the flood duration and the peak flows are smaller especially in the upper courses of the streams. There are no detailed studies on the changes in evaporation/transpiration in the Podlasie region, but it can be assumed that the higher winter temperatures and shorter snow covered periods will have led to some increase in winter evaporation. Experience, however, suggests that the impact of these changes on the water balance will be negligible.

Changes in water retention in the catchment area are difficult to investigate. In most studies, it is assumed that the ground water level in the upper horizon of the soils is the best parameter for this purpose. It is evident from many studies that, for the Upper Narev, the trend is negative - the water level in observation wells is falling. The drainage schemes undertaken in mid-1970s have added to the drying effect caused primarily by the climate changes.

It can be surmised, that the area of the Bialowieza National Park is affected primarily by the same factors that are important for all river valleys in north-east Poland. These processes can, in general, be seen as the diminishing of the snow cover during the winter leading to a decrease in the flood impulse during the spring. The drainage schemes are worsening this situation by increasing the speed of water outflow.

Prior to rehabilitation works, which can, to some extent, improve the situation, the above hypothesis should be verified for the Bialowieza Forest conditions. The study should be carried out using:

SUGGESTIONS FOR A RECOVERY PLAN

The Bialowieza Forest is famous for its huge ancient trees. Over 2000 giant trees in the Bialowieza Forest are classified as monuments of nature (
Figure 24. The Bialowieza Forest is
famous for its huge
ancient trees. Over 2000.
giant trees in the Bialowieza Forest
are classified as monuments of
nature ("Natuurmonumenten") (NC)
Having this assessment as a basis, and using the simplified regional groundwater model, some proposals for technical activities can be drawn up. There are two main types of actions which can be undertaken, namely restricting the outflow from a subcatchment area, and increasing the flood event in the river valleys. The first type of action is mostly appropriate for the highest parts of the catchment area and would require hindering the outflow from the drainage canals and small creeks. This would result in a groundwater level increase. The second type of action is more suitable for downstream and would require damming a particular river section in order to more easily flood the valley. It should be also noted, that blocking the outflow from the upper part of a catchment, will naturally make flooding the downstream part of the valley more difficult.

Detailed analysis of the hydrological conditions coupled with an ecological study would assist in selecting the best sites for these types of technical conservation actions.