Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Calculation and Forecast of the Annual Discharge of the Neman River in
Byelorussia
Alexander A. Volchak
The Department of Polesye Problems of the National Academy of Sciences of Belarus, Moskovskaya St., 204, Brest, 224020,
Belarus, e-mail Volchak@tut.by
1. Introduction
The Neman, one of the principal Belorussian rivers, is a
typical European transboundary river which flows on the
territory of Belarus and Lithuania and discharges into the
Baltic Sea. A distinct influence on the formation of the Neman’s
flow is exercised by climatic factors. The mechanism
of long-term fluctuation of Neman’s annual flow (as registered
in the city of Grodno) is determined by the asynchronous
fluctuation of water balance processes. The annual
fluctuation of atmospheric precipitation and total evaporation
in the Neman basin reaches 20%, which leads to the alternation
of relatively stable precipitation periods and periods
of variable water balance processes. As a result, cyclicism
(i.e. a tendency to group the years of high / low water
level without a certain regularity of the process) is a very
important feature of the long-term fluctuation of the Neman’s
annual flow, distinguished from the so-called “white
noise”.
2. Initial Data
This research aims at understanding the annual outflow of
Neman (Grodno) and at suggesting forecast models. For this
purpose the full time series available of the Neman’s annual
flow is used. The period in question spans 193 years (from
1808 to 2000) and consists of direct observations (1877 –
2000) and an estimated part (1808 – 1876) using the model
“Hydrologist”, and data from other analogous rivers – the
Smalininkai and the Rhine (Volchak A.A., 1998).
The hydrographer clearly traces the cyclicism of fluctuation:
During the period from 1870 to 1885 a fall in water level
was observed, from 1885 to 1930 – its rise, then from 1955
– its decrease, in the late 50s – a maximal increase over the
whole period of observation, then from the mid-60s a decline
in water level with a slight rise in the 80s was observed.
The reduction of the fluctuation range has been noticeable
since 1960.
Table 1 represents a selected estimation of the principal statistic
parameters for two periods.
Periods of observation,
years
Duration, years
Normal outflow,
Coefficient of
variation (Cv)
Coefficient of
asymmetry (Cs)
Coefficient of selfcorrelation
(r(1))
1877 – 2000 124 197 0,18 0,87 0,15
1808 – 2000 193 194 0,19 0,63 0,19
Table 1. Principal statistic characteristics of the Neman’s
annual flow at Grodno. Units of “Normal outflow” are
m 3 /sec.
Empirical curves correspond to the Pearson distribution of
the third type in which Cs=3C v. Provided that the probability
distribution function of the annual outflow does not differ
considerably from the normal distribution, the application of
parameter criteria for verifying the statistic hypothesis may
well be allowed.
3. Methodology of Research and its Outcome
At present, the practical methods of hydrology and water
economy calculations are based on the hypothesis of constant
variability of the annual flow. Although the practice of
modelling and exploiting hydrotechnical and water economy
objects has shown the applicability of this method, the statistic
conception of describing the long-term fluctuation of the
river flow in its traditional interpretation cannot be recognized
as a perspective for establishing methods for flow
forecast. Firstly, the predictability limit of stochastic models
of the annual outflow based on the Markov chain of the first
order is equal to 1-2 years under forecast condition ≤60%
(Ismailov G.H, Fedorov V.M., 2001). Secondly, resulting
from the growing anthropogenic pressure, global climate
change and other factors, the statistic parameters of time periods
may alter.
Multi-dimensional empirical-statistic models, employing
equations of multiple regression, have signalled the further
development of the contingency conception and its application
in analyzing and forecasting the time correlations between
the annual outflow in multi-dimensional space of the
predicting vector. At the same time it appears necessary to
prove the possibility of applying the revealed dependencies
for the forecast period. It is also essential to forecast the predicting
vector itself, which is a more complicated task, especially
for a specific period (Ismailov G.H, Fedorov V.M.,
2001).
Together with the contingency concept of the long-term
fluctuation of the annual outflow, the concept of cyclicism
is also used. Moreover, while there is no uniformity in the
nature of these cycles, an objective methodology of distinguishing
and analyzing cycles of river water level is also absent.
In addition to the irregular occurrence of cycles, the
possibility of the physical (genetic) interpretation belongs to
the weaker points of such an approach.
As both approaches yield a comparable result, the cyclicism
assumption may well be applied for the analysis and forecasts
of long-term fluctuation of the annual flow.
The analysis of the average annual water flow for the 3 periods
in question (1808 – 1876, 1877 – 1964, 1965 – 2000)
shows that the zero hypothesis, that the absolute differences
should be considered insignificant, is not accepted. At the
same time the zero hypothesis may be recognized for dispersions
only between the periods of 1808 – 1876 and 1877 –
1964, for other calculations the zero hypothesis of dispersion
equality must be neglected. The fluctuation range of the
annual flow in the Neman river at Grodno from 1964 to
2000 differs statistically from the previous 2 periods: It is
definitely lower.
Different periods were selected to estimate the statistic parameters
of the flow and to study their alteration between
periods. Apart from this, the statistic parameters of the initial
period were calculated with the help of 20, 30, 35, and
50 year long average estimation procedure (Table 2).