Runoff in Danish rivers
Niels B. Oversen
Technical report from NERI, no. 340
Abstract in English
The report provides a characterization and analyses of the
runoff in Danish rivers. The basis of the report is the archive of hydrometric
data collected over the last century. The runoff is described by means of
characteristic parameters, duration curves, extreme value analyses, trend
analyses and mapping of runoff and water balance. The report presents an
overwiev of the national hydrometric data archive providing maps, tables and key
data.
Runoff monitoring In Denmark, runoff has been
systematically registered since around 1917. Runoff is currently measured
continuously at almost 400 monitoring stations distributed in the app. 64,000 km
small and large streams. The total catchment of the monitoring stations
constitutes app. 55% of the total area of the country. Denmark’s largest
stream is the river Gudenå (149 km) draining an area of more than 2,600
km2 . The
river with the most water, however, is the river Skjern Å with an average
discharge of app. 37,000 l/sec.
Data collection For many years the water
stage has been registered by means of manual readings of a water level gauge
placed at many of the stations, but also automatic chart recorders have been in
operation since the beginning of the 20th century. Nowadays, registrations are
normally performed by dataloggers and occasionally with a telemetry link to the
stations. At most stations, however, the actual registration is still performed
by the use of float and pulley in a stilling well. Flow measurements are
performed by the use of a propeller instrument.
Flow velocity and river gradients Fall gradients of Danish streams fall almost
exclusively within the interval 0.1-10 per thousand. The gradient tends to
decrease downstream in the river system (the closer it gets to the coast) but
there are marked deviations between the various parts of the country. The
streams on Bornholm, for instance, have relatively large gradients.
The flow velocity in streams varies between app. 0.2 to 0.5 m
s-1 and the velocity is often greater in large streams. The average
stream velocity is app. 0.3 m s-1.
Trends in runoff The annual mean runoff shows an increasing trend over the past 80
years in most of the Danish streams. This relates primarily to a corresponding
increase in the quantity of precipitation. Some streams, however, show a falling
trend caused by for example water abstraction from the stream or from
groundwater within the catchment. Also the annual maximum and minimum runoff
show a predominantly increasing trend, but the results are not unambiguous. The
indicated trends are only significant in a small proportion of the analysed time
series.
Lowflow Median
minimum runoff is used as a key parameter in river management and water
resources administration. Year-to-year minimum runoff may differ considerably
from the median minimum. During very dry years the level will be less than half
of the median minimum and in some streams even as little as a quarter of the
median minimum. Consequently, the determination of median minimum runoff is
sensitive to the choice of length, starting and ending time of the calculation
period.
Extreme events By means of long time series combined with adjusted distribution
functions, the probability of certain runoff events are estimated. In Denmark,
T-year values are often used that are exceeded or reduced every T-th year on
average. T could be, for example, 2, 10, 20, 50 or 100-year events and the
values are used in connection with agricultural and draining interests, bridge
structures, fish ladders and river restoration projects.
Period of reference On the basis of the analyses and assessments of this report as well as
earlier investigations it is recommended that the median minimum, median maximum
and mean runoff be calculated on the basis of data for the period 1971-1998, and
that the period is increased to a 30-year period from 2001 (1971-2000). In the
long term it is recommended that the period of reference be forwarded at the
beginning of each new decade. The national topic centre of hydrometry has
previously recommended the period 1971-1990. By changing the period of reference
to 1971-1998, the level for the average median minimum runoff will be reduced by
app. 4 per cent.
Runoff characteristics There are considerable variations in the runoff pattern in Danish
streams. Duration curves are an excellent tool for describing and analysing
these variations. In some streams the runoff is very constant with little
variation between winter and summer water flow and without significant increases
in connection with for example heavy precipitation. Others have a very varying
water flow with large runoff during certain periods of the year and a very
marked response to precipitation events and may still almost dry up during
summer.
Relation between water quality index and runoff
characteristics Correlation analysis of
flow-ecological parameters and biotic water quality index shows that poor water
quality is in general related to streams with large flow-variations. However, it
is not revealed if the correlation is caused by the influence of flow-variations
on the fauna, or if other factors correlate to both flow-parameters and biotic
index. This could apply for organic matter. The flow-ecological parameters are
closely correlated to each other, and it is uncertain which values could be most
important for the river ecology.
Small versus large streams Analyses of the relationship between runoff parameters that may be
used as indicators for ecological conditions show that there are well-defined
variations between small and large streams and between the various regions of
the country. The variations are generally less significant in large streams than
small ones. A level of 7 times the median water flow is only exceeded between 0
and 15 times a year in large streams whereas it is exceeded between 1.5 and 5
times in small streams. The ratio between median minimum and the median water
flow is between app. 0.1 and 0.4 in small streams and between app. 0.3 and 0.6
in large streams.
Mean runoff in Denmark The average annual mean runoff
for Denmark as a whole is 318 mm (10.1 ls-1 km-2). In
the eastern part of the country it is app. 200 mm, and in the western part 400
mm per year. In central and southern parts of Jylland 600 mm are exceeded. The
considerable differences in runoff are related to precipitation pattern,
evaporation, regional groundwater migration and abstraction. In years with
sparse precipitation such as e.g. 1976 and 1996, the runoff is less than 200 mm,
and in years rich in precipitation, such as e.g. 1981 and 1994, it is more than
450 mm.
Extreme values – regional patterns Low-flow values
vary considerably in the various parts of the country. In the eastern part of
the country the average level is around 0.5 l s-1
km–2, and in the western part 10 times higher. The low-flow
pattern is mainly related to soil type and groundwater reservoirs. Flood values
also reveal large variations over the country. In regions with heavy
fine-grained soils, where infiltration of precipitation is restricted, the
2-year flood is in the interval 50 – 100 l s-1
km–2. In regions with sand-dominated soils the 2-year flood is
within the interval 20 – 40 l s-1 km–2.
Hydrological regimes Cluster analysis is performed on a set of flow variables, and separate
groups can be identified. The analysis shows that there is generally an unstable
water flow regime on the Danish islands, whereas the conditions in streams in
northern and western Jylland are more moderate.
Water balance When considering the water balance for the country as a whole over a
period of many years, there is a good fit between precipitation, evaporation and
runoff. As an average for the period 1971 – 1998 the annual precipitation
is 854 mm, the potential evaporation 520 mm and the runoff 322 mm. For small
regions or short periods there are considerable deviations caused mainly by
ground water movements across the topographic water divides, water abstraction
or fluctuation in the ground water level.
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