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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.

Report in PDF format (1265 KB)
Appendices in PDF format (1868 KB)
Maps in PDF format (3322 KB)

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