A Water Quality Simulation Model for a Lowland River-Lake System

A Water Quality Simulation Model for a Lowland River-lake System

Bernhard Luther1, Albrecht Gnauck1 and Alexander Meisch-Peschew1

Abstract Polluted water affects not only the functioning of freshwater ecosystems and various water uses, but risks human health. One of the long-term environmental problems of freshwater ecosystem management is the control of eutrophication. It is characterised by an intensive increase of dissolved nutrients in water bodies, by excessive growth of plants, mainly algae, and by restricted water uses due to anoxic water conditions as well as by odour problems. It re- fers to intensive man-made activities in the river catchment. The simulation framework presented consists of a combination of the MATLAB based simulation models SpreeMod and HavelMod. The simulator SpreeMod describes water quality changes along a river stretch while the simulator HavelMod covers water quality changes in shallow riverine lakes. Both simulators are coupled by interfaces which switch the model type from river to lake and vice versa. This simulation framework covers not only a long distance but also different types of freshwater ecosystems with changing dynamic water quality behaviour. Model state variables are phytoplankton, zooplankton, orthophos- phate phosphorus, ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, dissolved oxygen and biological oxygen de- mand. External driving forces are photoperiod, solar radiation and water temperature. For simulation input sampled raw data are treated by interpolation methods to generate equidistant data. In this paper simulation results of the lowland river-lake system formed by the rivers Spree and Havel close to the Berlin/Potsdam region are discussed.

1. Introduction The functioning of freshwater ecosystems and various water uses are affected by natural and man induced nutrient loads. While natural pollution is mostly small the nutrient input due to land erosion and intensive anthropogenic activities in river basins force the eutrophication process. It is characterised by an increase of dissolved nutrient concentrations, by excessive growth of plant biomass, and by restricted water uses due to anoxic water conditions as well as by odour problems. Especially, internal pollution by nutrient remobilisation from sediment plays an important role in lowland rivers and shallow lakes. A sustainable management of such freshwater ecosystems can be achieved by using simulation models. To evaluate the nutrient dynamics of a lowland river-lake system a stationary simulation framework was worked out. It consists of a combination of the models HavelMod (Gnauck et al. 2003) and SpreeMod (Meisch-Peschew 2005) to simulate the eutrophication process in the lowland rivers Havel and Spree as well as for shallow lakes of the Havel catchment close to the Berlin/Potsdam area. Originally, both models were developed independly to simulate the effects of phosphorus remobilisation from sediment in shallow lakes and matter changes in the rivers Spree and Havel caused by urbanisation processes of the City of Berlin. Applying this simulator on the Spree-Havel river stretch from Berlin to City of Brandenburg it will be possible to get statements on changes of the water quality status caused by human impact on different freshwater ecosystems. It can be expected that simulation results form a base to derive management op- tions for the river catchment.

1 Bernhard Luther, Albrecht Gnauck, Alexander Meisch-Peschew, Brandenburg University of Technology at Cottbus, Dept. Eco- systems and Environmental Informatics, Konrad-Wachsmann-Allee 1, D-03046 Cottbus, Germany, Phone: +49-355-692713, Fax +49-355-692743, E-mail: [email protected]

573 2. Modelling framework The model concept given in figure 1 was derived from the model HavelMod (Gnauck et al. 2003) where the state variables of DO, BOD and nitrite nitrogen are added. The model consists of eight state variables where phosphorus remobilisation from sediment is expressed by a special differential equation. It is included in the phosphorus balance. Time changes of each state variable are expressed by a differential equation. Site constants and model specific parameters used are valid for the whole experimental area. Qin and Qout characterise the inflow from the river segment above and the outflow to a river segment down. External driving forces are photoperiod (FOTOP), solar radiation (I) and water temperature (TEMP) (Straškraba/Gnauck 1985).

FOTOP I TEMP

DO A Z

NH4-N PO4-P

Qin Qout

BOD NO2-N

NO3-N

Sediment

Fig, 1: Model concept of the simulation framework

3. Experimental area The River Havel belongs to the greatest tributaries on the right hand site of the River Elbe. It is strongly influenced by the River Spree. Rivers Spree and Havel are characterised by small elevation differences. The watershed is characterised by shallow lakes, wetlands and marshy country, as well as by high evaporation rates. Hydraulic works and banked-up water levels influence water flow and the intensity and kinetics of nutrient dynamics along the course of the river. For the water quality simulation framework the river stretches of interest were divided into several segments of different length (figure 2). The simulation framework covers 19 segments of different length. But the number of segments is not restricted. The model concept is valid for each river segment. Data sets from 2001 are used as input information.

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Havel Westhafen River Channel WWTP Landwehr Ruhleben Channel Spree River

325 160 155 150 145 140 135 130 330 350 355 Havel River Sacrow-Paretzer Channel 345

Hv 0200 Hv 0190 SPK 0020 SPK 0010 Teltow Channel 430 Hv 0180 Havel Hv 0110 River

Hv 0170 Hv 0120 Nu 0120

Hv 0160 Hv 0130 Nuthe

Hv 0140

Fig. 2: Schematic representation of the experimental area

4. Results Simulation results obtained show changes in biomass and nutrient contents along the course of rivers Spree and Havel. As can be seen from figure 3 two algal blooms take place during a year.

0.1 130 0.09 140 150 0.08 160

0.07

0.06

0.05

CHA (mg/l) 0.04

0.03

0.02

0.01

0 JFMAMNJJA SO D time (M)

Fig. 3: Biomass productions along the course of the river Spree at Berlin

A slight shift of maximal biomass concentration from starting point of simulation to the mouth of river Spree is observed. This result is in accordance with increasing water quantity due to waste water input. Continued simulation runs up to the City of Brandenburg lay out that biomass increase is caused by

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Bibliography Gnauck, A., Heinrich, R. und B. Luther (2003): Modellierung und Simulation von Seen in der Unteren Havel. Forum der Forschung 8(2003)15, 43-48. Straškraba, M. and A. Gnauck: Freshwater Ecosystems – Modelling and Simulation. Elsevier, Amsterdam, 1985.

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