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Water Balance Modelling

In Germany groundwater is the most commonly used ressource for drinking water supply. According to Genesis-online (2005) in 2001 84% of the drinking water demand in North Rhine-Westphalia and 86% in Lower Saxony were covered by groundwater (incl spring water, bank filtrate and charged groundwater). In Hamburg even 100% of the drinking water demand is covered by groundwater. Therefore an important field of activity in water management is regional groundwater planning. One of the basic variables for planning is the water availability, which is defined by natural replenishment and seeping precipitation water. The groundwater recharge is also decisive for the flow conditions of rivers in times of low water and therefore very important for the ecological status of the rivers. Without area-wide knowledge about recharge rates decisions within water law procedures (e.g. regulation of permitted groundwater tapping quantity) but also accomplishment of the reporting commitment to the EU in terms of a quantitative status description of the groundwater bodies according to WRR (EU, 2000) are limited. The portion of groundwater recharge in total runoff is an important parameter to consider in the regional water quantity management. For water management purposes regions with high recharge rates are prior regions for water supply. On the one hand there is a quick replenishment of groundwater resources, otherwise measures for runoff regulation, which is important in regions with high parts of direct runoff, can be set aside.

Given that the discharge of nutrients into the groundwater respectively surface water is always bound to runoff, the distribution pattern of the water balance in a river catchment is beyond the water management aspects always the basis for analysing diffuse nutrient input into ground- respectively surface water.



Mean longtime groundwater recharge in North Rhine-Westphalia, Lower Saxony, Hamburg and Bremen

Cooperation with the Lower Saxon regional authority for Mining (LBEG), the geological service North Rhine-Westphalia (GD NRW) and the environmental authority Hamburg (since 1999)


For the federal states North Rhine Westphalia, Lower Saxony, Hamburg as well as Bremen the mean long-term groundwater recharge was calculated area-covering with the water balance model GROWA ("GROßräumiges WAsserhaushaltsmodell") (Kunkel & Wendland, 2002). In collaboration with the Geological Surveys this model was further developed in the last years. The results for each federal state were already released in a couple of publications (cp. Bogena et al., 2003, Tetzlaff et al., 2004, Wendland et al., 2003)

After the project completion there was the question how the results could be combined into a comprehensive digital map of groundwater recharge rates in North Rhine-Westphalia, Lower Saxony and Hamburg. As each federal state has a different data basis the challenge was to unify this data to one homogeneously data set which should be used to model the water balance (total runoff, real evapotranspiration, groundwater recharge, direct runoff). The groundwater recharge modelling was carried out to analyse and evaluate the sustainable disposable water quantity. Therefore the water balance model GROWA (Kunkel & Wendland, 2002), developed in the Research Centre Jülich, was used and evolved to quantify areadifferentiated water balance parameters real evapotranspiration, total runoff, direct runoff and groundwater recharge.

The results of the modelling are shown in digital maps. The figure shows exemplarily the calculated longtime mean groundwater recharge rate (1961 - 1990).



The calculated groundwater recharge rates range from less than 25 mm/a to more than 250 mm/a. This reflects the diversity of climatic, pedological and geological conditions. In plain unconsolidated rock areas of at some distance from the water table (e.g. on glacial outwash), the groundwater recharge largely corresponds to the total runoff and generally amounts to more than 150 mm/a.
In unconsolidated rock areas influenced by the groundwater and water logging (e.g. in floodlands) the groundwater recharge is less than 50 mm/a. The major runoff fraction (more than 80 %) is discharged in the form of direct runoff and reaches the receiving waters via the soil surface or the unsaturated soil zone. The same is true of areas on Palaeozoic and crystalline rocks where, although the base flow can amount to 250 mm/a and above, the groundwater recharge contributes less than 40 % to the total runoff. Only the karstified carbonate rocks of the Cretaceous in the Münsterland region and of the Middle Devonian in the Rhenish Slate Mountains have higher groundwater recharge rates of over 300 mm/a.

In order to validate the calculated groundwater recharge, it was assumed after Wundt (1958) that the mean long-term runoff fraction originating from the groundwater is represented with sufficient accuracy by the mean of the smallest daily runoff per month (MoMNQ) of the time series. 307 gauging stations were available for validating the groundwater recharge. As can be seen from the figure above, the deviations from the calculated groundwater recharge differ from the measured values for most gauging stations between 0 and ± 15 %. Errors in this order of magnitude lie within the usual variation range of an empirical model.

The major focus of the GROWA model is placed on calculating the mean long-term groundwater recharge in large areas, such as the Federal State of Lower Saxony. It was not the aim of the project to consider interannual variabilities. The groundwater recharge observed for individual years (e.g. wet/dry years) or interannual reference periods (e.g. summer/winter six months) may therefore deviate from the calculated values shown in the maps. In its existing form the GROWA water-balance model is therefore suitable for GIS?based modelling of the water balance in large areas for water management planning on a state and regional basis, e.g. with respect to analysis and evaluation for the sustainable use of the groundwater supply. Therefore, the application of the GROWA approach for practical water resources management issues like the determination of the long term groundwater recharge in river catchment areas as required by the EU water directive can be recommended.



Selected publications:
Bogena, H., Kunkel, R., Schöbel, T., Schrey, H. P. und Wendland, F. (2005): Distributed modeling of groundwater recharge at the macroscale. Ecological Modelling 187(1), 15 - 26.

Bogena, H.; Kunkel, R.; Schöbel, T.; Schrey, H.-P.; Wendland, F. (2003): Die Grundwasserneubildung in Nordrhein-Westfalen. Schriften des Forschungszentrums Jülich, Reihe Umwelt, 37, Forschungszentrum Jülich GmbH, Jülich, Germany, 148 S..

Dörhöfer, G.; Kunkel, R.; Tetzlaff, B.; Wendland, F.: Der natürliche Grundwasserhaushalt in Niedersachsen Arbeitshefte Wasser 1 (2001), S. 109-167.

Kunkel, R. ; Wendland, F. (2002): The GROWA98 model for water balance analysis in large river basins - the river Elbe case study.- Journal of Hydrology, 259, S. 152-162.

Kunkel, R., Bogena, H., Tetzlaff, B. und Wendland, F. (2006): Digitale Grundwasserneubildungskarte von Niedersachsen, Nordrhein-Westfalen, Hamburg und Bremen: Erstellung und Auswertungsbeispiele. Hydrologie und Wasserbewirtschaftung 50, H5, 212-220.

Tetzlaff, B.; Dörhöfer, G.; Kunkel, R.; Wendland, F. (2003): GIS-gestützte Ermittlung der Grundwasserneubildung in Niedersachsen.. Wasser und Boden 7/8, 53-57.

Tetzlaff, B., Kunkel, R., Taugs, R., Doerhoefer, G. und Wendland, F. (2004): Grundlagen für eine nachhaltige Bewirtschaftung von Grundwasserressourcen in der Metropolregion Hamburg. Schriften des Forschungszentrums Jülich, Reihe Umwelt/Environment, Band 46, Forschungszentrum Jülich GmbH, Jülich, Germany.

Wendland, F., Kunkel, R., Tetzlaff, B. und Doerhoefer, G. (2003): GIS-based determination of the mean long-term groundwater recharge in Lower Saxony. Environmental Geology 45(2), 273-278.

Wendland, F. und Kunkel, R.: Der Landschaftswasserhaushalt im Elbeeinzugsgebiet (deutscher Teil). Hydrologie und Wasserbewirtschaftung, H. 5, S. 226-232; 1999.

Das Modell mGROWA-BOWAB zur Simulation des innerjährlichen Wasserhaushaltes

Mit dem mGROWA-BOWAB wurde ein flächendifferenziertes rasterbasiertes Modells zur innerjährlichen Simulation des Wasserhaushaltes entwickelt. Hierzu wurde die BOWAB-Methodik des LBEG zur Simulation des Bodenwasserhaushalts ins GROWA – Modell implementiert. War BOWAB ursprünglich für die Simulation des Bodenwasserhaushalts von Ackerstandorten beschränkt, so war nach Anpassung der vegetationsspezifischen Parameter eine Anwendung auf Standorte mit anderen Vegetationsarten (z.B. Wald oder Grünland) möglich.
Im mGROWA-BOWAB erfolgt zunächst eine Berechnung der Verdunstung und der Abflussbildung in Tagesschritten. Hierzu werden das im Boden gespeicherte Wasser, das pflanzenverfügbare Bodenwasser und die tägliche Sickerwasserrate berechnet. Anschließend erfolgt eine Abflussseparation in die Hauptabflusskomponenten Direktabfluss (natürlicher Interflow und Abfluss aus Dränagen) und Grundwasserneubildung auf Basis des aus dem GROWA Modell bekannten empirischen Verfahrens. Die Räumliche Auflösung von mGROWA-BOWAB ist variabel und liegt derzeit bei 100 m x 100 m. Die Darstellung der Ergebnisse erfolgt in Kartenform, üblicherweise in Monaten.
Erste Vergleiche von beobachteten Abflussspenden mit den von mGROWA-BOWAB simulierten Abflussspenden in einer Vielzahl verschiedener Einzugsgebiete in Niedersachsen lassen bereits die Schlussfolgerung zu, dass mit mGROWA-BOWAB realistische Verdunstungs- und Sickerwasserraten berechnet werden können.
Eine Anwendung des mGROWA-BOWAB – Modells für ganz Niedersachsen für die Periode 1971 – 2000 steht kurz vor dem Abschluss (HERRMANN et al., in Vorbereitung).  Das GROWA-BOWAB-Modell wird in diesem Zusammenhang nicht nur zur Simulation des aktuellen innerjährlichen Wasserhaushaltes auf Landesebene eingesetzt, sondern auch zur Prognose der Auswirkungen von Klimaänderungen auf das nachhaltig bewirtschaftbare Grundwasserdargebot und die Beregnungsbedürftigkeit.

Forschungsprojekt zu diesem Thema:

2009 - 2012Niedersächsisches Landesamt für Bergbau, Energie und Geologie (LBEG) Entwicklung eines Wasserhaushaltsmodells zur Vorhersage der innerjährlichen Variabilität der Grundwasserneubildung in Niedersachsen

Eine Anwendung des mGROWA-BOWAB – Modells für ganz Niedersachsen für die Periode 1971 – 2000 steht kurz vor dem Abschluss (HERRMANN et al., in Vorbereitung).  Das GROWA-BOWAB-Modell wird in diesem Zusammenhang nicht nur zur Simulation des aktuellen innerjährlichen Wasserhaushaltes auf Landesebene eingesetzt, sondern auch zur Prognose der Auswirkungen von Klimaänderungen auf das nachhaltig bewirtschaftbare Grundwasserdargebot und die Beregnungsbedürftigkeit.
Simulationsergebnisse liegen derzeit flächendeckend für den Südteil der Metropolregion Hamburg sowie für die Regionen vor, in denen auch die Evaluierung der Simulationsergebnisse erfolgt: