a b s t r a c tStudy region: The Federal State of North Rhine-Westphalia (NRW), Germany.Study focus: On behalf of the Federal State Agency for Nature, Environment and Consumer Protection of North Rhine-Westphalia, Germany (LANUV) the mGROWA model is applied, in order to simulate the water balance components actual evapotranspiration, total runoff, direct runoff and groundwater recharge at Federal State level. mGROWAsimulations were performed in daily time steps for the hydrological reference period 1971-2000 and in a spatial resolution of 100 by 100 m. mGROWA results for groundwater recharge and State-wide recorded groundwater withdrawal rates have been used to assess the extent of groundwater exploitation in NRW.New hydrological insights: Simulated groundwater recharge levels are presented as long-term annual averages and as long-term monthly values in order to indicate the seasonal fluctuation of groundwater recharge rates. Quantitative statistics indicate that mGROWA enables the simulation of total runoff and groundwater recharge without significant tendency of over-or underestimation. Against this background mGROWA simulation results are used by LANUV to support regional water resources management, e.g. for determining the status of groundwater exploitation in NRW. The majority of groundwater bodies in NRW are at present not at risk of unsustainable groundwater usage. A small number of groundwater bodies along the river Rhine, however, have been assessed as being close to unsustainable groundwater exploitation.
Commissioned by Germany's Working Group of the Federal States on Water Problems (LAWA) the authors developed a procedure to define natural groundwater conditions from groundwater monitoring data. The distribution pattern of a specific groundwater parameter observed by a number of groundwater monitoring stations within a petrographically comparable groundwater typology is reproduced by two statistical distribution functions, representing the "natural" and "influenced" components. The range of natural groundwater concentrations is characterized by confidence intervals of the distribution function of the natural component. The applicability of the approach was established for four hydrochemically different groundwater typologies occurring throughout Germany. Based on groundwater monitoring data from 7920 groundwater monitoring stations, 15 different hydrochemical parameters were evaluated for each groundwater typology. For all investigated parameters the range of natural groundwater concentrations has been identified. According to the requirements of the EC Water Framework Directive (article 17) (WFD) this study is a basis for the German position to propose criteria for assessing a reference state for a "good groundwater chemical status".
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