Flooding in the northern part of The Netherlands has caused serious economic threats to densely populated areas. Therefore a project has been carried out in a pilot area to assess the retention of water in two river basins as a way to reduce flooding. The physically-based groundwater and sur-face water model SIMGRO was used to model the hydrology of the basins. The model was calibrated using discharges and groundwater levels. Scenarios of measures to assess the possibility of retaining water in the basin were then defined and tested. The first measure was the retention of higher dis-charges using culverts or gates in the upstream part of the basin. The second measure was to make the streams shallower and thereby, increase flood plain storage. The last measure was flood water storage in a designated area in the downstream part of one basin. The analysis indicates that holding water in the upstream parts of the basins proved to be feasible and can result in significant reductions of peak flows.

The very wet conditions of recent years in Europe have made it clear that measures will have to be taken in this century to prevent flooding. The question is how to manage groundwater in order to reduce the anticipated increased hydrological risk. Furthermore the surface water quality in the Netherlands is insufficient to meet the standards of the Water Framework Directive. The required improvements are difficult to reach, because the diffuse loads of nutrients from agricultural land can not be easily reduced. This demands for innovative solutions with respect to improve the surface wa-ter quality. In this pilot study the focus is on the purification in reed fields and use it as well to reduce the effects of the anticipated climate change. An experimental evidence on a practical scale is lacking and therefore in the woodland area of Lankheet in the eastern part of the Netherlands, 3 ha has been planted with reeds to purify the river water. The aim of the study is further to store the purified water in the groundwater in order to reduce climate change effects. For the hydrological situation a scenario study was set up, using a regional hydrological model to simulate the groundwater flow together with the water flow in a network of water courses. The analysis will give knowledge on the multifunc-tional use of such a system.

The Drentsche Aa catchment in The Netherlands, which has nearly untouched natural river valleys, is a designated Natura 2000 area. Agriculture is practiced on the adjacent higher-lying ground. A set of measures was drafted to achieve climate-proof solutions in the short term by reducing the effects of a drier climate on nature and agriculture. These measures must have no adverse effects. In order to check this, the Hunze and Aa’s Water Board investigated the feasibility of using groundwater for sprinkler irrigation in parts of the catchment. In the study, the SIMulation of GROundwater and surface water levels (SIMGRO) hydrological model was used in order to model future scenarios with different water level strategies and climate scenarios. The modelling examined various measures in the nature and agricultural areas to optimise the hydrological situation for both land use functions. In addition, the effect on the nature areas of abstracting groundwater for irrigation was determined for buffer zones of different widths. The findings have indicated the policy direction to be taken by both the water board and the province, as well as offer them opportunities to deal with the requests for withdrawals in the near future by the means of future-proof general rules.