Strengthening the reliability of the Netherlands Hydrological Modelling Instrument
KWR has developed a method to test the groundwater-level calculations made by the Netherlands Hydrological Modelling Instrument (NHI). The NHI is a groundwater and surface water model that covers all of the Netherlands. A number of partners, including the Dutch Directorate for Public Works and Water Management, the Foundation for Applied Water Research (STOWA), water managers and engineering consultancies, contribute their knowledge and data to the NHI. With this instrument, the government is in a position to make solid policy decisions.
One important issue in the Netherlands concerns the management of its fresh water resources during the summer. Agriculture and drinking water companies need water, but so do electric power stations. In periods of water shortages, an allocation formula needs to be established for the optimal distribution of the available water. This is why it is essential that the NHI results be reliable. To strengthen the NHI’s reliability, KWR has developed a method to test the groundwater-levels the model calculates.
Testing in practice
The NHI lays out, in a theoretical model, a grid that covers all of the Netherlands with cells measuring 250 by 250 meters. The model calculates the groundwater level for each cell. It does not recognise any variations in these groundwater levels within the cells, even though such variations naturally occur. For example, the presence or absence of ditches in an area will have an influence on how rapidly the water there is discharged.
KWR’s method tests the model from a practical perspective. There are tens of thousands of groundwater monitoring wells throughout the Netherlands. Drawing on the measurements from these wells and knowledge of the specific environment – e.g., the number of ditches – the method can make forecasts for the whole 250 by 250 meter area. These results are then compared with those of the NHI, thus effectively testing the latter. This procedure is applied to three test areas in the Netherlands which, together, reflect the different groundwater conditions in the country.
Unravelling the causes
The measurements reflect the effect of all the phenomena that have actually occurred, like rain, evaporation and variations in river levels. KWR’s method unravels the different causes behind the fluctuations in groundwater levels: instead of a complex assessment of all the causes combined, the method provides a single value for each cause. This makes it possible to check whether the NHI can calculate the influence of specific causes, such as the reaction of groundwater levels to precipitation.
The contribution of KWR to the NHI is financed by the Association of Dutch Water Companies (Vewin).
© 2018 KWR Watercycle Research Institute
Willem Jan Zaadnoordijk