Assumptions & limitations for drought

The groundwater maps are based on model calculations involving 250 x 250 metre cells, uniformly characterised to the best possible extent. This has generated nationwide, sound pictures for the current and future situations. However, the results cannot be used to underpin general local-level statements; this requires more detailed models that factor in local variations in, for example, land use and surface-level altitude.

Several assumptions underlie the maps for pole rote and differential settlement, which also are mentioned in the technical notes (PDF). The datasets include assumptions on the probability of certain foundation types based on general location, building age and general soil type (peat/clay). It is assumed that all buildings located on a sandy subsoil have a shallow foundation which is not subject to settlement/ damage. Assumptions in the datasets are based on input from a session with foundation experts held in Gouda on 16-5-2019, in the course of the National Knowledge Water and Innovation Programme Water and Climate (NKWK KBS) project.

The drought susceptibility nature map does not show any groundwater-independent types of nature, such as the Veluwe forests, nor wet species of vegetation based on very deep groundwater levels, as these cannot be regulated through groundwater management. Such species grow on so-called fluctuating groundwater levels. Furthermore, the map only reflects the generic drought sensitivity of the type of nature; it does not address local developments expected at a specific location. Moreover, the sensitivity is based on vegetation only.

The map showing the annual yield loss of grass is made to provide a univocal picture of the differences in risk of drought stress for the whole of the Netherlands. These maps are consistently based on grass fields. In actual practice, many surfaces will be bare or covered by vegetation that is either more, or less, vulnerable than grass. Obviously, vegetables such as lettuce will entail higher yield losses compared to, for example, grains.