The Dutch soil physical units map: BOFEK

Soils and their properties play an important role in land evaluation studies. Often such studies focus on larger scales ranging from watersheds up to the national scale or even larger. Soil properties are often known at smaller scales, sometimes at the level of individual soil samples. The aim of this study is to show how point information on soil hydraulic properties, i.e., water retention and hydraulic conductivity characteristics, can be upscaled via soil textural classes to a soil physical units map of a region or nation. Base information is the Dutch soil map (1:50,000) and the hydraulic properties of individual soil samples. All individual soil samples for which hy-draulic properties were measured were divided based on their texture into eighteen top-soils and eighteen sub-soils. For each of these thirty-six groups geometric average water retention and hydraulic conductivity charac-teristics were derived. In total 368 derived soil profiles are distinguished in the Dutch soil map consisting of soil layers that are linked to the thirty-six texture groups. For each soil profile eight static hydraulic properties were calculated. The soil profiles were then clustered based on these properties into seventy-nine clusters or units, which then make up a soil physical units map for the Netherlands. It has been demonstrated that dynamically simulated transpiration reduction for the clustered situation is similar to obtained for all individual soil profiles. At the Dutch national scale, the difference in simulated transpiration reduction between runs using all 368 soil profiles or the 79 soil physical units was less than 2.5 % percentage-points in 96 % of all plots or less than 5 % percentage-points in 99 % of all plots. Similar good correspondence was obtained for other water balance terms as well, including actual transpiration, actual evaporation at the soil surface, degree of saturation at 15 and 30 cm depth, the integrated water flux at 100 cm depth and surface runoff.