Updating legacy soil maps through digital delineation of homogeneous land units based on soil formation factors: Experiences from Portugal

Abstract

Land use planning and the sustainable management of natural resources rely on updated and harmonised soil information systems. Meanwhile, in Portugal, soil mapping remains highly heterogeneous regarding scales, classification systems, and mapping methodologies. Accordingly, a fast and robust approach is needed to update soil maps at a same scale (1:100,000), while using an international classification system. For that, the delineation of Homogeneous Units (HU), based on relevant soil formation factors, is proposed to predict and frame soil units derived from the legacy soil map of Portugal (1:25,000). In the project area, digital tools were used to create and intersect three primary factor layers (climate, lithology, and relief) to generate the HU. The capacity of HU to differentiate WRB Reference Soil Groups (RSGs) was then assessed by calculating the proportion of soil units from the legacy soil maps within each HU. The study identified six climatic units, five relief classes, 23 lithological groupings, and 100 HU were obtained in two study areas. HU proved to be a simple and highly effective approach in differentiating RSGs, and some qualifiers, in their landscape. Lithology was a primary factor as mafic igneous rocks were mostly associated with Vertisols, intermediate with Luvisols, and felsic with Regosols. Relief contributed to distinguishing RSGs in schists. Both lithology and relief also explain the distribution of key qualifiers (e.g. Leptic, Stagnic, Skeletic, Calcic). The proposed methodology offers a powerful basis for updating and harmonising soil mapping in Mediterranean regions, enhancing soil data consistency, and supporting the integration of national soil mapping into international frameworks.