Operational harmonisation of soil maps in Portugal using legacy data and WRB classification

Accurate and harmonised soil maps are essential for sustainable land management and achieving the Sustainable Development Goals (SDGs). In Portugal, existing soil maps remain inconsistent in scales (1:25,000 vs. 1:100,000), classification systems (national vs. WRB), and methodologies. Based on combinations of soil-forming factors (climate, lithology, and relief), the present study develops a fully operational workflow to update and harmonise soil mapping at a 1:100,000 scale using the World Reference Base for Soil Resources (WRB). Following the delineation of provisional land units, the methodological approach included: (1) compiling, validating and harmonising legacy soil profiles to support classification conversion and methodology validation; (2) converting the Soil Classification of Portugal (SCP) into WRB Reference Soil Groups (RSGs) and key qualifiers; (3) refining provisional LUs to improve their internal consistency; (4) defining Soil Mapping Units (SMUs) through identification of dominant, co-dominant, and associated soil units within different LUs; and (5) quantitatively validating SMUs against 165 legacy soil profiles using a presence/absence scoring method. The SMUs achieved 76–82% correspondence with legacy profiles under two validation scenarios. Despite legacy data constraints, the defined SMUs clearly differentiate WRB RSGs and qualifiers (principal and supplementary) across a Mediterranean landscape, enabling the identification of key soil functional properties often absent from legacy maps. The present approach provides a reproducible, scalable, operational framework for harmonised soil mapping and land evaluation. It also supports evidence-based soil management and can be readily adapted to other regions with comparable legacy data and environmental conditions. • Soil-forming factors drive WRB RSGs and qualifiers distribution in Mediterranean areas. • Updated soil mapping units show high agreement (76–82%) with legacy soil profiles. • Success of soil map updating depends on the accuracy of national-WRB conversion. • Reliable soil mapping with limited legacy data is enabled by land units' delineation. • The developed approach supports soil mapping harmonisation and land evaluation.