INCORPORATING VARIABILITY INTO SOIL MAP UNIT DELINEATIONS

Published soil surveys comprise a nationwide soils data base available to users. Although adequate for farm planning and agricultural produc tion, the surveys lack information on spatial and temporal variability of soil attributes. Such information is increasingly needed for model ing of water flow and contaminant transport, in geographical informati on systems (GIS) applications, and for environmental impact assessment . The proposed approach combines spatially interpolated distributions of measured values with soil map unit delineations within a GIS framew ork. Data analysis provides insights into the potential continuity and variability of attributes within and between the map units. The resul t is a map that preserves the map unit boundaries but incorporates spa tial variability of attribute data into map unit delineations. The app roach is illustrated using bulk density and hydraulic conductivity dat a for surface soil horizons at a farm scale. Semivariograms based on m easured values are constructed and cross-validated. Subsequently, GIS overlays of soil survey attributes and kriged overlays of measured dat a can be prepared. These GIS overlays can be combined pixel by pixel a ccording to a simple rule. Results indicate that spatial distributions of bulk density and hydraulic conductivity are similar to original ma p unit delineations, but include aspects of the attribute continuity a nd variability from geostatistical analysis. Such combined representat ions of bulk density and hydraulic conductivity are superior, because they contain more information than either the soil survey map, or the kriged interpolation of discrete data.