Soil geomorphology: Identifying relations between the scale of spatial variation and soil processes using the variogram

Abstract

Practitioners of geostatistics often fail to make associations between the patterns of variation on their maps of kriged predictions and the physical processes that might generate them. Geostatistical approaches that use knowledge of the underlying process have been proposed to reduce the number of soil samples required. Also, if the scale of variation identified by the variogram can be linked to process, it could be incorporated as a scale factor into existing deterministic models. To attempt to relate the scale and pattern of variation in topsoil properties to processes, the topsoil of four field sites in southern England on different parent materials and with a variety of topography was sampled. Relations between variogram range, topography and parent material were examined. The variogram ranges for parent materials that result in heavier soil textures tended to be longer as were those for plateau and valley areas compared with slopes. Possible links between these patterns and soil processes were investigated by examining: 1) changes in soil characteristics with depth, 2) moving correlations, 3) variation in soil properties with topography within fields, 4) directional variograms and 5) derived topographic attributes. The predominance of vertical rather than lateral movements of water through the soil in plateau and valley positions was identified as responsible for the longer range of variograms in these locations. Hydraulic conductivity was suggested as a possible underlying cause of differences in the scale of variation in the topsoil between parent materials.