Coastal plain soils and geomorphology: a key to understanding forest hydrology.
In the 1950s, Coile published a simple classification of southeastern coastal soils using three characteristics: drainage class, sub-soil depth, and sub-soil texture. These ideas were used by Warren Stuck and Bill Smith to produce a matrix of soils with drainage class as one ordinate and subsoil texture as the second for the South Carolina coastal plain. Soils with sandy clay loam sub-soils (the most widely distributed soils on the coastal plain) were further divided by sub-soil depth into three categories: >40 inches, 20-40 inches, and <20 inches. In 1974 Donald Colquhoun classified geomorphology of the lower SC coastal plain by relationship to seven former marine terraces. Sediments were associated with beach, offshore, or back barrier deposits while river valleys were associated with either fluvial or estuarine deposits. Using GIS, soils in the matrix can be mapped to the geomorphic features revealing a geomorphic explanation for the distribution of soils across the coastal plain. Beach and offshore deposits have sand throughout the soil profile, while back barrier deposits tend to have clay or clay loams. Fluvial terrace deposits have sandy clay loam sub-soils while some estuarine valleys have entirely organic soil profiles. Classification of drainage class is directly related to the average water table depth of soils. Within a single sub-soil type (sands), average water table depth is directly predicted by drainage class. Soil subsurface type also greatly influences drainable porosity (the porosity that is filled or emptied by a small change in water table). Geochemical analysis of flows on sandy subsoil (near Georgetown) and clay sub-soils on Turkey Creek and Watershed 80 (near Charleston) show this difference in drainable porosity and water table fluctuations to be related to the source of storm runoff. Sandy sub-soils have higher drainable porosity, smaller water table fluctuations, and a prevalence of soil water chemistry in runoff. Clay sub-soils have lower drainable porosity, greater water table fluctuation as a response to rainfall and ET, and stream runoff chemistry more similar to that of rainwater.