Interpretation of soil features produced by ancient and modern processes in degraded landscapes .7. Water duration

Rising saline groundwater and fresh perched water have increased over 120 years (modern processes) in the Mediterranean region (>600 mm per annum) of the Mt Lofty Ranges, South Australia. This was caused by replacement of native vegetation with pastures which use less water. Relationships between morphological features (mainly soil colour) and field measurements (e.g. watertable duration, salinity, and sodicity) were studied to 3 m at 12 sites down a representative toposequence of red-yellow-grey duplex soils (Palexeralfs-Natraqualfs), which are common from crest to flat in these catchments. Three soil systems were identified from groupings of soil features: (i) brown/grey topsoil system (e.g. A and E horizons), (ii) red subsoil system (e.g. Bt), and (iii) yellow/white subsoil system (e.g. Btng or Cg). A water duration index was developed to quantify water duration in the brown/grey topsoil system from perched water levels measured in 12 dipwells installed to 0.5 m. Nested piezometers were used to determine if unsaturated zones occurred within the subsoil systems. Measured water duration and levels were compared with the presence pr absence of inferred redoximorphic features. Most features in the brown/grey topsoil system and the red and yellow/white subsoil systems reflected differences in water duration. A conceptual model was constructed to match successive changes in hydrology with differences in soil morphology and thus distinguish between modern and relict (late Mesozoic) soil features that developed under past and present hydrological conditions.