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 rep lacement of native vegetation with pastures which use less water. Rela tionships between morphological features (mainly soil colour) and fiel d 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 f rom crest to flat in these catchments. Three soil systems were identif ied 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) yel low/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 occurr ed within the subsoil systems. Measured water duration and levels were compared with the presence pr absence of inferred redoximorphic featu res. Most features in the brown/grey topsoil system and the red and ye llow/white subsoil systems reflected differences in water duration. A conceptual model was constructed to match successive changes in hydrol ogy with differences in soil morphology and thus distinguish between m odern and relict (late Mesozoic) soil features that developed under pa st and present hydrological conditions.