THE CAUSES OF WATERLOGGING IN SHALLOW SOILS AND THEIR DRAINAGE IN SOUTHWESTERN AUSTRALIA

Sandy loam topsoils overlying sandy clay subsoils on hillslopes in southwestern Australia are often waterlogged. The relative importance of soil, landform, climatic and drainage factors on waterlogging intensity was assessed from 1984 to 1986. The SEW(30) index was used to quantify waterlogging intensity, as measured in 162 shallow wells in five subcatchments. Waterlogging is caused by the development of a perched watertable on the subsoil clay. Gravel within the soil profile reduces the porosity and thus the water storage capacity of the soil. Waterlogging intensity is low where topsoils are deeper than about 0.6 m; where the soil profile has an A(2) horizon with high hydraulic conductivity (up to 7.8 m day(-1)) and the site is sloping, facilitating lateral flows. The hydraulic conductivity of the B horizon can change by as much as two orders of magnitude over a 10 m lateral distance. The sites most susceptible to waterlogging have subsoils with a hydraulic conductivity less than 0.003 m day(-1) and topsoils which are less than 0.4 m thick. Susceptible soils became waterlogged in early winter, after rainfall exceeded pan evaporation by just 50 mm whereas others remained well drained even in the wettest years. Q(0) = 2.5 x 10(-3) m(2) day(-1) per m cross-section of the soil is a critical value of transmission capacity that induces severe waterlogging in these catchments.