Engineers Australia

The Thornthwaite Moisture Index and seasonal soil movement in Adelaide

Sarah A. Jewell and Peter W. Mitchell

Abstract

In regions with highly expansive clay soil profiles, climate is a critical determinant of the behaviour and magnitude of seasonal surface soil heave. In this paper, the correlation between soil heave and soil moisture availability, as determined using the C.W. Thornthwaite method of estimating potential evapotranspiration, is investigated for two exposed vegetated sites near Adelaide. The study sites were characterised by highly reactive soil profiles, and were influenced by extremely variable moisture conditions due to the semi-arid climate of the Adelaide region. The effects of vegetation on soil moisture availability are demonstrated, with soil shrinkage observed to occur quite rapidly during the late spring when moisture uptake by maturing annual grasses is high. Maintaining vegetation ground cover as mulch was shown to reduce noticeably soil shrinkage during the dry summer months, thus limiting the seasonal variation in soil volume. The study results also provided good experimental justification for assuming a soil moisture capacity of 100 mm of rainfall, as recommended by Thornthwaite. Climate change predictions for Adelaide have been used to estimate future trends in soil moisture availability and Thornthwaite Moisture Index, and indicate that Adelaide’s climate will become increasingly arid. This may have significant implications with respect to footing design requirements in regions characterised by highly expansive soils.