Thornthwaite Moisture Index (TMI) was introduced in 1948 as an index to classify the climate condition of different areas. Since then it has been widely used in many fields such as agricultural and environmental geotechnics. The Australian residential footing design procedure is associated with TMI in order to relate the effect of climate condition on the variation of soil moisture profile.
This paper illustrates the different methods commonly used to calculate TMI and compares their different outcomes. The different calculation methods result in different TMI values for the same climate condition which consequently affects the subsequent correlations. The sensitivity of the primary climate data used to calculate TMI is also discussed in this paper. TMI is an index highly dependent on the rainfall variation and is less sensitive to temperature variations and so the annual TMI can be reliably related to annual rainfall. The change in TMI with change of number of years which the average rainfall is based on has also been presented in this paper. The higher the numbers of years used to obtain the average TMI, the less reliable it is to describe the effects of extreme climate events.