Evaluation and comparison of methods for calculating Thornthwaite Moisture Index

The use of Thornthwaite Moisture Index (TMI) as a means of classifying climatic zones and estimating the depth of design soil suction changes is now widely accepted in Australia. Over the last two decades, TMI-based contour maps for various parts of Australia have been published by a number of authors. However, reconciling these TMI maps is often difficult because different assumptions and equations were adopted in the study. There is no doubt that there are discrepancies between the various maps and their associated sets of TMI values due to the adoption of different TMI equations and methods for calculating potential evapotranspiration (PET) as well as differences in the quality, quantity and time span of data used in the computations. In this study, TMI values calculated using various assumptions and methods are compared and evaluated. It is hoped the results and discussion presented in this paper can promote a higher level of understanding of the use of TMI equations and methods and provide useful guidance for future TMI users.