Engineers Australia

On the use of the Thornthwaite Moisture Index to infer depths of seasonal moisture change

Stephen Fityus and Olivier Buzzi


The use of the climatic parameter, the Thornthwaite Moisture Index (TMI) as a predictor of the depth of seasonal moisture change (Hs) is now widely accepted. Many workers have published studies which include TMI-based maps of Hs for specific regions of Australia. The use of the TMI for this purpose involves many compromises and assumptions which can significantly affect the maps that are produced. These include differences in the methods used to estimate potential evapotranspiration, differences in the quality and quantity of data used as a basis for the map, judgment exercised in making interpolations of the point data and the limited basis for the correlation between TMI and Hs. This paper explores the significance of these factors in regard to their likely effects on Hs prediction maps, and it recommends that published studies include sufficient information so that the background and reliability of maps can be assessed and appreciated. This information includes data on the length of the climatic records used, details of the potential evapotranspiration model used and a map showing the position of the data points upon which interpolated maps are based. It concludes that where such information is not provided, inconsistencies between outcomes/maps produced by different authors may be difficult to resolve.