Development of a Thornthwaite Moisture Index Map for Trinidad and Tobago

The Thornthwaite Moisture Index (TMI) has been traditionally used as a climatic index parameter to estimate annual moisture status of soils (deficit/surplus), primarily in the agricultural domain. Engineers have also adopted the TMI in efforts to estimate volume change potential in expansive clays, where seasonal (temporal) moisture changes can be correlated to soil matric suction and ultimately volume change via appropriate mechanistic models. In Trinidad and Tobago, approximately 60 % of the islands are covered with over-consolidated clays of medium to high plasticity. When combined with extreme variations in moisture status, these plastic clays have exhibited high volume change potential. This paper investigates the spatial distribution of this climatic index for Trinidad and Tobago, intending to develop an index map for the islands. Within the post-colonial era in Trinidad and Tobago (1962 ~ present), the availability of consistent climatic data is limited to just two recording stations within the islands. The Meteorological Services of Trinidad and Tobago (MET) manages both stations: Piarco, Trinidad and Crown Point, Tobago, where consistent data exists for 36 years (1981 ~ 2018). These two points and their limited data timeframe cannot support the development of a spatial TMI map for the islands. This research addresses this shortcoming by collecting and analysing historical climatic data collected at 28 stations over Trinidad and Tobago over the British Colonial era (1931 ~ 1964). These data are recorded in publications of the Land Capability Surveys of The Imperial College of Tropical Agriculture (ICTA). Data sets of monthly rainfall data from the historical and at present for the Piarco location showed strong statistical coherence, examined through a t-test. Having justified the historical data, TMI values are calculated at all locations. Using the calculated TMI values, a map was developed using the GIS software, Surfer, and interpolation method of Kriging. In Trinidad, the map indicates high TMI at the north-eastern side of the island, with a significant decrease going into the south-western side. Low TMI values are observed in the most western side of Trinidad indicating substantially long dry season period, during which the underlying expansive clay can experience significant shrinkage.