The study of hydro-climatic cycles in Southeast Asia is carried out by analysing time series of rainfall, streamflow, evapotranspiration and net radiation from three representative regions: Northern Thailand, Peninsular Malaysia and Borneo. The method used is a data-based mechanistic tool called the Unobserved Component – Dynamic Harmonic Regression (UC-DHR) model, used here to identify both the diurnal and inter-annual variability patterns of existing plot studies data sets.