Boochabun, K. Tych, W., Chappell, N.A., and Carling, P.A. 2002. Statistical modelling of rainfall and river flow in Thailand. Paper presented at that Global Continental Paleohydrology (GLOCOPH) 2002 Conference, Pune, India, 2-7 December 2002.



Thailand experiences severe floods and droughts that have been shown to impact on the nation's agriculture. New technologies are being developed to study and forecast these events to form the basis for improved flood and drought alleviation policies and practices. Data-Based-Mechanistic modelling is a new approach, which can be used to study the rainfall and riverflow behaviour.

In this paper we use Dynamic Harmonic Regression (DHR) models to analyze the rainfall and the discharge time series across Thailand to find the evolution of annual and supra-annual cycles, (which reflect the character of seasonality, trends and forecasted rainfall and discharge. The aims of the study are first, to identify changes in rainfall regime and in river flow and their spatial distribution. Secondly, to identify statistical patterns in the frequency of extreme rainfall and flow periods with a view to improving predictions of medium and longer-term rainfall and river flow patterns.

The results indicate the existence of both temporal and spatial variation within the annual rainfall pattern in the 8 study catchments. For example, the seasonality of the rainfall in the South is less pronounced (i.e. more equatorial). The discharge seasonal pattern show stronger semi-annual cycles, with the weakest pattern in the South of country (station X.113), whereas the strongest discharge seasonality is observed in upper North (P.14). The overall areal rainfall trend has not changed significantly over the last 20 years. The identified dry years (1982-1983, 1991, 1997-1998) can be associated with the ENSO events. The discharge trend has also tended to drop in 1982 and 1987 ENSO years.

The DHR forecasts of rainfall and river flow data for 1998-1999 using data up to 1997 have relatively low prediction errors.

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Copyright Dr. Nick Chappell, Lancaster Univsersity 2000. ALL RIGHTS RESERVED.