The magnitude of data (i.e. daily, 1-day, monthly, annual) is important in hydrology and water resources analysis. In hydrology analysis those data were classified based on time base and event base data. The length of data also play important role in hydrology analysis. Based on WMO, 30 years of data is sufficient to quantify the changes in hydrology variables.
In climate change analysis, 30 years of high reliability daily precipitation is needed and for Malaysia condition data range from 1961 to 1990 is selected. Previous study shows that this range of data is good enough to investigate the current and future climate change.
Observed large-scale NCEP (National Centre for Environmental Prediction) reanalysis atmospheric variables for the same time period (1961 to 1990) have been used as predictors. The NCEP/NCAR Reanalysis produced a retroactive 51-year (1948–1998)record of global atmospheric fields derived from a Numerical Weather Prediction model kept unchanged over the analysis period and constrained by observations (Kalnay et al., 1996). NCEP daily global analyses data provided by the NCEP/NCAR internet site http://dss.ucar.edu/pub/reanalysis/.
The GCM simulations used for this study are from Hadley Centre 3rd generation (HadCM3) coupled oceanic-atmospheric general circulation model (Wilby et al., 2001). The Hadley circulation provides a useful framework for understanding the nature of large scale flow, the actual circulation in the tropics involves substantial zonal and regional variations (Manton and Bonell, 1995). This data is available at http:// www. cics. uvic.ca /scenarios / sdsm/ select. cgi. These experiments forced with changes in greenhouse gas (GHG) concentrations alone and those forced with greenhouse gas and sulphate aerosol changes. In terms of precipitation, control runs from HadCM3 transient simulations indicate approximately a 3% increase in global precipitation by the end of the 21st century (IPCC, 2001). The GCM data from 1961 to 2099 were extracted for 30-year time slices. For consistency description the scenarios data will be named as follow; the baseline period, 1961-1990 (current observed), 2010 to 2039 (the 2020s), 2040 to 2069 (the 2050s) and 2070 to 2099 (the 2080s). The justification for this division was based om a substantial change in rainfall. This point also coincide with a change justified by WMO. In climate change analysis, it is important that equal time segments are used for contrast and comparison purposes.
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