.homogenous area, a 5% difference seems very insignificant. However, if we account for spatial variations in rainfall, the outcome can be dramatically different. The 2008 precipitation in the east was 16% higher than the 2007 value; in contrast, the 2008 precipitation in the south was 10% lower than the 2007 value. In terms of temporal variability, the average annual value hides inter-annual fluctuations and could lead to faulty interpretations about the water situation, especially if the figure is further used in the operation of reservoirs or resource management. For instance, in 1993-2004, the Tab Ma sub-basin in Rayong province had an average annual stream flow of 95 mm3. If the inter-annual variation in stream flow is only 5%-10%, there may be no need to have a secondary or emergency water storage facility because of the very low uncertainty in the amount of water supply (HAII, 2008-a). However, the actual findings show that the inter-annual variability can be as high as 30%, emphasizing the need for water storage as part of the risk management. With climate change, we do not know precisely the magnitude of the changes or their associated impacts, but we can be sure that the uncertainty of spatial and temporal variability will rise.