Abstract
Under the umbrella of GAME-T (GEWEX Asian Monsoon Experiment-Tropics), an on-going project is monitoring rainfall in the Mae Chaem watershed in Thailand. The main purpose of this study is to estimate the temporal and spatial distribution of rainfall in the mountainous area of the upper Chao Phraya River basin, in order to validate rainfall-runoff models. Thirteen rain gauges were installed at altitudes from about 380 to 2, 500m above sea level (a.s.l.) in the Mae Chaem watershed, which has an area of 3, 853km2. Data from these rain gauges allowed us to analyze the rainfall record with a fine time resolution (one second) from June to November 1998. In this watershed, it is clear that the amount of rainfall varies with elevation throughout the rainy season. We used two methods to investigate its effect on orographic rainfall enhancement, and calculated the duration and mean intensity of rainfall using two formulae. The effects of orographic rainfall enhancement vary with the time interval or mean time used for the calculations. It is clear that the amount of rainfall is correlated with the duration of rainfall, regardless of the method of calculation, whereas the amount of rainfall is not correlated with the mean rainfall intensity. In this mountainous area, the change in the amount of rainfall with altitude is correlated with the duration of rainfall and is independent of the mean rainfall intensity.
