Abstract
This paper describes a simple runoff analysis method by using only spreadsheet software (Excel) that allows counterpart personnel in developing countries to easily predict the amount of runoff to a dam. A case study was done for the Pasak-Jolasid dam basin which has a long and wide area and also has operation problems. A single runoff does not fill the reservoir. It is difficult to separate the runoff by each rainfall due to the long duration of runoff, and therefore period runoff (runoff for some duration) should be taken into account. It was found that the runoff duration is approximately one month. Runoff ratios should be calculated by this monthly runoff depth and the areal rainfall of the previous month. The rainy season and the dry season are clearly separated in the dam basin. There are two peaks of rainfall, one in May and the other one in August/September. Most of the rainfall percolates into dry soil at the start of the rainy season, so the runoff ratios are low. As the soil moisture increases, the runoff ratios increase and become highest in September, after which they decrease.
