The cali-bration tests indicated that the Christiansen uniformity (Christiansen, 1941) of the rainfall intensities over the test plot was 81e89%. The nozzles were installed at a height of 7.0 m along the centerline of the tested plot (Fig. 1). The drops could reach a vertical distance of at least 0.5 m above the nozzle before starting to fall as rain, so that the raindrops fell on the soil surface from a vertical distance of at least 7.5 m. They had kinetic energy of at least 95% of natural rainfall (Morgan, 1995). In this study, three rainfall in-tensities of 35 mm/h, 65 mm/h and 95 mm/h were selected with a duration of 1 h for a 2e200 yr return period in Northern Thailand.