Large-scale afforestation of arid land aiming at fixation of atmospheric carbon dioxide requires the effective use of a small amount of rainwater by control of runoff water. Thus it is necessary to estimate the quantity and quality of water available place by place in order to determine a suitable site for afforestation, and then to introduce technologies for improvement of soil structure in order to apply the water to tree growth. However, measured figures are limited, especially for arid land. In the present study, the amount of runoff water was measured to evaluate the water balance at a rainfall event in an area with annual precipitation of around 200 mm. The runoff ratio was determined to be 17.5% for a rare precipitation of 29 mm. The present results will be used to examine validity of our originally developed model.