As a countermeasure against global warming, large scale afforestation of arid land has been performed as a means to fix atmospheric CO(2) into plants. Arid land and semi-arid land account for about one third of all land area and are characterized by extremely low productivity, which is mainly caused by low precipitation. In these areas, a considerable amount of rainwater is lost due to runoff and evaporation, but runoff water is available for tree growth when it is collected and allowed to penetrate into the deep root zone. Therefore, the selection of afforestation area suitable for effective use of rainfall/runoff water for tree growth is required for arid land afforestation. In this study, we constructed an original runoff model to represent water movement in a watershed area of several km by several km in an arid area of Western Australia. For numerical calculation with ten meter grids, the maximum required time difference value was determined to give negligibly small calculation errors. Suitable fitting parameters in our model were determined by comparing the numerical results with actual data of runoff behavior after rainfall. As a result, the water movement in the flow area could be expressed by our model calculation.