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In Okinawa, red soil runoff from developed lands and consequent ocean pollution soil has been a major issue since the 1970s. The Okinawa Prefecture Department of Agriculture must move to introduce some soil runoff prevention measures. For such countermeasures to be properly formulated, we must be able to estimate their efficiency in the early planning stages. To this end, the USLE has been in use but this model has shortcomings in its inability to calculate short-term runoff or runoff on the scale of entire watersheds. The USDA also developed the WEPP model in 1985. In the WEPP model, watersheds are composed of slopes and channels. Changes of vegetation and soil condition on the slopes are calculated to yield seasonal changes in erosion or soil loss. Calculations for the channel include considerations of detachment and deposition in consonance with flow rates. Therefore, the WEPP model can be used to calculate spatial and temporal soil erosion processes. However, there are few examples of applications of the WEPP model in Japan.

Then, WEPP model have been applied to an agricultural watershed in Okinawa and affectivity of the model is checked. In the application, parameters on soil and farming schedules were drawn from observations made in the watershed and the other parameters were defaults shown in the WEPP model. As the results demonstrate, the WEPP model can be used to calculate changes of crop and soil conditions clearly. Calculated soil erosion was almost the same as actual observed parameters in the event scale.