SOIL EROSION ANALYSIS BY WEPP MODEL AT SUGARCANE FIELDS IN OKINAWA

Abstract

 In Okinawa, Japan, soil erosion at sugarcane fields is a serious problem. Turbid water due to soil erosion flows into the coastal area has and cause a great negative impact on coral and other aquatic ecosystems. In this study, we analyzed the sediment discharge from sugarcane fields in Okinawa by using the Water Erosion Prediction Project, WEPP to improve the prediction and the control of the soil erosion. First, the accuracy of WEPP calculation was verified under several conditions. In each calculation, the soil erodibility was represented by the coefficient which reflects the soil properties. From the comparison between the field study and WEPP calculations, the high accuracy of coefficients which are identified from laboratory experiments was verified. Next, some virtual fields were set up for different sugarcane cultivation methods. The results for each cultivation methods showed that the amount of sediment discharge was large in the order of summer planting, spring planting, and ratooning. The results also indicate the period when soil erosion is likely to occur and the events which cause significant soil erosion. Moreover, we analyzed sediment discharge from virtual fields with different topographical conditions for 4 soils in Okinawa. The results showed that the greater sediment discharge was resulted in the longer and higher slope condition. Futhermore, the impact of different topographys to the sediment discharge amount differed depending on the soil types.