Application of a physically based soil erosion model, GUEST, in the absence of data on runoff rates - II. Four case studies from China, Malaysia, andThailand

Runoff rates were estimated from rainfall rates and runoff amounts for 4 ex perimental sites in China, Malaysia, and Thailand before a physically based erosion model GUEST was used to determine the soil erodibility parameter a nd evaluate the potential to use the erosion model to predict the amount of soil loss on an event basis. We also examined 3 different ways of determin ing the soil erodibility parameter for the same storm event using: (i) hydr ographs estimated from rainfall intensities and runoff amounts; (ii) an eff ective runoff rate calculated from the hydrograph; (iii) an estimate of the effective runoff rate based on a scaling technique involving the peak rain fall intensity and the gross runoff coefficient. All 3 methods can produce consistent soil erodibility parameters for a given runoff event. The calcul ated soil erodibility for individual storm events for all sites shows consi derable temporal variation and for most sites a decreasing trend over time, as observed elsewhere in the same region. Among the 4 soils examined, the average soil erodibility tends to decrease as the ratio of coarse to fine m aterials decreases. When the erosion model GUEST is used to predict event s oil loss using estimated soil erodibility parameters, an average model effi ciency of 0.68 is achieved for the sites tested.