It is important to apply proper dispersivity in unsaturated condition when solute transport near the soil surface is assessed. However data on unsaturated dispersivity are limited because dispersion in unsaturated condition is more complicated than in saturated condition. In the previous study, we tried to evaluate unsaturated-dispersivity by tracer experiment in the laboratory seepage tank. However, it was difficult to investigate clear results because observed values were scattered. In this study, numerical tracer simulation was conducted under unsaturated water flow condition. The numerical tracer simulation results were confirmed that longitudinal and transverse dispersivities have dependency on soil water content. When the water content is lowered, dispersivity becomes larger. The ratio of transverse dispersivity to longitudinal dispersivity was in the range of 1/1000-1/6000. Numerical results showed that longitudinal dispersivity may have almost constant value in the small scale as seepage flow tank used in this study.
