A three-dimensional finite element model was developed to calculate the rain-fog convection and sedimentation in valley during the flood discharge. By the proper solution of the concentration gradient vector, the nonnegative type and monotonic changes of the concentration is maintained during the transportation. With the concept of raindrop size distribution and settlement velocity, the formulas was derived for the quantitative conversion between the rainfall intensity and water concentration, which is available to couple the boundary with other inner field numerical models. The distribution of rainfall intensity from the cavity discharge was simulated under the different wind fields in Pubugou hydropower station, the calculated results accord with the real physical mechanism well.
