Abstract: Rainfall-runoff processes are important driving factors of nitrogen loss in cropland. During the processes, dissolvable nitrogen in the topsoil of cropland gets dissolved and migrates into surface runoff. As a main way of dissolved nitrogen exporting from cropland, nitrogen dissolution from topsoil needs to be precisely estimated, which may help improve and perfect the estimation of non-point source nitrogen pollution load. Based on the concept of scouring and oozing coefficient in the calculation method of dissolved pollutant loads in the large scaled models, an approach to estimating nitrogen dissolution in plain river network areas was developed. A plot of dryland field typical of the plain river network area of Dafeng City in the north of Jiangsu Province was selected for test run of the proposed approach. The scouring and oozing coefficient with respect to topsoil nitrogen dissolution during a complete rainfall event was determined through real-time observation and modeling, and comparison was done with previous studies. The proposed approach is found to be a useful supplement for modeling and experimental studies on estimation of nitrogen release from topsoil of the dryland fields in the plain river network areas.