Abstract: Abstract: Hydrological processes are the driving force of transportation, transformation, and accumulation of agricultural non-point-source pollutants in a paddy irrigation district. The aim of this study was to simulate hydrological processes and non-point source pollution transport processes in a complex irrigation and drainage system in north-east China. Field experiments were conducted to measure quality and quantity of leakage and irrigation return water in field and drainage system in the Qianguo irrigation district (Jilin, China) during the period from 2009 to 2011. The hydrological processes in the various drains were different. In the field canals, the subsurface flow and leakage with high N concentrations were diluted by irrigation return water. The storage process in the lateral drains impacted the drainage process and the convection and mixing processes of the non-point pollutants significantly. In a lateral irrigation canal controlled region, the subsurface flow and the direct seepage flow from paddy fields through the side walls of field drains to the drains were simulated using unsteady flow equation. The water and contaminants in the field drains converged to lateral drains, and finally reached the main drain. A modified Muskingum method was proposed to calculate the processes of water flow and chemical transport in the drainage system. The results from the field experiments and simulations indicated that the fate of ammonium (NH4+), nitrate (NO3-), and chemical oxygen demand (COD) in the system were primarily controlled by the drainage processes. The NH4+and NO3- transport processes were mainly affected by the surface leakage and the deep leakage process, respectively. Besides from irrigation return water, the COD mass discharged from paddy fields to drains was through both subsurface flow and deep leakage. The results clearly showed the contributions of different drainage processes to non-point source pollution in a complex drainage system during the rice-growing period. Bromide (Br-) was used as tracer to investigate water flow and solution transport in the soil during the freezing and thawing periods. The redistributions of soil water, temperature, and Br- tracer were monitored. In the frozen soil, water movement was caused by the temperature gradient, matric potential gradient and gravity. The matric potential of unfrozen water in frozen soil at an equilibrium state was estimated using a temperature-based function. A linear relationship was observed between solution flux and soil water flux. The SWAT was modified and applied successfully to simulate the drainage and contaminants (NH4+, NO3- and COD) transport processes. The methods and results from this study should be useful to characterize non-point source pollutions in paddy irrigation district of north-east China.