The processes of water movement and leaching of nitrates within the soil profile are complex, but important to agricultural production and the environment. The effects of our modern agricultural production technology on nitrate movement within and from watersheds need to be determined. The objective of this study was to develop mathematical models to describe the movement of nitrates in the soil profile and to predict the nitrate losses with tile drainage water;A mathematical model was developed to describe the movements of nitrates in a soil profile based on a zero-order reaction rate of nitrate reduction. This model is capable of predicting the concentration of nitrates at any time within the soil profile from a single time application of fertilizers on the soil surface. A second mathematical model was developed based on a first-order reaction rate of nitrate reduction. The validity of these models has been supported by comparison with experimental data;Also, a computer simulation model of nitrogen transformations and transport in soil was developed to predict nitrate concentrations in tile effluent as a function of farm management practices and climatic conditions. This model is based on a previously developed model and uses daily pan evaporation, and daily precipitation records as inputs. Other inputs to the model include planting and harvest dates, dates of fertilization, and initial moisture content nitrate concentrations in the soil profile. The various outputs from the model are tile flow, nitrate concentration in the effluent, evapotranspiration, and nitrogen uptake by plants;Predicted values of tile flow and concentration of nitrates in the tile effluent compared favorably with the values measured for the years from 1970 to 1978 at Agronomy and Agricultural Engineering Research Center in Boone County, Iowa. A sensitivity analysis of the model parameters related to tile drainage and nitrate concentrations in the tile water was completed. Water content in the unsaturated region and initial nitrate concentrations in the soil profile are the two important factors influencing the effluent concentrations.