Field-Scale Nitrate Simulation

Models are useful tools for integrating different processes involved in nitrogen transport in soil and can be used in forecasting how a system will behave without actually making measurements in the physical system. Lately, computer simulation models have become a useful tool in understanding the transport of water and solutes through the soil into the groundwater. Transport of water and solute in soil under field conditions is a research area of continuous interest. In this study, LEACHN, a sub-module of LEACHM (leaching estimation and chemistry model), was used to simulate nitrate leaching using an existing experimental data set collected at a land treatment site of Carterton District in New Zealand. The LEACHN model was calibrated with the field data collected at the site and then subjected to a sensitivity analysis against several parameters. The calibrated model was applied to the Carterton land treatment site to predict the leachate nitrate concentration in the unsaturated zone (0 to 500 mm depth) in the low, medium, and high irrigation treatments of tree and pasture plots. It was difficult to calibrate the model because of a lack of measured nitrate data in the root zone, nitrate movement below the root zone to the water table was not evaluated, and the need to relate the predicted concentration in the root zone to measured results in the top few meters of the groundwater table. Nitrate-N values predicted with LEACHN were relatively sensitive to certain soil physical and chemical properties, such as bulk density, air entry value (a), the exponent for Campbell's equation (BCAM, b), mineralization rate, base temperature, and Q10, which is the factor change in rate with a 10° change in temperature. The LEACHN application to the tree and pasture plots (control plots) revealed that the cumulative drainage was higher in the pasture plots than in the tree plots. There was a drainage effect on the leachate nitrate-N concentration in all treatments of the tree and pasture plots. The model gave a good estimate of the nitrogen dynamics for different crops (tree and pasture) during different periods of the year.