Validation of the Overseer Cropping Model for Estimating Nitrate Leaching Losses in Precision Agriculture

The Overseer model is widely used in New Zealand as a precision-agriculture-related tool for estimating nitrate (NO3−) leaching losses in agricultural systems. This study evaluated the accuracy of the Overseer model in predicting nitrate (NO3−) leaching through a two-year lysimeter experiment conducted at Woodhaven Gardens, New Zealand, under beetroot and pak choi cultivation. Seven distinct nitrogen (N) fertilizer treatments were applied to assess model performance. In year 1, Overseer overestimated NO3− leaching by an average of 45.2 kg N/ha (15.7%), and in year 2, the model overestimated by 35.2 kg N/ha (43.5%). A sensitivity analysis highlighted soil texture, impeded layer depth and crop residue incorporation as key drivers of leaching variability, underscoring the need for improved model calibration. Overseer performed reasonably well under lysimeter conditions, with a strong linear relationship (Pearson’s correlation coefficient r = 0.89, p < 0.0001) between measured and predicted values and explaining 77% of the variance (R2 = 0.77) in the observed data. The model predicted a baseline leaching loss of 39.4 kg N/ha/year even when measured losses were zero. Overseer demonstrates moderate reliability in predicting NO3− leaching under vegetable cropping systems but exhibits notable limitations in handling crop-specific N dynamics, soil hydrology, and fertilizer timing.