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

The Overseer model is widely used in New Zealand for estimating nitrate (NO₃⁻) leaching losses in agricultural systems. This study evaluated the accuracy of the Over-seer model in simulating nitrate (NO₃⁻) leaching through a two-year lysimeter experi-ment conducted at Woodhaven Gardens, New Zealand, under beetroot and pak choi cultivation. Seven distinct nitrogen (N) fertiliser 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%), due to underestimated crop uptake. Similarly, overestimations were observed in Year 2, with overprediction rates reaching up to 63.5%. Sensitivity analysis highlighted soil texture, impeded layer depth and crop residue incorporation as key drivers of leaching variability, underscoring the need for improved model cali-bration. 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 simulating NO₃⁻ leaching under vegetable cropping systems but exhibits notable limi-tations in handling crop-specific N dynamics, soil hydrology, and fertiliser timing.