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The effectiveness of growing winter cover crops to mitigate nitrogen (N) leaching can be widely variable, even within a single climatic zone. We sought to investigate key drivers of this variability for the Canterbury Plains of New Zealand. First, through an analysis of local field experiments, we quantified the local variability in the effectiveness of cover crops to reduce N leaching. We then calibrated and applied a biophysical model to isolate the impact of potential drivers of this variability. Crop management (cover crop sowing date), soil water holding capacity (WHC) and inter-annual weather variability (30 years of historical climate) were selected as main factors to be investigated. The analysis of local literature showed that, compared to fallow treatments, winter cover crops reduced N leaching by an average (±95% CI) of 17 ± 8.2 kg N ha −1 . This represented a median N leaching reduction of ∼50% with a wide interquartile range (6–75%). The modelling study showed that the delay in sowing dates consistently reduced the average effectiveness of cover crops, from >80% for March- to <25% for June-sown crops. For any “sowing date by soil WHC” scenario, there was also a large year to year variability. This was caused by the stochastic effect of inter-annual weather variability on the dynamics of crop N demand and soil N supply. For the conditions assumed in the modelling study, a sensitivity analysis of simulated results showed that sowing dates were the main contributor to total variability in the effectiveness of cover crops, followed by weather, factor interactions and soil WHC. These results suggest the need for caution when interpreting data from individual field trials due to the impact of inter-annual variability and interactions among multiple drivers on cover crop effectiveness. In addition, our analysis highlights the value of complementary methodologies, such as biophysical modelling, for extending the inference space of individual field studies. [ABSTRACT FROM AUTHOR]