Calculation of emission factors (EF) for nitrous oxide (N₂O) is complicated by their large spatial variability. The objective of this study was to test the hypotheses that spatial variation in N₂O emissions can be explained by spatial and temporal variation in soil water-filled pore space (WFPS), among topographic positions that shed or collect water according to topographically-driven water movement. This hypothesis has been incorporated into a detailed processed-based, three-dimensional mathematical model of terrestrial ecosystems, ecosys. We simulated emissions using ecosys at different spatial scales – meter, fetch and field, using a 20 x 20 matrix of 36m x 36m grid cells from a DEM to represent topography of a fertilized agricultural field. Modeled results were compared to measured data from chambers (m² scale spatial variability), and from micrometeorological flux towers equipped with tunable diode lasers (TDL) using flux-gradient technique (tower scale spatial variability).

Coefficients of spatial variation (CSVs) amongst chamber replicates (2 x 3 m grid) during emission events were 28 to 195 %, indicating that spatial variation of N₂O occurs at a very small spatial scale. CSVs of N₂O modeled from the grid cells in which chambers were located were 11 to 49% when uniform soil properties were assumed across the field. This variation was attributed in the model only to spatial variation of WFPS, due to different flow accumulation within the field caused by topographically-driven water movement (~1.8m over 600m). Consequently, EF for 112 kg N ha^-1 was larger in an area of the field with lower topography (1.27%) compared to one with higher (0.73%). These results show the importance of the use of 3-dimensional models such as ecosys at an hourly time-step with input from DEMs, to fully capture large spatial and temporal variability of N₂O at different spatial scales even in seemingly flat (0.2% slope) landscapes.