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Rainfall simulation experiments are widely used to study erosion and contaminant transport in overland flow. We investigated the use of two rainfall simulators designed to rain on 2‐m‐long (2‐m²) and 10.7‐m‐long (32.6‐m²) plots to estimate overland flow and phosphorus (P) transport in comparison with watershed‐scale data. Simulated rainfall (75 mm h⁻¹) generated more overland flow from 2‐m‐long (20 L m²) than from 10.7‐m‐long (10 L m²) plots established in grass, no‐till corn (Zea mays L.), and recently tilled fields, because a relatively greater area of the smaller plots became saturated (>75% of area) during rainfall compared with large plots (<75% area). Although average concentrations of dissolved reactive phosphorus (DRP) in overland flow were greater from 2‐m‐long (0.50 mg L⁻¹) than 10.7‐m‐long (0.35 mg L⁻¹) plots, the relationship between DRP and Mehlich‐3 soil P (as defined by regression …