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Context Runoff estimation is an important aspect of pesticide environmental behaviour and is the major loss pathway to the environment. Aims To improve understanding of pesticide runoff. Methods Data from three rainfall simulator studies was used. Twelve pesticides were studied ranged from tightly sorbed (DDE, soil sorption coefficient (K D ) ~15 000Lkg−1 ) to weakly sorbed (dimethoate, K D Key results Event runoff pesticide concentrations were closely related to soil concentrations (0–25mm depth). The ratio of runoff to soil concentration (the runoff extraction ratio, E RO ), was similar for pesticides with a wide range of sorption and across the three soils: runoff concentration (μgL−1 )=28×soil concentration (mgkg−1 ). E RO decreased with time after spraying, presumably due to lower concentrations in the top few mm of soil. Conclusions This model provides improved or similar estimates of pesticide runoff than previous models. Similar E RO values between sites was probably due to similar hydrology (high rainfall intensity, surface sealing, moist subsoils) and erosion, and because the same masses of soil and water are involved in mixing. Reduction in runoff concentrations by leaching was not influential, because infiltration was small and soil sorption too high. Implications Conditions studied apply during summer storms on most cotton and grain land on clay soils in the northern grain and cotton lands in eastern Australia. The model should be applicable under these conditions.

Context Do some pesticides run off more than others? How does pesticide runoff vary with pesticide properties? Aims Improve understanding of pesticide runoff from croplands. Methods Concentrations in surface soil and in runoff from three Australian rainfall simulation studies and three rainfall simulation and five catchment studies in North American croplands were used. The ratio of event averaged runoff concentrations and the surface soil concentrations is the runoff extraction ratio. Key results Pesticide runoff concentrations were closely related to soil surface concentrations at the start of rainfall. Runoff extraction ratios were not significantly different for 13 pesticides with a wide range of properties, on gentle slopes (0–3%), but were significantly lower for three pesticides. On steeper slopes, runoff extraction was significantly greater for atrazine but lower for glyphosate and metolachlor. Low sloping, furrow irrigated fields had low sediment concentrations and low pesticide runoff concentrations for more tightly sorbed pesticides, but not for less sorbed pesticides. Runoff extraction was not significantly different for simulated and most catchment studies. Conclusions Similar runoff extraction ratios were due to similar hydrology and limited sediment concentrations. Different runoff extraction occurs on bare soil if (a) pesticides are leached from the runoff-mixing layer, requiring sorption coefficients less than two and significant infiltration, and no interflow, (b) sediment concentrations are either low (−1 ) or high (>100gL−1 ) and (c) pesticides have different concentration profiles in the runoff-mixing layer. Implications Conditions studied apply for croplands in the North American mid-west on silty soils and for Australian clay soils.