[1] Although nitrous oxide (N 2 O) emission from agricultural runoff is thought to constitute a globally important source of this greenhouse gas, N 2 O flux from polluted aquatic systems is poorly understood and scarcely reported, especially in low-latitude (0°-30°) regions where rapid agricultural intensification is occurring. We measured N 2 O emissions, dissolved N 2 O concentrations, and factors likely to control rates of N 2 O production in drainage canals receiving agricultural and mixed agricultural/urban inputs from the intensively farmed Yaqui Valley of Sonora, Mexico. Average per-area N 2 O flux in both purely agricultural and mixed urban/agricultural drainage systems (16.5 ng N 2 O-N cm À2 hr À1 ) was high compared to other fresh water fluxes, and extreme values ranged up to 244.6 ng N 2 O-N cm À2 hr À1 . These extremely high N 2 O fluxes occurred during green algae blooms, when organic carbon, nitrogen, and oxygen concentrations were high, and only in canals receiving pig-farm and urban inputs, suggesting an important link between land-use and N 2 O emissions. N 2 O concentrations and fluxes correlated significantly with water column concentrations of nitrate, particulate organic carbon and nitrogen, ammonium, and chlorophyll a, and a multiple linear regression model including ammonium, dissolved organic carbon, and particulate organic carbon was the best predictor of [N 2 O] (r 2 = 52%). Despite high per-area N 2 O fluxes, our estimate of regional N 2 O emission from surface drainage (20,869 kg N 2 O-N yr À1 ; 0.046% of N-fertilizer inputs) was low compared to values predicted by algorithms used in global budgets.