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Irrigation practice has increased considerably recently and will continue to increase to feed a growing population and provide better life standards worldwide. Numerous studies deal with the hydrological impacts of irrigation, but little is known about the temporal evolution of the affected variables. This work assesses the effects on a gully after irrigation was implemented in its hydrological basin (7.38 km2). Flow, electrical conductivity, nitrate concentration and exported loads of salts and nitrates were recorded in Lerma gully (Zaragoza, Spain) for eight hydrological years (2004–2011), covering the periods before, during and after implementation of irrigation. Non-parametric statistical analysis was applied to understand relationships and trends. The results showed the correlation of irrigation with flow and the load of salts and nitrates exported, although no significant relationship with precipitation was detected. The implementation of irrigation introduced annual trends in flow (3.2 L s-1, +23%), salinity (–0.38 mS cm-1, –9%), and nitrate concentration (5.4 mg L-1, +8%) in the gully. In addition, the annual loads of contaminants exported increased (salts and nitrates, 27.3 Mg km-2year-1, +19%, and 263 kg NO3–-N km-2year-1, +27%, respectively). The trends presented a strong seasonal pattern, with higher and more significant trends for the irrigation season. The changes observed were different from those of larger irrigation districts or regional basins, due to the differences in land use and irrigation management. It is important to understand these changes in order to achieve an adequate management of the environment and water resources. EditorZ.W. Kundzewicz CitationMerchán, D., Causapé, J., and Abrahão, R., 2013. Impact of irrigation implementation on hydrology and water quality in a small agricultural basin in Spain.Hydrological Sciences Journal, 58 (7), 1400–1413. [ABSTRACT FROM AUTHOR]