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Maintaining the integrity of riparian ecosystems whilst continuing to reserve and extract water for other purposes necessitates a greater understanding of relationships between riparian vegetation and water availability. The Normalised Difference Vegetation Index (NDVI) is a good indicator for identifying long-term changes in vegetated areas and their condition. In this study, we use regression tree analysis to investigate long term NDVI data (23 years) at semi-arid riparian areas in the Namoi catchment, Australia. Climatic factors (temperature and rainfall), surface water (flow and flooding) and groundwater levels are analysed collectively. We find that in general maximum temperature is the variable that primarily splits NDVI values, followed by antecedent 28-day rainfall and then inter-flood dry period and groundwater levels. More rain is required in the warmer months compared to cooler months to achieve similar mean NDVI values in tree patches or areas of high NDVI in riparian zones, presumably because of higher evaporation. Inter-flood dry period is shown to be important for maintenance of NDVI levels, particularly when rainfall is limited. Shallower groundwater levels sustain the NDVI and hence vegetation greenness when conditions are cooler and wetter. (C) 2014 Elsevier Ltd. All rights reserved.