Identifying and measuring the ecological impacts of water extraction from free-flowing river systems in altered landscapes is challenging as multiple stressors (e.g. flow regime alteration, increased sedimentation) may have simultaneous effects and attributing causality is problematic. Furthermore, patterns of water extraction (e.g. pumping timing and rates) may govern the characteristics of hydrologic impacts, further complicating ecological responses. Understanding the effects of flow regime alteration is, however, critical to water resource management, especially implementation of environmental flows.

We examined the impacts of land and water use in the upper Ringarooma River catchment in Tasmania (Australia), which contains intensively-irrigated agriculture, to support water planning. Temporal trends in river condition were assessed using a 19-year dataset of macroinvertebrate community monitoring. Physico-chemistry, benthic algae and macroinvertebrates were sampled at 19 sites before and after a dry irrigation season in 2012/13, while spatial datasets (e.g. land and water use, geology, etc.) and habitat measurements were used to derive a suite of environmental predictor variables for study sites. In addition, the response of macroinvertebrate communities to diel flow variability was investigated. Univariate and multivariate analyses indicated that water extraction-related stressors exacerbated impairment (e.g. reduced macroinvertebrate density) associated with agricultural land use. Furthermore, increased diel flow variability relating to water extraction appeared to contribute to the degradation. The findings of this study are assisting the implementation of a water management plan in the Ringarooma catchment and the analytical approach may provide a framework for assessing water use impacts on rivers at regional scales.