Assessing long term climate change impacts on environmental flow compliance and shortfalls in the Yarra River

Climate change has the potential to cause significant management challenges with overall reduction in rainfall and increases in temperature and rainfall intensities. Managers have the responsibility to develop long term environmental watering plans to maintain and preserve river condition. One method to assess condition is through analysing compliance with environmental flow recommendations and shortfalls to achieving full compliance. To investigate climate change implication, DELWP guidelines (Approach 1) apply scaling factors for several climate change outlooks. However, this approach does not consider changes in the variability and intensity of rainfall and runoff events. Another approach that stochastically generates rainfall and can develop multiple possible flow scenarios offers an alternative method (Approach 2). In this paper we demonstrate these two methods and its impact on compliance and shortfalls in the Yarra River. Results suggests that compliance with flow recommendations using Approach 1 will decline in the long term. Compliance with freshes events is poorer, but the current Yarra River Environmental Entitlement (EE) of 17 GL is generally sufficient to meet the priority flow recommendations. Under the longer climate change outlooks, compliance decreases and shortfall volumes increase. Using Approach 2, compliance also reduces as the impact of climate change increase, but the extent of non-compliance is less and shortfall volumes smaller. The difference in outcomes between the two approaches is based on the methods and assumptions used to derive the future flow time series. Overall, the assessment reveals the difficulty in predicting climate change impacts on streamflows and consequent complexity in making decisions. There still remains uncertainty associated with future climate change outlooks. However, by continuing to use and refine new methodologies, tools and the best science available, a better understanding of the long term ecological condition can be obtained, thus making more informed decisions.