Aims
River floodplains are among the most dynamic and diverse ecosystems on the planet and are at risk of degradation due to river regulation and climate change. Environmental water can be delivered to floodplains to maintain the environmental health of vegetation and other floodplain species. It is important to understand the long-term impacts of environmental flows on vegetation to improve environmental water management and restore floodplain ecosystem health.
Methods
This study was undertaken using vegetation data from Hattah Lakes, a semi-arid connected floodplain-lakes system located in north-western Victoria. Thirty-year Landsat satellite data were used to derive Normalized Difference Vegetation Index (NDVI) and modified Normalized Difference Water Index (mNDWI) data to assess vegetation and water condition. Generalized additive mixed models (GAMMs) were used to spatially model vegetation response to environmental water, natural floods and other climate factors.
Results
Vegetation in most areas of Hattah Lakes responds more slowly to environmental water compared to natural floods, where a widespread response is often seen within one month. In some fringing areas, natural floods have positive influences with a longer lag time, from 4 to 12 months. Vegetation in most areas shows a positive response to environmental water 1 to 3 months after the flow event, but a negative response within the first month after inundation. Some vegetation classes respond more slowly to environmental watering, with a lag time of 4 to 12 months.
Conclusions
It is clear that by pumping environmental flows into floodplain ecosystems, we can improve vegetation health, and with far less water than is necessary for a natural flood. However, our results show that the benefits are different, both spatially and temporally, than those of natural floods. As we move to increase the amount of regulated floodplain watering under the Basin Plan, these differences need to be acknowledged.