Assessment of potential drought refuges in the Upper Ovens River based on UAS thermal imagery

The Upper Ovens River is one of the last major rivers in the Murray-Darling Basin which retains a near natural flow regime. It has highly-connected groundwater and surface water resources, supporting groundwater dependent ecosystems with unique ecological values and it is classified as high priority for protection within the North East Catchment Management Authority (NECMA) Waterway Strategy (2013). Although largely unregulated, the Upper Ovens River experiences stress during summer months when flows are at their minimum but demand for surface and groundwater are at their greatest. The projected prolonged durations of warmer weather and increased variability of rainfall and streamflow under climate change may lead to heightened anthropogenic groundwater extraction, hence consumptive demand in dry years has the potential to significantly reduce or stop flow in the Upper Ovens. Given this threat and the uniqueness of the system, the Upper Ovens River Water Supply Protection Area was the first in Victoria to implement a groundwater-surface Water Management Plan (WMP) with the objective to maintain river ecosystem health, whilst maintaining the balance between the various demands for water in the area and ensure the long-term sustainability of water resources. The Plan is the basis for managing water extraction licenses in a way that enables a water regime sustaining in-stream ecological objectives, including the preservation of native fish species such as the Murray cod, Macquaire perch and Trout cod, and recreational fisheries species such as Brown and Rainbow trout. The key environmental goal of the WMP is to reduce the risk to aquatic life during critical low flow periods by restricting water diversions. While the importance of protecting the ecological values of the upper Ovens River under extreme low flows is recognised, there is little understanding of the natural capacity of the system to promote thermally differentiated (cold-water) areas that could act as drought refuges during periods of low flow. The highly-connected groundwater and surface water sources in the Upper Ovens have the potential to provide thermally differentiated areas due to relatively cooler groundwater upwelling. Such areas may provide important ecological refuges for native fish during both drought and extremely hot weather. Therefore, as a first step, a detailed mapping of the thermal variability in the Upper Ovens was key to start comprehending the system’s resilience potential and to both support decision-making for sustainable water use and help meet ecological objectives of NECMA’s WMP. In this project, we tested the possibilities of using novel Unmanned Aerial Systems (UAS)-based Thermal InfraRed (TIR) and optical (RGB) imagery to map and characterize thermally differentiated areas (Cold Water Patches, CWPs), and we used it as a basis for further assessments.

In a first study, we mapped stream temperature and characterised CPWs along 90 km of linear river length in the Ovens through the acquisition, processing and interpretation of UAS-based TIR and RGB imagery. We detected and classified a total of 260 CWPs. Both stream and CWPs temperatures increased but presented considerable variability with downstream distance. CWPs were non-uniformly distributed along the riverscape, with emergent hyporheic water types dominating, followed by deep pools, shading, side channels, and tributaries. We found associations between CWPs and key physical controls including land use, riparian vegetation, longitudinal and lateral CWP location, and CWP area size, illustrating processes acting at multiple spatial scales. With this initial study, we highlighted the major challenges of the use of UAV-based TIR and optical imagery, and provided a basis for future works on the thermal associations with physical controls over a riverscape. In a follow-up study, we developed a semi-automatic supervised approach to classify the key riverscape habitats and associated thermal properties of the Upper Ovens at a pixel-scale accuracy, based on spectral properties. We selected five morphologically representative reaches along the river to (i) illustrate and test our combined classification and thermal heterogeneity assessment method, (ii) assess the changes in CWP numbers and distribution with different metric definitions, and (iii) model how climatic predictions would affect thermal habitat suitability and connectivity of the cold-adapted fish species Rainbow trout. Our method was successfully tested, showing mean thermal differences between shaded and sun-exposed fluvial mesohabitats of up to 0.62 °C. CWP metric definitions substantially changed the number and distance between identified CWPs, and they were strongly dependent on reach morphology. Warmer scenarios illustrated a decrease in suitable fish habitats, but reach-scale morphological complexity helped sustain such habitats. Overall, both studies illustrated several methodological approaches to detect, identify and characterise CWPs, demonstrating the sensitivity of different methods and metric definitions to assess CWPs and thermal heterogeneity in rivers.

Further research is underway focusing on assessing the links between longitudinal thermal variability and different morphological channel types, as well as the links to ecological processes. In conjunction, these studies can help prioritize river restoration measures as effective climate adaptation tools. Regarding specific management approaches related to CWPs, o further management steps have been taken to date, partly given that in the past years very few flow restrictions have been imposed due to regular rainfall events and sufficient flows in the system. Average rainfall over the past couple of years meant the flow rates in the Ovens River did not fall below the trigger level and therefore minimal restrictions on the take of water were implemented in the WSPA. Groundwater monitoring data indicate that groundwater pumping in 2020/21 did not result in observable impact on aquifer-wide groundwater levels. The total volume of licence transfers has continued to be low, reflecting the limited demand for transfer associated with low volume of use. However, the North East CMA have a project on the Upper Ovens which has a focus to revegetate and improve riparian zones with continuous native vegetation. This action has the potential to improve shading of the waterway, buffering stream temperatures in the long term, considering that the removal of exotic willow removal may have the opposite effect in the short-term.

NECMA’s website:
www.necma.vic.gov.au/
The management of the Upper Ovens Water Supply Protection Area (WSPA) and a copy of the WMP can be found here:
www.g-mwater.com.au/water-resources/ground-water/management/upperovenswspa
Publications:
Kuhn J, Casas-Mulet R, , Pander J,Geist J.2021.Assessing stream cold-water patches fromUAV-based imagery: a matter of classification method and metrics.Remote sensing. www.mdpi.com/2072-4292/13/7/1379
Casas-Mulet R, Pander J, Ryu D, Stewardson MJ, Geist J. 2020. Unmanned Aerial Vehicle (UAV)-based ThermalInfra-Red (TIR) and optical imagery reveal multi-spatial scale controls of cold-water areas over a groundwater-dominated riverscape.Front. Environ. Sci.-FreshwaterScience. www.frontiersin.org/articles/10.3389/fenvs.2020.00064/full