Quantifying water use of various water consumers is an essential part of sustainable water management. Annual evapotranspirartion (ET) of plantation forests often exceeds that of dryland agriculture, which in South Africa and South Australia has resulted in restrictions on plantation development. In the latter case, water licenses are issued to commercial forestry plantations to account for higher ET compared to dryland pasture. Unlike irrigated crops, it is not practicable to measure water use of plantations directly and so a set of ‘deemed’ average water use rates has been applied based on species and depth to groundwater. Since the ‘deemed’ rates were calculated in 2013, additional plot-scale measurements of annual ET from plantations < 2 years old and post-canopy closure have been used to quantify various components of evapotranspiration (ET). This has enabled development of two empirical ET models for plantations in the region, and facilitated an advanced understanding of the effect of plantations on hydrological processes, particularly in relation to groundwater use. In this study, we applied these models to estimate rotation-averaged annual ET and net groundwater impacts (net groundwater extraction plus recharge reduction compared to pasture) of plantations, driven by climate and groundwater depth, for comparison with the deemed rates. The modelling suggests that the groundwater impacts of plantations vary in space and time and that the deemed rates over-estimate these impacts, on average. Accounting for variation in the effects of climate on the various components of ET, both spatially and temporally, may allow for more flexible rules for water resource allocation than using the current rule-of-thumb approach.

Management of water, regionally, nationally and globally will continue to be a priority and complex undertaking. In riverine systems, biotic components like flora and fauna, play critical roles in filtering water so it is available for human use and consumption. Preservation of ecosystems and associated ecosystem functions is therefore vital. In highly regulated large river basins, natural ecosystems are often supported through provision of environmental flows. Flow delivery, however, should be underpinned by rigorous monitoring to identify and prioritise biotic water requirements. Broadscale monitoring solutions are thus integral and for woody tree vegetation species, this is can be via measurement of field evapotranspiration, regionally scaled using remote sensing. However, as there is generally a mismatch between field data collection area and remote sensing pixel size, new methods are required to proportion tree evapotranspiration based on tree fractional canopy area per pixel. Within, we present a novel method to derive tree fractional canopy cover (FTCC) at 20 m resolution, in semi-arid and arid floodplain areas. The method employs LiDAR as a canopy area field measurement proxy (10 m resolution). Sentinel-1 and Sentinel-2, radar and multispectral imagery, were used in Random forest analysis, undertaken to develop a predictive FTCC model trained using LiDAR for two regions in the Murray-Darling Basin. A predictor model, combing the results of both regions, was able to explain between 85-91% of FTCC variation when compared to LiDAR FTCC, output in 10% increments. Development of this method underpins the advancement of woody vegetation monitoring to inform environmental flow management in the Murray-Darling Basin. The method and fine scale outputs will also be of value to other catchment management concerns such as altered catchment water yields related to bushfires and as such, has application to water management worldwide.