This document reports on a CEH (UK) and Bureau of Meteorology (Australia) collaborative project to apply the G2G distributed hydrological model for forecasting water flows to the Great Barrier Reef. It forms a component of the 鈥楥atchment Monitoring and Modelling鈥?stream of the eReefs project concerned with protecting and preserving the Reef environment. The primary purpose here is to quantify the water discharges along the coastline of the Reef. These in turn, through modelled water quality constituent concentrations, can be used to forecast sediment and nutrient loads for input to Coastal Ocean Models and other Marine models of the Reef marine environment. The approach used results in a water forecasting model of the land area draining to the Reef that has utility also for forecasting both water availability and flooding in Queensland.G2G (Grid-to-Grid) is a physical-conceptual grid-based distributed hydrological model used by environment agencies in Britain to support 24/7 flood guidance at countrywide scale out to 5 days. It is also used to produce Hydrological Outlooks months ahead using predictions of seasonal precipitation, to produce spatial flood risk assessments under climate change, and to assess water discharges into the sea through combining gauged and modelled river flows. Its area-wide formulation allows G2G to provide real-time flow forecasts everywhere on a 1km model grid and thus is particularly well suited for application to areas that are ungauged. This is achieved through use of a model formulation that is underpinned by spatial datasets on terrain, land-cover and soil/geology properties. These landscape properties act to shape a storm pattern into a flow response providing a coherent space-time map of river flows from catchment to coast and from flood to drought. Depending on application, G2G can be configured for a range of model domains (catchment, river basin, regional, national, global) and grid-resolutions (from 50 m through 1 km and 25 km have been used).G2G has previously been evaluated in Australia for water forecasting at a range of time-scales, for flood warning and water availability applications, through a pilot case study on the Upper Murray. The present study builds on this experience. The report provides background to the G2G model and its use in the UK. Challenges in setting up G2G using the spatial datasets available for Australia are discussed in some detail as part of configuring the G2G to the land area draining to the Reef.An approach to model setup, calibration and assessment is developed that is tailored to the hydrometric network of the Reef land area and the impact of reservoir controls judged to be significant. The performance of the G2G model is first assessed in simulation-mode, with a focus on how well G2G transforms gridded rainfall and potential evaporation to river flow. The performance of G2G in forecast-mode using full data assimilation (a combination of state-updating, direct flow insertion and error-prediction) of river flows is then assessed. The forecast-mode assessment shows how performance varies with lead-time and the dependence on rainfall forecasts is illustrated using thee alternative input scenarios: perfect foreknowledge of raingauge rainfall, NWP rainfall and zero rainfall. Finally, the G2G model at 1km grid resolution is applied to produce hourly time-series of water discharge volumes along the Reef coastline in a form suitable for use with the eReef marine models.The report closes with a summary of its content, a setting down of the project鈥檚 main achievements and a list of recommendations - both of a research and operational nature. These recommendations focus on guiding the Bureau of Meteorology in its future planning of G2G activity in relation to Great Barrier Reef and Queensland applications.
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