AWRA-G: A groundwater component for a continental scale land surface model

The Australian Water Resources Assessment (AWRA) system of models is currently being developed by CSIRO for BOM under the Water Information Research and Development Alliance (WIRADA) to enable BOM to meet its requirements under the Water Act 2007 for undertaking annual water resource assessments and national water accounts. AWRA consists of several linked component models, namely: AWRA-L, a landscape model; AWRA-R, a river routing model; and, AWRA-G, a groundwater component model. AWRA is a hind-casting model; it can use observations of available elements of the water balance to inform other components for which no data exists. Most continental scale land surface models do not have the capacity to allow water to flow between cells as groundwater and so misrepresent this element of the water balance. The conceptualisation of groundwater within AWRA-L is consistent with other continental scale landscape models in that recharge and discharge occur within the same grid cell. This is likely to be a suitable assumption in upland areas with local groundwater flow systems but is not appropriate for regional systems. The sources of groundwater storage changes that are not accounted for in AWRA-L are from groundwater extraction, losing streams, overbank flooding, flow from adjacent cells, discharge to the ocean and interactions between the surficial aquifer and deep confined systems. Groundwater extraction is accounted for in AWRA-G through the metering or estimation of extraction within a grid cell of AWRA-L. This information is to be collected by BOM as part of their regulations data under the Water Act 2007. Recharge from losing streams is accounted for within the node-link AWRA-R model. The losses from the river are accounted for at a reach scale and then need to be apportioned in space back to the gridded landscape model. Once this recharge is added back into the landscape model it is then available to be transpired by vegetation or extracted via pumping. Recharge from overbank flooding is estimated from open water likelihood mapping derived from MODIS imagery combined with floodplain soil type, aquifer transmissivity and depth to water table. The finer scale estimates of recharge on the MODIS grid (~500 m) are then aggregated to the AWRA-L grid (~5 km) for each time step. Groundwater flow between AWRA-L cells is estimated using Darcy's Law from modelled water table surface derived from AWRA-L, this will be supplemented with groundwater level observations through data assimilation. Groundwater discharge to the ocean is treated similarly. The fluxes from groundwater extraction, losing streams, overbank flooding, lateral flow and ocean discharge are aggregated at the AWRA-L grid cell scale by AWRA-G and then passed to the groundwater storage bucket within AWRA-L for each time step. The AWRA system of models (including AWRA-G) will provide, for the first time, the ability to model the water balance at a continental scale with the accuracy required to report results at a local scale.