Murray-Darling Basin Sustainable Yields Project. SIMHYD Daily Grid Cell and Subcatchment Runoff - Future Global Warming Scenarios

The lumped conceptual rainfall-runoff model, SIMHYD, with a Muskingum routing method is used to estimate daily runoff for 0.05° x 0.05° grid cells (~5 km x 5km) across the entire MDB. The adopted rainfall-runoff modelling method provides a consistent basis (that is automated and reproducible) for modelling historical runoff across the Murray-Darling Basin (MDB) and for assessing the potential impacts of climate change and development on future runoff.\n\nThe future climate scenario (Scenario C) is used to assess the range of likely climate conditions around the year 2030. Forty-five future climate variants, each with 112 years of daily climate sequences, are used. The future climate variants come from scaling the 1895 to 2006 climate data to represent ~2030 climate, based on analyses of 15 global climate models (GCMs) and three global warming scenarios.\n\nAs the future climate series (Scenario C) is obtained by scaling the historical daily climate series from 1895 to 2006 (Scenario A), the daily climate series for Scenarios A and C have the same length of data (112 years) and the same sequence of daily climate. Scenario C is therefore not a forecast climate at 2030, but a 112-year daily climate series based on 1895 to 2006 data adjusted to match projected global temperatures at ~2030 relative to ~1990.\nLineage: For all the gauged catchments within the MDB, SIMHYD was calibrated against observed daily streamflow data from 1975 to 2006. The calibrated parameters were used to simulate one set of 112 years of daily historical runoff for all grid cells within MDB (Scenario 1A), 100 replicates of runoff assuming that the last 10 year drought will last for 112 years (Scenario B) and 45 sets of future runoff (Scenario C, downscaled rainfall from 15 GCM’s and three global warming scenarios). The runoff for all non-calibration or ungauged 0.05° grid cells was modelled using optimised parameter values from the geographically closest 0.05° grid cell from a calibration catchment. The daily cell runoff for all cells within a subcatchment was aggregated to estimate the subcatchment daily runoff, the annual average was then calculated.\n\nIt is possible that in data-rich areas, specific calibration of SIMHYD or more complex rainfall-runoff models based on expert judgement and local knowledge as carried out by some agencies, would lead to better model calibration (and runoff estimates) for the specific modelling objectives of the area.