ECOHYDROLOGICAL CHANGES IN THE MURRAY-DARLING BASIN .3. A SIMULATION OF REGIONAL HYDROLOGICAL CHANGES

1. Regional scale changes to the hydrological cycle of the Murray-Darling Basin (MDB) in Australia have occurred as a result of European settlement 200 years ago. Replacement of deep-rooted perennial plants (trees) by shallower rooting plants (pastures and cropping) is of particular significance in altering water-tables and causing waterlogging and secondary salinization. 2. The purpose was to locate the areas at risk of waterlogging and salinization as a result of tree clearing. To achieve this, present-day evaporation (ET) from 0.8% of the MDB (7750 km2) is compared with ET from a reconstruction of the pre-European condition. 3. The spatial geographical database for the 155 x 50 km study area consisted of vegetation, soils, climate and topographic information at 1.6 x 1.6 km cell resolution (3072 individual cells for each data layer). 4. Leaf area index (LAI) was used to define the amount of photosynthesizing and transpiring tissue. Present-day LAI was estimated for each cell using the Normalized Difference Vegetation Index calculated from NOAA-9 Advanced Very High Resolution Radiometer data. Pre-European LAI was obtained using the assumption that an equilibrium exists between LAI and climate-soil conditions. Daily climate was interpolated across the study area from Australian Bureau of Meteorology weather stations using topographic information and a microclimate simulator (MTCLIM). 5. A water balance model previously described (Part II) was used to calculate past and present-day ET for each cell assuming that only the tree vegetation had changed. The simulation was confined to a 30-day period in autumn when a uniform senescent ground layer occurs. Maps of water use by trees for pre-European and present-day vegetation were produced. 6. Differences in ET between pre-European and present-day vegetation were displayed spatially across the study area. These areas are interpreted as representing areas at risk from waterlogging and salinization.