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-Darl ing Basin (MDB) in Australia have occurred as a result of European set tlement 200 years ago. Replacement of deep-rooted perennial plants (tr ees) by shallower rooting plants (pastures and cropping) is of particu lar significance in altering water-tables and causing waterlogging and secondary salinization. 2. The purpose was to locate the areas at ris k of waterlogging and salinization as a result of tree clearing. To ac hieve this, present-day evaporation (ET) from 0.8% of the MDB (7750 km 2) is compared with ET from a reconstruction of the pre-European condi tion. 3. The spatial geographical database for the 155 x 50 km study a rea consisted of vegetation, soils, climate and topographic informatio n 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 pho tosynthesizing and transpiring tissue. Present-day LAI was estimated f or each cell using the Normalized Difference Vegetation Index calculat ed from NOAA-9 Advanced Very High Resolution Radiometer data. Pre-Euro pean LAI was obtained using the assumption that an equilibrium exists between LAI and climate-soil conditions. Daily climate was interpolate d across the study area from Australian Bureau of Meteorology weather stations using topographic information and a microclimate simulator (M TCLIM). 5. A water balance model previously described (Part II) was us ed to calculate past and present-day ET for each cell assuming that on ly the tree vegetation had changed. The simulation was confined to a 3 0-day period in autumn when a uniform senescent ground layer occurs. M aps of water use by trees for pre-European and present-day vegetation were produced. 6. Differences in ET between pre-European and present-d ay vegetation were displayed spatially across the study area. These ar eas are interpreted as representing areas at risk from waterlogging an d salinization.