Use of modelling to explore the water balance of dryland farming systems in the Murray-Darling Basin, Australia

The Agricultural Production Systems Simulator (APSIM) modelling framework was used to explore components of the water balance for a range of farming systems in the Murray-Darling Basin (MDB) of Australia. Water leaking below the root zone of annual crops and pastures in this region is leading to development of dryland salinity and delivery of salt to waterways. Simulation modelling was used to identify the relative magnitude of transpiration, soil evaporation, runoff and drainage and to explore temporal variability in these terms for selected locations over the 1957-1998 climate record. Two transects were used to explore the impact of climate on water balance, with all other factors held constant, including the soil. An east-west transect at approximately latitude 33°S demonstrates the primary effect of annual average rainfall ranging from 300 to 850 mm. A north-south transect along approximately the 600 mm rainfall isohyet demonstrates a secondary effect of rainfall distribution, with the fraction of annual rainfall received in winter months rising from 40% in the north to 70% in the south. Water excess (i.e. runoff plus drainage) is strongly episodic, with 60% simulated to occur in 25% of years. Longer term cycles are also evident in the time series simulations, with strong below average periods from 1959-1968 and 1979-1988 interspersed with extended periods of above average water excess from 1969-1978 and 1989-1993. Water excess was highest for the annual wheat farming system and lowest for perennial lucerne pasture. Other systems that mix summer and winter annuals (opportunity cropping) or include wheat and lucerne pasture in different temporal combinations (phase farming and companion cropping) were intermediate in their simulated water excess. These differences in water balance of the farming systems simulated were associated with differences in grain and forage yields that will affect their economic viability. The predictions of annual water excess derived from the dynamic, daily time-step modelling using APSIM for a wheat based farming system were of similar magnitude as those predicted by the Zhang et al. (2001) static model for shallow rooted pasture catchments, whilst continuous lucerne was similar to predictions for deep rooted forest catchments. To capture the effect of rainfall distribution between winter and summer an additional term was added to the Zhang model. This modified function captured 88% of the variation in the APSIM predictions of annual average water excess from annual wheat systems for 78 locations in the MDB.