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The basic principle in this model is that the main determinants of crop production in
dryland cropping systems are water availability and water use efficiency (Rodriguez
and Sadras, 2007 ). Figure 1 shows this concept as a single boundary line for the high-
est value of WUE observed from a range of sites across four continents.
In the dryland cropping systems of Australia, water availability is limited by low
and highly-variable rainfall (Nicholls 1986 ; Stephens and Lyons 1998 ; Potgieter
et al. 2002 ), often in combination with soils where plant-available water is restricted
by physical and chemical constraints to root growth and function (Rengasamy 2002 ;
Nuttall et al. 2003 ; Sadras et al. 2003 ; Rodriguez et al. 2006 ). The influence of
latitudinal gradients in vapor pressure deficit and fraction of diffuse radiation on
WUE were later recognized by Rodriguez and Sadras ( 2007 ) while changes in non-
productive evaporative losses in the cropping cycle as a function of latitudinal gra-
dients in the size and structure of rainfall were described by Sadras and Rodriguez
( 2007 ). Highlights from that work showed that the size and frequency of rainfall
events vary significantly across eastern Australia with important implications for
resource availability i.e. water and nitrogen, and for matching crop traits to the envi-
Fig. 1 Scatter plot of grain yield and seasonal evaporation in four mega-environments. The line
uses the French and Schultz ( 1984 ) frontier concept, with x-intercept = 60 mm and boundary line
= 22 kg grain/ha.mm (Source: Sadras and Angus 2006 )
D. Rodriguez et al.