Analysis of water- and nitrogen-use efficiency of wheat in a Mediterraneanclimate

Water-use efficiency (WUE [g grain yield m(-2) mm(-1) ET]) and nitrogen-use efficiency (NUE [Delta g grain yield g(-1) N applied]) are important measu res that can affect the productivity of crops in different environmental sy stems. However, measurement and interpretation of WUE and NUE in the field are often hampered by the high degree of complexity of these systems due to season-to-season variability in rainfall, the variation in crop responses to soil types and to agronomic management. To be able to guide agronomic pr actice, experimentally-derived measurements of WUE and NUE need to be extra polated across time and space through appropriate modelling. To illustrate this approach, the Agricultural Production Systems Simulator (APSIM), which has been rigorously tested for wheat (Triticum aestivum L.) in a Mediterra nean environment, was used to estimate and analyse the WUE and NUE of wheat crops in the Mediterranean-climatic region of the central Western Australi an agricultural zone. The APSIM model was run for three locations (average annual rainfall of 461 mm [high rainfall zone], 386 mm [medium] and 310 mm [low]) and two soil types that had contrasting plant-available water-holdin g capacities in the rooting zone (sand: 55 mm, clay soil: 109 mm). Simulati ons were carried out with historical weather records (82-87 years) assuming current crop management and cultivars. The modelling analyses highlighted the inherently high degree of seasonal variability in yield, WUE and NUE of wheat, depending on soil type, N fertiliser input, rainfall amount and, in particular, rainfall distribution. The clay soil tended to be more product ive in terms of grain yield, WUE and NUE in the high and medium rainfall zo nes, but less productive in most years in the low rainfall zone. The sandy soil was less productive in the high rainfall zone due to the high nitrate leaching potential of this soil type, but more productive than the clay in the low rainfall zone due to poorer pre-anthesis growth and less water use, less water loss by soil evaporation and relatively more water use in the p ost-anthesis phase. When a wheat crop was sown early on clay soil in the lo w rainfall zone, it yielded as high as in the other rainfall zones in seaso ns when rainfall was above average or there was a good store of water in th e soil prior to sowing. The simulations confirmed findings from a limited n umber of field experiments and extended these findings both qualitatively a nd quantitatively across soil types, rainfall regions and crop management o ptions. Furthermore, by using long-term historical weather records, the sim ulations extended the findings across the wide range of climatic scenarios experienced in mediterranean-climatic regions.