The influence of crop management on the water balance of lupin and wheat crops on a layered soil in a Mediterranean climate

The influence of time of sowing and sowing density on evapotranspiration an d drainage loss beneath wheat (Triticum aestivum cv. Spear) and lupin (Lupi nus angustifolius cv. Gungurru) crops grown on a layered soil was investiga ted for three seasons in a Mediterranean climate in Western Australia. The aim of the study was to investigate whether managing crops to maximise thei r canopy growth would increase their water use and minimise groundwater rec harge contributing to dryland salinity. A soil water balance approach was u sed to estimate evapotranspiration, with changes in soil water content meas ured with a neutron water meter. The study was carried out on a layered soi l typical of agricultural soils in the region with variable depth to clay ( 0.22-0.38 m) and a marked contrast in hydraulic properties between the tops oil and subsoil. As a result of the low permeability subsoil, a perched wat er table occurred in the sandy topsoil in each of the three seasons under s tudy during winter when rainfall was high and potential evaporation low. Pe rched water tables persisted for 2-3 months, with hydraulic gradients consi stently downward causing drainage losses to occur. Although crop management had a large influence on shoot and root development, evapotranspiration fr om the different treatments was generally similar. Drainage losses were not influenced by either crop type, time of sowing or sowing density, because potential evaporation and hence evapotranspiration was low during the perio d when drainage losses occurred. The total drainage loss measured in each s eason was different, with losses ranging from 20.1 to 22.2 mm in 1990, from 40.4 to 46.7 mm in 1991 and from 49.4 to 66.6 mm in 1992. The increase in drainage loss from 1990 to 1992 was a result of progressively more seasonal rainfall in 1990, 1991 and 1992. It was concluded that there was little sc ope to increase water use and decrease deep drainage through crop managemen t for sites with climatic conditions where winter rainfall exceeds potentia l evaporation.