www.publish.csiro.au/journals/ajar Australian Journal of Agricultural Research, 2007, 58, 303–315 Impact of subsoil water use on wheat yield

Abstract. Water stored deep in the soil profile is generally considered valuable to crop yield because it becomes available during grain filling, but the value of subsoil water for grain yield has not been isolated and quantified in the field. We used rainout shelters with irrigation to control the water supply to wheat crops that had different amounts of subsoil water available to isolate and quantify the efficiency with which the subsoil water was converted to grain yield. Under moderate post-anthesis stress, 10.5mm of additional subsoil water used in the 1.35–1.85m layer after anthesis increased grain yield by 0.62 t/ha, representing an efficiency of 59 kg/ha.mm. The additional yield resulted from a period of higher assimilation 12–27 days after anthesis and was related to an increase in grain size rather than other yield components. Under more severe stress with earlier onset, extra water use below 1.25m was accompanied by additional water use in upper soil layers and it was more difficult to isolate and quantify the benefit of deep water to grain yield. The additional water used from all layers from the time the stress was imposed was converted to grain at 30–40 kg/ha.mm, but this increased to 60 kg/ha.mm for water used after anthesis. The high efficiency for subsoil water use is 3 times that typically expected for total seasonal water use, and twice that previously estimated for total post-anthesis water use in a similar environment. The results demonstrate that relatively small amounts of subsoil water can be highly valuable to grain yield