Abstract: Supplementary irrigation is an important approach for increasing crop yield in dryland agriculture.However,its effects on the crop yield,crop water use efficiency and soil water consumption are different in previous researches.In this study,the effects of supplementary irrigation on the yield,population,water use efficiency and root distribution of winter wheat and the soil moisture content were investigated.The field experiments were conducted in continues two years at Changwu Research Station in the Loess Plateau,China.The experiments included four different irrigation treatments: no irrigation (W0),30 mm irrigation at jointing stage (W1),30 mm irrigation at booting stage (W2),and 30 mm irrigation at jointing stage +30 mm irrigation at booting stage (W3).The results showed that the hsupplementary irrigation could be used to increase wheat yield,but not water use efficiency.The wheat yield could be increased more under the irrigation treatments of W1 and W3 than that under the W0 and W2.The proportions of water consumption during the jointing and flowering stages were 42% and 29% of the total water consumption during the whole growth period of winter wheat in the continue two years,and the supplemental irrigation increased the proportion of water consumption during the jointing-flowering stage in normal flow year,reduced the tillering polarization,and increased the population of winter wheat during its flowering stage.Supplementary irrigation affected significantly the root distribution by increasing shallow roots and decreasing deep roots.Compared with W0,the root densities at 1-2 m depth under the irrigation treatments of W1,W2 and W3 were decreased by 12%,7% and 16% in normal flow year,and by 31%,16% and 39% in high flow years.As a results,the supplementary irrigation failed to make the full use of deep soil water storage.In addition,the effect of supplementary irrigation on yield and water use efficiency of winter wheat was also significantly affected by rainfall during the growth period.To make the full use of deep soil water storage by deep roots is key approach in increasing crop yield and water use efficiency in dryland agriculture.The results of this study showed that the supplemental irrigation did not regulate the deep roots and deep water consumption,which brought about a yield increased without increasing water use efficiency.The result suggested that supplementary irrigation should be applied in the areas with less rainfall for increasing the crop water use efficiency,and how to regulate the deep root distribution with supplementary irrigation should be considered in the future research.

Key words: supplemental irrigation, winter wheat, root, water use efficiency, yield, water consumption, semiarid area