ROOTZONE PROCESSES AND THE EFFICIENT USE OF IRRIGATION WATER

The need for more-efficient agricultural use of irrigation water arise s out of increased competition for water resources, and the greater pr essure on irrigation practices to be environmentally friendly. In this review for the 25th Jubilee volume of Agricultural Water Management w e focus on three rootzone processes that determine water-use efficienc y in irrigation. Firstly, we discuss the role of macropores in prefere ntially-transporting irrigation water to depth during infiltration und er both sprinkler and flood systems. It is suggested that more-uniform entry of irrigation water into the rootzone will result either by mat ching the sprinkler rate to the soil's matrix hydraulic conductivity, or by modifying the soil-surface's macroporosity prior to flood irriga tion. Secondly, the environmentally-deleterious leaching of chemicals by irrigation is shown to be reduced if the applied fertilizer is firs t washed into dry soil by a small amount of water. This first pulse of water is drawn by capillarity into the soil's microporosity, and it c arries with it the dissolved fertilizer which becomes resident there. These nutrients are then available for plant uptake, yet less prone to susbsequent leaching by heavy rains. Meanwhile, initially-resident so lutes in the dry soil, such as salts, will be more-effectively displac ed by the infiltrating irrigation water. Finally, our time domain refl ectometry (TDR) observations of the changing soil water content in the rootzone of a kiwifruit vine, and our direct measurements of sap flow within individual roots, both reveal that plants can rapidly change t heir spatial pattern of water uptake in response to the application of irrigation water. The prime uptake role of near-surface roots is high lighted. Consideration of all three of these rootzone processes reinfo rces the claim that more-efficient and environmentally-sustainable wat er management will arise through higher-frequency applications of smal ler amounts of irrigation.