The effect of changing patterns in soil-moisture availability on grapevine root distribution, and viticultural implications for converting full-cover irrigation into a point-source irrigation system

Improvements in the efficiency of water use in Australian vineyards have included a move away from diffuse or 'total cover' irrigation techniques to drip irrigation systems. This change in irrigation method has a significant effect on moisture distribution within the soil profile, with implications for root growth, root-system architecture, and soil-water acquisition. Those issues are addressed in this paper. Our report is based on grapevines that had been established under a diffuse irrigation system (overhead sprinklers or microjets) and subsequently converted to drippers. Results are also presented to show the influence of soils with different water holding capacities on root distribution within a single vineyard. Variations in soil texture within a vineyard were found to influence the vertical distribution of roots, whereas irrigation history had more influence on horizontal variations in root density within a depth range. Conversion of vines from sprinkler irrigation to drippers resulted in a marked increase in total root mass (volume) under the drip line, particularly 25-50 cm below the surface. Roots were differentially influenced by irrigation history according to their diameter class. Under drip irrigation, the largest increase in root-length density occurred with roots in diameter classes between 1 and 4 mm diameter. Grapevines established under sprinklers, and subsequently converted to drip irrigation, had significantly larger root systems (within the volume of soil sampled) than did vines maintained under sprinklers throughout. Therefore, vines established under sprinkler irrigation and then converted to drippers may be better equipped to cope with deficit irrigation during drought (via either RDI or PRD), by virtue of those additional roots.