Response of vegetative and fruit growth to the soil volume wetted by irrigation in a super-high-density olive orchard

In most fruit tree orchards under localised irrigation, only a fraction of the rhizosphere is wetted by irrigation. As this may reduce growth and production, such that there is a demand for information on the optimum number and location of emitters. In a mature 'Arbequina' olive orchard with 1667 trees ha(-1), we analysed the impact of applying the same amount of water with one vs. two drip irrigation pipes (laterals) per tree row. We had trees under full irrigation, for which irrigation supplies amounted to 80-100% of the crop water needs (ETc), and two deficit irrigation treatments supplying ca. 40% of ETc. We used the HYDRUS 2D/3D (v. 1.11) model to simulate the shape and volume of the fraction of the rhizosphere wetted by irrigation (irrigation bulbs), as well as drainage and soil evaporation. For both full irrigation and deficit irrigation, differences in the soil volume and the soil surface wetted by irrigation were not sufficient to affect drainage and soil evaporation. The greatest values of root length density and root surface per unit volume were found in trees under deficit irrigation with two laterals, and the lowest in fully irrigated trees with one lateral. This, however, was counterbalanced by both the greater size of the irrigation bulbs and the greater soil water contents in the bulbs of the fully irrigated trees, such that water uptake was not enhanced by the second lateral. For 2 consecutive years we found no impact of irrigating with one or two laterals per tree row on water potential and stomatal conductance, shoot and fruit growth, leaf area and production, either for full irrigation or deficit irrigation conditions. Therefore, our findings suggest no benefits of irrigating super high density olive orchards with two laterals instead of one.