Assessment of the root water uptake pattern of a partial drip irrigated pear tree

Drip irrigation is used in ‘Conference’ pear tree in Belgium and the Netherlands to maximize fruit size in dry years. Besides regulated deficit irrigation, partial root zone irrigation (PRI) is an irrigation strategy used for increasing the water use efficiency. Trees under PRI take up more water in the irrigated part compared to the non irrigated part. However the importance of this “compensation effect” and the magnitude of this effect on tree transpiration on an irrigated tree in a temperate climate is unknown. To evaluate the impact of compensation in the pear tree root system, the root water uptake pattern was obtained in HYDRUS 2D using the dimensionless water stress index ωc (Jarvis 1989), recently implemented in HYDRUS. For the simulation detailed observations of soil hydraulic properties and root distribution were carried out. Simulation results were compared to measurements of soil hydraulic head and soil water content as well as sap flow rate in the trunk. Evolution of soil hydraulic head was successfully simulated for a non irrigated and partially irrigated tree for a period of 87 days. During this period, soil hydraulic head did not drop below 450 cm while it decreased to -800 cm for the non irrigated treatment. Water stress was observed in the non irrigated treatment but not in the partial irrigated treatment. Simulations with HYDRUS indicate that compensation did not interfere with the course of sap flow nor with the soil moisture content evolution.