Young lime tree evapotranspiration measurements in lysimeters with automated irrigation

Lime tree growing area is increasing in Mediterranean temperate regions. In these areas, climate change scenario is expected to raise air temperature and water shortages. Such scenario requires new approaches to implement a precision irrigation in agriculture. In order to use water more efficiently, it becomes necessary to accurately determining the crop water needs, which are estimated by crop evapotranspiration computations (ETc). In this study the ETc of young lime trees grown under Mediterranean conditions were determined using the soil water balance method. For this purpose, two-year old lime trees (Citrus latifolia Tan., cv. Bearss) grafted on C. macrophylla rootstock were cultivated in pot-lysimeters, equipped with capacitance and granular matric sensors for real-time monitoring of the soil water status. Irrigation, drainage, and pot weight were also monitored continuously. All measurements were integrated into a telemetry platform. Agro-meteorological variables, plant water status (stem (Ψ_stem) and leaf (Ψ_leaf) water potentials), and leaf gas exchange parameters (stomatal conductance (g_s) and net photosynthesis (P_n)) were measured. Along the experiment, an automated irrigation protocol based on volumetric soil water content (θ_v) threshold values were programmed, guaranteeing an adequate lime tree water status. Irrigation dose was calculated based on a feed-back strategy maintaining θ_v within 30% management allowed depletion.

During the experimental period, the lime trees were well irrigated as revealed midday Ψ_stem values that were maintained above -0.8 MPa. Also, the mean seasonal values of ≈ 7 µmol m⁻² s⁻¹ and 80 mmol m⁻² s⁻¹, for P_n and g_s, respectively, indicated optimal gas exchange values. The computed water balance parameters yielded values for the crop evapotranspiration from 0.25to 2.56 mm day^-1, in winter and summer months, respectively, with maximum values in July when evaporative demand conditions were the highest. This soil water balance was daily validated by the pot weight balance through the year.

In conclusion, the automated irrigation of young potted lime trees, using soil water content as a control system variable, has ensured an adequate lime tree water status. A simple, robust weighing/drainage lysimeter, with real-time monitoring of the soil water balance parameters, has been proved practical and economical tool for crop evapotranspiration measurements.

Acknowledgments: This work was funded by Spanish Agencia Estatal de Investigación (PID2019-106226RB-C2-1/AEI/10.13039/501100011033) and Fundación Séneca, Región de Murcia (19903/GERM/15) projects.