Coupled water–carbon flux processes of date palms under lysimeter-based saline irrigation: CoupModel validation and water productivity optimization

Date palm trees are widely cultivated in arid regions and are commonly irrigated using saline groundwater. The coupled water-carbon flux processes of date palms under saline irrigation have been calibrated and evaluated using CoupModel. This study aims to further validate the ability of CoupModel to simulate daily evapotranspiration and annual yield across various cropping systems, and optimize the irrigation water productivity (IWP) under different irrigation regimes and salinity conditions. The daily evapotranspiration and annual yield from mature trees grown in weighing lysimeters irrigated with water with an electrical conductivity (EC) 8.0 dS m −1 (EC8.0) were used to validate CoupModel parameterization. Similarly, the daily evapotranspiration was simulated for juvenile trees grown in weighing lysimeters under three irrigation levels at EC3.1. In addition, CoupModel was used to simulate the yield of fully developed trees under two irrigation regimes at EC2.4 in a conventional orchard. The IWP was optimized for palm trees by evaluating the simulated outputs based on combinations of 6 growth periods, 36 irrigation amount ratios, and 15 water salinities in CoupModel. The results show that both daily evapotranspiration and annual yield of palm trees irrigated with EC8.0 were underestimated using CoupModel. Nevertheless, the model accurately reproduced the daily evapotranspiration and annual yield of trees irrigated with EC3.1 and EC2.4, respectively. The growth period from May to June was crucial for maximizing the IWP under low salinity (≤EC3.0). For the salinity range of ≥EC3.5 to ≤EC8.0, adjusting the irrigation amount in February to June should be considered to leach salts from the root zone and maintain soil water and salinity under relatively suitable conditions. In summary, CoupModel parameterization is recommended for IWP optimization in the case of water salinity of ≤EC5.0. These findings are expected to contribute to improvement in IWP of date palms under different irrigation water amounts and salinities.