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This article presents findings from a field research study conducted in the San Joaquin Valley of California in 2016–2018 to appraise the effects of soil salinity and sodicity on evapotranspiration and energy balance components of micro-irrigated pistachio orchards. Actual evapotranspiration (ETa) and tree physiologic parameters were measured during consecutive growing seasons in mature orchards grown on non-saline and saline/sodic soils. Salinity and sodicity decreased pistachio water use by about 30%, with ETa reductions varying along the growing season. Accurate information on the dynamics of ETa and energy balance components along the growing season can improve water management for nut orchards exposed to long-term saline-sodic conditions. Results show that the main driver of ETa was the net radiation (Rn), which supplied most of the energy to vaporize water, irrespective of the growth period and level of salinity/sodicity. Field observations revealed that Rn was lower for salt-affected trees due to smaller canopies, which intercepted less light than non-saline trees. Secondarily, the exchange of sensible heat (H) between the ambient air and tree canopies was affected by the interaction between salinity–sodicity and seasonality. Early in the season, salinity and sodicity affected ETa mainly through the reduced canopy growth, which decreased the available energy (Rn–G) for ETa and reduced the water uptake as a result of the lower soil water potential. Late in the season, an increase in H and a decrease in the contribution of the aerodynamic component (β coefficient) to the latent heat flux (LE) occurred, which determined a further reduction of ETa due to a physiological response. The decrease in the β coefficient during the late season was associated with a direct impact of ion accumulation on leaf functionality. Collecting data on the contribution of the aerodynamic component to LE offers a low-cost method to detect and quantify physiological stress, while providing useful information for managing irrigation in salt-affected orchards. The results presented in this article provide insights to improve irrigation management of salt-affected pistachio through integration of weather measurements, energy balance components, and plant-based parameters. [ABSTRACT FROM AUTHOR]

• A user-friendly simulator for estimating tree PAR interception under different latitudes, canopy dimensions, leaf densities, tree spacing, and row orientations. • Predicting the solar radiation interception by mechanically pruned citrus trees. • PAR interception by citrus trees based on the canopy geometry and its shading patterns. • To inform irrigation planning and management decisions for different site-specific conditions. Diurnal and seasonal patterns of photosynthetically active radiation (PAR) intercepted by tree canopies in production orchards have an important role for eco-physiological processes that determine tree water use and productivity. Under well-watered conditions, PAR intercepted by trees is a reliable predictor of crop consumptive water use, which is regulated by tree spacing, tree row orientation, and by the tree canopy dimensions and geometry. Despite its advantages, only rare attempts have been made by the scientific and fruit production communities to develop canopy PAR interception simulators that rely on analytical principles to inform irrigation planning and management decisions. In this article, a novel model is presented that simulates PAR interception by citrus trees based on the canopy geometry and its shading patterns. The article also illustrates preliminary field testing and validation of the model with data collected in commercial citrus production orchards located in the San Joaquin Valley of California. The testing and validation could enable citrus growers and orchard managers to predict the net radiation for mature, micro-irrigated Navel orange and Page mandarin orchards grown with north–south and east–west tree row orientations and different tree densities. Results from the simulations and field validation showed that the canopy PAR light interception contributed 58.8 (± 0.02) and 71.1 (± 1.60) percent to the overall net radiation for the Navel orange orchards during the periods around summer solstice and autumnal equinox, respectively, whereas for the Page mandarin orchards, PAR light interception contributed 63.0 (± 5.60) and 85.7 (± 18.87) percent to the overall net radiation during the same time periods. Good correlations were found between daily values of PAR and net radiation for these citrus production orchards grown with the two alternative row orientations, which suggest that the proposed model could be used to inform irrigation planning and management decisions for different site-specific citrus orchard features and growing conditions. [ABSTRACT FROM AUTHOR]

California growers face challenges with water shortages and there is a strong need to use the least amount of water while optimizing yield. Timely information on evapotranspiration (ET), a dominant component of crop consumptive water use, is critical for growers to tailor irrigation management based on in-field spatial variability and in-season variations. We evaluated the performance of a remote sensing-based approach, Mapping Evapotranspiration at high Resolution with Internalized Calibration (METRIC), in mapping ET over an almond orchard in California, driven by Landsat satellite observations. Reference ET from a network of weather stations over well-watered grass (ETo) was used for the internal calibration and for deriving ET at daily and extended time period, instead of alfalfa based reference evapotranspiration (ETr). Our study showed that METRIC daily ET estimates during Landsat overpass dates agreed well with the field measurements. During 2009–2012, a root mean square error (RMSE) of 0.53 mm/day and a coefficient of determination (R2) of 0.87 were found between METRIC versus observed daily ET. Monthly ET estimates had a higher accuracy, with a RMSE of 12.08 mm/month, a R2 of 0.90, and a relatively small relative mean difference (RMD) of 9.68% during 2009–2012 growing seasons. Net radiation and Normalized Difference Vegetation Index (NDVI) from remote sensing observations were highly correlated with spatial and temporal ET estimates. An empirical model was developed to estimate daily ET using NDVI, net radiation (Rn), and vapor pressure deficit (VPD). The validation showed that the accuracy of this easy-to-use empirical method was slightly lower than that of METRIC but still reasonable, with a RMSE of 0.71 mm/day when compared to ground measurements. The remote sensing based ET estimate will support a variety of State and local interests in water use and irrigation management, for both planning and regulatory/compliance purposes, and it provides the farmers observation-based guidance for site-specific and time-sensitive irrigation management [ABSTRACT FROM AUTHOR]