Carbon–water coupling in California almond orchards: a multi-scale assessment of ecosystem water use efficiency using eddy covariance and remote sensing

Water use efficiency (WUE) at the ecosystem level is a critical ecophysiological indicator that integrates carbon-water coupling processes and provides essential insights for sustainable agriculture in water-limited environments. This study investigated the dynamics of WUE in California almond orchards, a high-value and water-intensive crop system of global economic significance, by combining eddy covariance (EC) measurements with satellite remote sensing to analyze seasonal and interannual patterns from 2020 to 2022. Gross primary productivity (GPP) was estimated using the Vegetation Photosynthesis Model (VPM), while evapotranspiration (ET) was derived using the pySEBAL surface energy balance model applied to Landsat imagery. The ratio of these fluxes provided spatially distributed WUE estimates. Daily GPP ranged from ~0.5 to 11.5 g C m^-2 d^-1, while ET ranged from ~0.5 to 7.5 mm d^-1, with both fluxes peaking during mid-summer. WUE values exhibited distinct seasonal patterns (ranging from ~0.5 to 5.9 g C kg^-1 H₂O), with higher efficiency during dormancy and lower values during fruit development stages, averaging 2.14 g C kg^-1 H₂O over the study period. GPP and ET showed similar seasonal patterns and responded in tandem to key climatic variables (solar radiation, air temperature, and humidity), suggesting common environmental drivers govern these processes. WUE exhibited more complex behavior: it increased slightly with greater precipitation and humidity and declined under high solar radiation, high vapor pressure deficit (VPD), and extreme temperatures. The remote sensing-derived GPP and ET agreed well with EC tower measurements (R² ≈ 0.87-0.88), affirming the reliability of the integrated approach. This study advances our understanding of carbon-water coupling in perennial almond orchard systems. It provides valuable ecological indicators for precision irrigation management in the face of increasing water scarcity and climate variability.