Stage-dependent response of maize canopy water use efficiency to groundwater depth: insights from ecosystem flux observations

Canopy water use efficiency (WUE_c) is an important indicator for understanding the coupling between water and carbon processes in agroecosystems. In arid irrigation districts with shallow groundwater which is an important source of evapotranspiration (ET), previous studies have demonstrated that groundwater table depth (WTD) influences the crop water use efficiency. The dynamic response of water use efficiency at the maize canopy scale to WTD remains unclear, particularly regarding the physiological differences across growth stages, which is critical for understanding plant water-use regulation within the soil-plant-atmosphere continuum under shallow groundwater conditions. Based on eddy covariance observations from 2017 to 2019, along with ET partitioning and statistical modeling, this study systematically analyzed the variations in WUE_c and its environmental drivers, with a focus on the stage-dependent responses of photosynthesis and transpiration to WTD. The results showed that average T/ET was 85.7% over the three growing seasons, while groundwater contribution to ET was 38.2%, 37.3%, and 29.9% in 2017, 2018, and 2019, corresponding to mean groundwater depths of 1.60 m, 1.76 m, and 1.81 m, respectively. Mean WUE_c was 2.28 ± 0.75, 2.22 ± 1.14, and 3.43 ± 1.01 g C kg⁻¹ H₂O in the three years. The fluctuations in WTD significantly affected WUE_c, especially in years with relatively low surface water input. The standardized WUE_c (WUE_z), which excluded the effects of crop development and atmospheric evaporative demand, decreased with deepening WTD during the vegetative growth stage but increased during the reproductive stage. This shift stemmed from the differential sensitivity of canopy photosynthesis and transpiration to WTD at each stage. During the vegetative stage, a deepening WTD caused the standardized photosynthesis (NEP_z) to decline more sharply than transpiration (T_z), reducing WUE_z. In contrast, during the reproductive stage, both NEP_z and T_z increased in response to a deeper WTD, but the greater increase in NEP_z led to an overall rise in WUE_z. This study reveals a previously unreported, stage-dependent pattern in how crop water-carbon coupling responds to variations in groundwater depth. Our findings provide critical empirical evidence for refining the representation of plant water use regulation under soil water stress in ecohydrological models.