Stage-specific responses of maize water use efficiency to shallow groundwater in an arid region

Canopy water use efficiency ( WUE c ) is a critical indicator for characterizing the coupling of water and carbon processes in agroecosystems. Although groundwater serves as an important water source for crops in arid districts, the response of WUE c to groundwater depth during crop growing season remains poorly understood. We examined this relationship in an irrigated maize field in the Hetao Irrigation District by combining three years (2017 −2019) of eddy covariance observations with evapotranspiration ( ET ) partitioning and environmental factors measurements. The results revealed that maize transpiration accounted for an average of 85.7% of ET and daily mean WUE c was 2.68 g C kg −1 H 2 O. WUE c was jointly influenced by meteorological conditions, canopy development, and water availability. And groundwater table depth ( WTD ) was significantly and negatively associated with WUE c when surface water input was relatively low. Normalized net ecosystem productivity ( NEP n ) and transpiration ( T n ) decreased as WTD increased, with NEP n showing greater sensitivity than T n . As a result, WUE c remained relatively stable within the observed smaller WTD range, but declined significantly as WTD increased beyond approximately 2.0 m in the late growth stages. In particular, when root-zone soil water content ( SWC ) remained below field capacity (0.31 m 3 m −3 in this site) for several consecutive days, a sharp decline in NEP n led to a rapid decrease in WUE c . This study demonstrates a stage-dependent response of WUE c to WTD , and highlights the importance of considering both groundwater depth and SWC in arid irrigation districts particularly in the late growing season to sustain WUE c and agricultural water productivity.