Multiple Environmental Factors Drive Variations in Sap Flow and the Ratio of Transpiration to Evapotranspiration of Maize

ABSTRACT

Crop water consumption in agroecosystems is a critical determinant of water sustainability in arid regions worldwide. However, the variability of crop sap flow (SF) and the ratio of transpiration ( T )  to evapotranspiration (T/ET) remain insufficiently explored in these environments. We conducted 2 years of synchronous measurements of stem sap flow (SF) and evapotranspiration (ET) in an irrigated maize field, using the sap flow technique and the eddy covariance method, respectively. Our results showed that the daily single‐plant stem SF averaged 533.3 ± 134.3 g. Across the typical growing seasons, the T/ET ratio ranged between 87.4% and 88.2%, with T dominating total water flux. On a single‐plant basis, the small stem diameter maize (SD2) consumed 2.9 kg less water per month on average than the large stem diameter maize (SD1). SF and T/ET were regulated by meteorology, soil, and leaf at different time scales. At the half‐hourly scale, net radiation ( R _n ), relative humidity (RH), vapour pressure deficit (VPD), air temperature ( T _a ), wind speed ( W _s ), irrigation, stomatal conductance (SC), leaf water potential (LWP), surface‐layer soil water content (SL‐SWC), deep‐layer soil temperature (DL‐ST), and soil heat flux (G) were the primary controls. At the daily scale, SF and T/ET were mainly governed by R _n , T _a , deep‐layer soil water content (DL‐SWC), surface soil temperature (SL‐ST), soil water potential (SWP), irrigation, leaf area (LA), and leaf area index (LAI). These findings highlight the multi‐scale environmental modulation of water fluxes in agroecosystems and underscore the importance of considering crop stem diameter in both hydrological modelling and agricultural water management, which will still have great potential for water savings under drip irrigation and smart precision irrigation for maize in arid zones worldwide.