<p>Evapotranspiration (ET) is an important part of the hydrological cycle and a key indicator for measuring water and energy exchanges in the soil-plant-atmosphere continuum. From 2018 to 2020, the eddy covariance measurement system and full-element automatic weather station were used to assess energy fluxes and ET as well as their environmental and biophysical controls over a conventional wheat/maize rotation farm in the ancient course of Yellow River. A surface conductance (Gs) of 12&#xa0;mm s^− 1 was identified as a critical threshold for the evapotranspiration response. When Gs was below 12&#xa0;mm s^− 1, ET exhibited a significant positive linear correlation with Gs, indicating that evapotranspiration was controlled by water supply. When Gs ≥ 12&#xa0;mm s^− 1, this positive linear correlation weakened. The identification of this threshold introduces a nonlinear response mechanism to farmland ET models, revising the simplified assumption of Gs in traditional evapotranspiration models. Daily fluctuations in midday Priestley–Taylor coefficient (α) and decoupling coefficient (Ω) indicated that ET alternated between water-limited and energy-limited conditions during wheat/maize growth stages, transitioning from aerodynamic control (i.e., governed by vapor pressure deficit (VPD) and Gs) to available energy control and back to aerodynamic control. Additionally, the fluctuations in daily average midday α (0.77/0.60) and Ω (0.41/0.36) throughout the entire growth stages of winter wheat/summer maize further demonstrated the water-limited conditions. These findings will contribute to developing precise irrigation plans, improving water resource utilization in winter wheat/summer maize ecosystems, and refining ET modeling.</p>