Responses of Ecosystem Water Use Efficiency to Diversified Crop Rotations in the Inland Pacific Northwest of the United States

With intensifying global warming and water scarcity worldwide, improving ecosystem water use efficiency (WUE) is critical for the sustainable development of rain‐fed dryland agriculture. Diversified crop rotations represent a promising agronomic strategy to optimize carbon‐water coupling and improve stress tolerance. Using 3‐year continuous eddy‐covariance observations for three representative rotation systems in the semi‐arid Inland Pacific Northwest, USA, this study investigated how diversified rotations regulate ecosystem‐scale WUE, with a focus on the mediating role of canopy conductance (Gc). Ecosystem WUE was jointly governed by crop‐rotation regime, air temperature, and soil water content, largely through Gc‐dependent physiological pathways. The four dominant crop species exhibited divergent hydrothermal adaptation thresholds and physiological response strategies. The winter wheat–spring wheat–winter pea rotation maintained the highest and most stable annual WUE (2.67 g C kg −1 H 2 O), accompanied by a moderate WUE decline during compound drought‐heat extremes. By comparison, the rotations incorporating garbanzo achieved lower WUE and diminished stress resilience. Vapor pressure deficit constrained Gc across all crop types, while soil moisture shaped WUE indirectly by altering canopy physiological conditions. Divergent hydrothermal adaptive traits among constituent crops offer a plausible mechanistic explanation for the superior functional performance of the optimal rotation system. These findings illustrate that well‐designed cereal‐legume rotations can optimize canopy‐level physiological functioning, strengthen ecosystem carbon‐water coupling, and improve the resistance of semi‐arid agroecosystems to compound drought‐heat stress. This study provides physiological insights and actionable rotational management guidance for developing water‐saving and stress‐adapted cropping systems across global water‐limited dryland regions.