Based on fifteen-year observations, we explored the interannual variations of five water use efficiency (WUE) indicators (i.e., standard ecosystem WUE (EWUE), plant-canopy WUE (PWUE), inherent WUE (IWUE), underlying WUE (uWUE) and stomatal slope (G₁)) over a typical winter wheat-summer maize rotation cropland in the North China Plain. The results showed that none of WUE indicators had a significant interannual trend. PWUE showed the lowest interannual variability and could be regarded as stationary for these two crops. $\text{G1}$ was stationary for wheat which was under an essentially constant soil water condition, whereas exhibited a strong fluctuation for maize experiencing more variable soil water content (SWC) than wheat. IWUE and uWUE were non-stationary for both crops. SWC and leaf area index (LAI) dominated EWUE for maize and wheat, respectively. PWUE was dominated by the shortwave solar radiation (R_g) in maize seasons and not significantly affected by any meteorological or biophysical factors in wheat seasons. For IWUE and uWUE, air temperature (T_a) and LAI were the most important drivers in maize and wheat seasons, respectively. Atmospheric CO₂ concentration (C_a) and vapor pressure deficit (VPD) were not revealed as important controllers. These findings reflected that the meteorological and biophysical regulatory mechanisms of WUE indicators at the ecosystem level were quite different from that at the leaf level. But G₁ was an exception due to that for both maize and wheat, G₁ was only significantly affected by SWC. Moreover, we found that none of the remote sensing (RS)-based global products performed satisfactorily on capturing interannual variations of EWUE for both crops. These results cautioned that using these global RS-based products to explore the interannual variations of EWUE for the cropland ecosystem may lead to substantial bias.

基于15年的观察，我们探索了5个用水效率指标（WUE）的年际变化（即标准生态系统WUE（EWUE），植物冠层WUE（PWUE），固有WUE（IWUE），基本WUE（uWUE）华北平原典型冬小麦-夏季玉米轮作农田的气孔坡度（G ₁））。结果表明，WUE指标均没有明显的年际趋势。PWUE的年际变化最小，可以认为这两种作物是静止的。$\text{G1}$对于基本处于恒定土壤水分条件下的小麦而言，它是静止的，而与小麦相比，土壤水分含量（SWC）变化更大的玉米表现出强烈的波动性。两种作物的IWUE和uWUE都不稳定。玉米和小麦的SWC和叶面积指数（LAI）分别占主导地位。在玉米季节，PWUE受短波太阳辐射（R _g）的支配，而在小麦季节，PWUE不受任何气象或生物物理因素的显着影响。对于IWUE和uWUE，气温（T _a）和LAI分别是玉米和小麦季节的最重要驱动因素。大气中的CO ₂浓度（C _a）和蒸气压不足（VPD）没有被揭示为重要的控制因素。这些发现表明，WUE指标在生态系统水平上的气象和生物物理调节机制与叶片水平上的差异很大。但是G ₁是一个例外，因为对于玉米和小麦，G ₁仅受SWC的显着影响。此外，我们发现基于遥感（RS）的全球产品都无法令人满意地捕获两种作物的EWUE的年际变化。这些结果提醒我们，使用这些基于RS的全球产品来探索农田生态系统的EWUE的年际变化可能会导致重大偏差。