中文摘要：

对近些年提出的两种生态系统水分利用效率指标——内在水分利用效率（IWUE）和固有水分利用效率（u WUE）与经典的水分利用效率（WUE）指标在黄土高原典型半干旱草地生态系统的应用情况进行了比较,主要分析了不同指标在小时、日和年尺度上的一致性和稳定性,并提出优化水分利用效率（o WUE）指标,比较了不同定义的水分利用效率指标在模拟总初级生产力（GPP）逐日动态过程上的表现。结果显示：IWUE和u WUE适合在日和小时尺度分析植被的碳水耦合特征,WUE适合分析年尺度的碳水耦合特征;o WUE指标在描述碳水耦合特征方面与u WUE相比并没有本质性改善,但在用于模拟GPP动态过程时模拟效果改善明显;采用o WUE和u WUE指标能改善植被生长盛期的GPP动态过程模拟,但在生长季早期和晚期的模拟效果比WUE差。IWUE指标不适合模拟GPP动态过程。

英文摘要：

Aims We evaluated the applicability of different measures of water use efficiency through analyzing the coupled dynamics of GPP and evapotranspiration in the semi-arid steppe in the Loess Plateau of China. Our objective is to explore the applicability of two quantitative measures of ecosystem water use efficiency--inherent water use effi- ciency （IWUE） and underlying water use efficiency （uWUE） --for the semi-arid steppe and to endeavor necessary modifications. Methods The consistency and stability of three indices of water use efficiency formulations （i.e. WUE, IWUE, uWUE） were calculated and compared at hourly, daily and annual time scales before proposing an optimal water use efficiency （o WUE）. These indices were additionally used to quantify their importances in modeling the diel change of gross primary production （GPP）. The yielded-accuracy of the prediction was used for justifying their uses. Important findings I~TJE and uWUE appeared suitable for examining the coupled water-carbon characteristics of vegetation at hourly and daily scales, whereas WUE was more plausible on the annual and interannual scales. The optimized water use efficiency index did not improve the prediction of the coupled water-carbon characteris- tics as compared with uWUE, but it improved the prediction of GPP and its dynamics, oWUE and uWUE im- proved the predictions of GPP in the peak growing period, while WUE predicted the GPP better at the early and late growing season. Interestingly, we found that IWUE was not suitable for predicting GPP and its dynamics. The results will be of great importance in modeling the effects of climate change on the carbon assimilation and water cycle for the future.