Revised algorithm of ecosystem water use efficiency for semi-arid steppe in the Loess Plateau of China

AimsWe 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 efficiency (IWUE) and underlying water use efficiency (uWUE) -for the semi-arid steppe and to endeavor necessary modifications. MethodsThe 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 (oWUE).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 IWUE 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 characteristics as compared with uWUE, but it improved the prediction of GPP and its dynamics.oWUE and uWUE improved 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.Key words water use efficiency; inherent water use efficiency; underlying water use efficiency; optimal water use efficiency; predicted gross primary production