Abstract. Fast-response infrared gas analyzers (IRGAs) have been widely used over three decades in many ecosystems for long-term monitoring of water vapor fluxes in the surface layer of the atmosphere. While some of the early IRGA sensors are still used in these national and/or regional eco-flux networks, optically-improved IRGA sensors are newly employed in the same networks. The purpose of this study was to evaluate the performance of water vapor density and flux data from three generations of IRGAs – LI-7500, LI-7500A, and LI-7500RS (LI-COR Bioscience, Inc., Nebraska, USA) – over the course of a growing season in Bushland, Texas, USA in an irrigated maize canopy for 90 days. The energy balance ratio, which is the sum of turbulent fluxes divided by the sum of surface available energy, was used to assess systematic biases of the IRGA sensors for evapotranspiration (ET). Water vapor density measurements were in generally good agreement, but temporal drift occurred in different directions and magnitudes. Means exhibited mostly shift changes that did not impact the flux magnitudes, while variances of water vapor density fluctuations were occasionally in poor agreement, especially following rainfall events. LI-7500 variances were largest compared to recent LI-7500RS and LI-7500A results manifesting in widened cospectra, especially under unstable and neutral static stability. Agreement among the sensors was best under the typical irrigation-cooled boundary layer, with a 14 % interinstrument coefficient of variability under advective conditions. Generally, the smallest variances occurred with the LI-7500RS, and high-frequency spectral corrections were larger for these measurements resulting in similar fluxes between the LI-7500A and LI-7500RS. Fluxes from the LI-7500 were best representative of growing season ET based on a world-class lysimeter reference measurement but using the energy balance ratio as an estimate of systematic bias corrected most of the differences among measured fluxes.

中文翻译：

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三代红外气体分析仪的水蒸气密度和湍流

摘要。快速响应的红外气体分析仪（IRGA）在三十年中已在许多生态系统中广泛用于长期监测大气表层中的水蒸气通量。尽管某些早期的IRGA传感器仍在这些国家和/或区域性的生态通量网络中使用，但光学改进的IRGA传感器又在同一网络中被新采用。本研究的目的是评估三代IRGA（LI-7500，LI-7500A和LI-7500RS（LI-COR Bioscience，Inc.，美国内布拉斯加州））的水蒸气密度和通量数据的性能。在美国得克萨斯州布什兰市的一个生长季节中，使用灌溉的玉米冠层进行90天的耕种。能量平衡比，即湍流之和除以表面可用能量之和，用于评估IRGA传感器的蒸散量（ET）的系统偏差。水蒸气密度的测量总体上吻合良好，但时间漂移发生在不同的方向和大小。平均数显示出大部分变化，但并不影响通量大小，而水蒸气密度波动的变化有时不一致，特别是在降雨事件之后。与最近的LI-7500RS和LI-7500A结果相比，LI-7500的方差最大，特别是在不稳定和中性的静态稳定性下，该结果表现出较宽的共谱。在典型的灌溉冷却边界层上，传感器之间的一致性最佳，在对流条件下，仪表间变异系数为14％。通常，LI-7500RS的变化最小，对于这些测量，高频频谱校正更大，导致LI-7500A和LI-7500RS之间的通量相似。根据世界一流的溶渗仪参考测量，LI-7500的通量最能代表生长季的ET，但使用能量平衡比作为系统偏差的估计值可以校正大多数测量通量之间的差异。