Department of Earth and Environment, Boston University, 675 Commonwealth Ave., Boston, MA, 02215, USA.
Glob Chang Biol. 2017 Mar;23(3):1140-1151. doi: 10.1111/gcb.13439. Epub 2016 Sep 20.
Evapotranspiration, defined as the total flux of water from the land surface to the atmosphere, is a major component of the hydrologic cycle and surface energy balance. Although evapotranspiration is expected to intensify with increasing temperatures, long-term, regional trends in evapotranspiration remain uncertain due to spatially and temporally limited direct measurements. In this study, we utilize an emergent relation between the land surface and atmospheric boundary layer to infer daily evapotranspiration from historical meteorological data collected at 236 weather stations across the United States. Our results suggest a statistically significant (α = 0.05) decrease in evapotranspiration of approximately 6% from 1961 to 2014, with a significant (α = 0.05) sharp decline of 13% from 1998 to 2014. We attribute the decrease in evapotranspiration mostly to declines in surface conductance, but also to offsetting changes in longwave radiation, wind speed, and incoming solar radiation. Using an established stomatal conductance model, we explain the changes in inferred surface conductance as a response to increases in carbon dioxide and, more recently, to an abrupt decrease in atmospheric humidity.
蒸散作用，定义为水从陆地表面到大气的总通量，是水文循环和地表能量平衡的主要组成部分。尽管预计随着温度的升高，蒸散作用会加剧，但由于直接测量的空间和时间有限，长期的、区域性的蒸散趋势仍然不确定。在这项研究中，我们利用陆地表面和大气边界层之间的一种新关系，从美国 236 个气象站收集的历史气象数据中推断出每日蒸散量。我们的结果表明，从 1961 年到 2014 年，蒸散量呈统计显著（α=0.05）下降约 6%，而从 1998 年到 2014 年则呈显著（α=0.05）急剧下降 13%。我们将蒸散量的减少主要归因于地表导度的下降，但也归因于长波辐射、风速和入射太阳辐射的抵消变化。使用一个已建立的气孔导度模型，我们解释了推断出的地表导度变化是对二氧化碳增加的响应，以及最近对大气湿度突然下降的响应。
