Abstract
Abstract. Nocturnal water loss (NWL) from the surface into the atmosphere is often overlooked because of the absence of solar radiation to drive evapotranspiration and the measuring difficulties involved. However, growing evidence suggests that NWL – and particularly nocturnal transpiration – represents a considerable fraction of the daily values. Here we provide a global overview of the characteristics of NWL based on latent heat flux estimates from the FLUXNET2015 dataset, as well as from simulations of global climate models. Eddy-covariance measurements at 99 sites indicate that NWL represents 6.3 % of total evapotranspiration on average. There are six sites where NWL is higher than 15 %; these sites comprise mountain forests with considerable NWL during winter that is related to snowy and windy conditions. Higher temperature, vapor pressure deficit, wind speed, soil moisture, and downward longwave radiation are related to higher NWL, although this is not consistent across all of the sites. On the other hand, the global multi-model mean of terrestrial NWL is 7.9 % of the total evapotranspiration. The spread of the model ensemble, however, is greater than 15.8 % over half of the land grid cells. Finally, NWL is projected to increase everywhere with an average of 1.8 %, although with a substantial inter-model spread. Changes in NWL contribute substantially to projected changes in total evapotranspiration. Overall, this study highlights the relevance of water loss during the night and opens avenues to explore its influence on the water cycle and the climate system under present and future conditions.
Highlights
Water is lost from the surface to the atmosphere through evapotranspiration (ET)
Nocturnal water loss (NWL) from the co-located lysimeter and EC system show a Pearson correlation of 0.5 or 0.57, depending on how dew is estimated from the lysimeter data (L1 vs. L2, see Fig. 1a and b, respectively)
The agreement between EC and lysimeter improves if the NWL monthly climatology is analyzed
Summary
Water is lost from the surface to the atmosphere through evapotranspiration (ET). This process interlinks the water, energy, and carbon cycles and, influences climate, ecology, agriculture, and economy (e.g., Betts et al, 1996; Fisher et al, 2017; Zhang et al, 2015). Daytime ET, which is mainly driven by solar radiation, represents the majority of the contribution to total water loss, nighttime ET is likely non-negligible. It is recognized that vapor pressure deficit, temperature, wind speed, longwave radiation, and surface resistance influence nocturnal ET (Monteith, 1965; Penman, 1948). The prevalence of nocturnal water loss and its significance for the surface water and energy balance, remains overlooked and unclear
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