Quantifying and predicting nighttime evapotranspiration for two distinct grassland ecosystems

Evapotranspiration (ET) is after precipitation the second largest flux at the land surface in the global water cycle and water losses occur mainly during daytime. Less attention has been given in the past to water losses from the land surface into the atmosphere during nighttime. Recent investigation suggests that significant amounts of water are lost during night by transpiration and/or evaporation processes. Nighttime ET may be predicted from models that use meteorological data, but due the lack of long-term experimental observations their verification is limited. Thus the prediction and measurement of ET at nighttime poses a major challenge in the land surface hydrology.

We used precision weighing lysimeter data (2013 - 2016) to quantify the amount of water loss during night (sunrise to sunset) for two grassland ecosystems in western Germany (TERENO SOILCan). We found that annual nighttime ET ranged between 3.5% and 9.5% of daytime annual ET. Nighttime ET occurred mainly during wet soil and canopy surface conditions and was positive correlated with wind speed, which suggest that nighttime water losses are largely related to evaporation. Dew formation was also analyzed and ranged between 4.8% and 6.4% of the annual precipitation. Predicted nighttime ET based on the Penman-Monteith model improved, when assuming that the nighttime stomatal resistance parameter was set to zero and when the aerodynamic and surface resistance parameters were based on measured plant heights. The occurrence of hot days during the observation periods showed to substantially increase nighttime ET rates. Overall our results suggests that nighttime ET can be observed with precision weighing lysimeter and was an important component of the terrestrial water cycle of the grassland ecosystem.