Comparison of nighttime with daytime evapotranspiration responses to environmental controls across temporal scales along a climate gradient

Understanding daytime (ET_D) and nighttime (ET_N) evapotranspiration is critical for accurately evaluating terrestrial water and carbon cycles. However, unlike ET_D, the factors influencing ET_N remain poorly understood. Here, long-term ET_D and ET_N data from five FLUXNET sites along a climate gradient in Northern Australia were analyzed to compare their responses to environmental drivers at different temporal scales. Across the sites, mean annual ET_N/ET_D ranged between 5.1% and 11.7%, which was mainly determined by ET_D variations. Both vegetation and climatic conditions were closely related to mean annual ET_D, while the primary controls on mean annual ET_N were air temperature and net radiation (Rn). At site levels, monthly ET_D and ET_N showed better correlations with meteorological and vegetation variables than annual ET_D and ET_N, and the coupling of ET_D and ET_N was also stronger at monthly timescales, particularly under drier climatic conditions. At daily timescales, leaf area index and soil water content (SWC) controlled ET_D with SWC being more important at drier sites; whereas, SWC was the dominant factor controlling ET_N. At half-hourly timescales, the boosted regression tree method quantitively showed that ET_D and ET_N were controlled by Rn and SWC, respectively. Overall, the results showed that ET_N was less responsive to environmental variables, illustrating that ET_D and ET_N responded differently to diverse climate regimes and ecosystems at varying temporal scales.