Comparison of Nighttime With Daytime Evapotranspiration Responses to Environmental Controls Across Temporal Scales Along a Climate Gradient

Abstract 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. We found that (a) across the sites, mean annual ET N / ET D ( / ) ranged between 5.1% and 11.7%, which was mainly determined by variations. Particularly, vegetation and meteorological variables mostly controlled , while was largely related to air temperature and net radiation ( Rn ) due to lower nighttime atmospheric water demands; (b) At individual sites, ET D and ET N exhibited higher correlations with meteorological and vegetation variables at monthly timescales than at annual timescales. Monthly ET D and ET N were also strongly coupled, especially 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. These findings provide a critical evaluation for contrasting ET D and ET N interactions in constantly changing environments, which has important implications for ecosystem water balance and land surface processes modeling.