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. We found that (a) across the sites, mean annual ET_N/ET_D ((Formula presented.) / (Formula presented.)) ranged between 5.1% and 11.7%, which was mainly determined by (Formula presented.) variations. Particularly, vegetation and meteorological variables mostly controlled (Formula presented.), while (Formula presented.) 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.