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

Abstract

As one of the key components in terrestrial water cycles, actual evapotranspiration (ET) links water, energy, and carbon cycles via plant physiological activities (Fisher et al., 2017;Katul et al., 2012). Although ET is primarily composed of daytime ET (ET D ), a growing body of observational and modeling evidence suggests that nighttime ET (ET N ) is also important for various ecohydrological and physiological processes from local to global scales (Forster, 2014;Padrón et al., 2020;Whitley et al., 2013). Globally, Padrón et al. (2020) showed that the average ET N /ET value was about 6.3% when calculated from the FLUXNET2015 data set and 7.9% from simulation results of global models. Moreover, ET N can exert significant impacts on the estimation of global water and carbon budgets, particularly in arid and semiarid regions (Lombardozzi et al., 2017;Resco De Dios et al., 2015), since the consideration of nighttime water use as indicated by ET N can noticeably reduce water use efficiency (WUE, defined as the ratio of CO 2 assimilation rate over water vapor loss) (Chaves et al., 2016). At field scales, neglecting ET N can cause the underestimation of total daily ET and the incomplete closure of energy balance, thus resulting in underestimated water requirements for crops (Skaggs & Irmak, 2011). Therefore, in addition to the role of ET D , a better understanding of nighttime water loss from terrestrial ecosystems also bears many important implications for hydrological, climatic, and agricultural studies (

Evaluation of Environmental Controls on Terrestrial Net Ecosystem Exchange of CO2: A Global Perspective From the FLUXNET Sites

“…Similar findings were also shown by Q. Han et al. (2021) along a climate gradient in northern Australia. Since NEE and ET a are tightly coupled (Samuels‐Crow et al., 2020; Wang et al., 2008), recent findings of ET a naturally raised the question as to whether/how the factors influencing NEE switch at different temporal scales.…”

“…The climatic dryness at each site was quantified by aridity index ($${\Phi}=\overline{{\mathrm{ET}}_{\mathrm{p}}}/\mathrm{MAP}$$, where $$\overline{{\mathrm{E}\mathrm{T}}_{\mathrm{p}}}$$ denotes mean annual potential evapotranspiration (ET p )) (Q. Han et al., 2020, 2021). Here, daily ET p was computed from the measurements at each site using the Penman‐Monteith equation (Allen et al., 1998) and then summed up to obtain $$\overline{{\mathrm{E}\mathrm{T}}_{\mathrm{p}}}$$.…”

“…Study sites from both networks are equipped with EC towers to measure water, carbon, and energy exchanges between the atmosphere and biosphere (Baldocchi, 2020; Novick et al., 2018). Data measured at the FLUXNET and AmeriFlux sites have been widely used to assess the factors that influence ecosystem processes, such as NEE and ET a (Fu et al., 2019; Q. Han et al., 2021; Wang et al., 2019). To compare the NEE responses to various environmental drivers among the FLUXNET sites and undisturbed AmeriFlux sites, a comprehensive data set was compiled from different data sources.…”

“…Study sites from both networks are equipped with EC towers to measure water, carbon, and energy exchanges between the atmosphere and biosphere (Baldocchi, 2020;Novick et al, 2018). Data measured at the FLUXNET and AmeriFlux sites have been widely used to assess the factors that influence ecosystem processes, such as NEE and ET a (Fu et al, 2019;Q. Han et al, 2021;Wang et al, 2019).…”

“…The climatic dryness at each site was quantified by aridity index ( 𝐴𝐴 Φ = ETp∕MAP , where 𝐴𝐴 ETp denotes mean annual potential evapotranspiration (ET p )) (Q. Han et al, 2020Han et al, , 2021. Here, daily ET p was computed from the measurements at each site using the Penman-Monteith equation (Allen et al, 1998) and then summed up to obtain 𝐴𝐴 ETp .…”