Canopy Resistance and Estimation of Evapotranspiration above a Humid Cypress Forest

Abstract

This study presented a two-year data set of sensible heat and water vapor fluxes above a humid subtropical montane Cypress forest, located at 1650 m a.s.l. in northeastern Taiwan. The focuses of this study were to investigate (1) the diurnal and seasonal variations of canopy resistance and fluxes of sensible heat and water vapor above this forest; and (2) the mechanism of why a fixed canopy resistance could work when implementing the Penman–Monteith equation for diurnal hourly evapotranspiration estimation. Our results showed distinct seasonal variations in canopy resistance and water vapor flux, but on the contrary, the sensible heat flux did not change as much as the water vapor flux did with seasons. The seasonal variation patterns of the canopy resistance and water vapor flux were highly coupled with the meteorological factors. Also, the results demonstrated that a constant (fixed) canopy resistance was good enough for estimating the diurnal variation of evapotranspiration using Penman–Monteith equation. We observed a canopy resistance around 190 (s/m) for both the two warm seasons; and canopy resistances were around 670 and 320 (s/m) for the two cool seasons, respectively. In addition, our analytical analyses demonstrated that when the average canopy resistance is higher than 200 (s/m), the Penman–Monteith equation is less sensitive to the change of canopy resistance; hence, a fixed canopy resistance is suitable for the diurnal hourly evapotranspiration estimation. However, this is not the case when the average canopy resistance is less than 100 (s/m), and variable canopy resistances are needed. These two constraints (200 and 100) were obtained based on purely analytical analyses under a moderate meteorological condition (Rn = 600 W·m−2, RH = 60%, Ta = 20°C, U = 2 m·s−1) and a measurement height around two times of the canopy height.

Responses in the Coupling of Carbon and Water Vapor Exchanges of Irrigated and Rainfed Andean Potato (<i>Solanum Tuberosum</i> Subsp. Andigenum) Agroecosystems

“…We observed a typical theoretically expected parabolic variation in the diurnal trend of Rc and Ra during all growth stages in both sites (Alves et al, 1998;Irmak and Mutiibwa, 2010;Lin et al, 2020;Monteith and Unsworth, 2013;Perez et al, 2006;Rana et al, 1994). At the FI and DI sites, Rc had high values (> 30 m s -1 ) in the early morning (5:00 -8:00), then tend to decrease toless than 15 m s -1 , remaining relatively constant from 10:00 to 15:00 After this, Rc increases gradually in the afternoon.…”

Carbon and water vapor exchanges coupling in irrigated and rainfed Andean potato (Solanum tuberosum subsp. andigenum) agroecosystems

“…At FI and DI, we observed a theoretically expected parabolic variation in the diurnal trend of Rc and Ra during all growth stages (Rana et al 1994;Alves et al 1998;Perez et al 2006;Irmak and Mutiibwa 2010;Monteith and Unsworth 2013;Lin et al 2020). The Rc values around 30 m s -1 in the early morning (5:00 -8:00), decrease to less than 15 m s -1 from 10:00 to 15:00 to again increase gradually in the afternoon.…”

Calibration of Land-Use-Dependent Evaporation Parameters in Distributed Hydrological Models Using MODIS Evaporation Time Series Data

“…However, this relationship becomes invalid under dry conditions. Further insights come from Lin et al [47], who noted the coupling of canopy resistance and water vapor fluxes with meteorological conditions. The behavior of canopy resistance in different seasons and under varying conditions plays a crucial role in modulating ETa patterns.…”