Dynamics of nocturnal, daytime, and sum-of-hourly evapotranspiration and other surface energy fluxes over nonstressed maize canopy

The magnitude and driving forces of nocturnal evaporative losses, ET _cnight , and the interactions of other surface energy fluxes and microclimatic variables under various climatic, soil, and management conditions are not well understood. Such relationships are important for ecophysiological studies. This research attempts to investigate such relationships. Furthermore, ET _cnight can be a sizable portion of the daily total evaporative losses. Most empirical equations, especially ones that use solar or net radiation to estimate daily evapotranspiration (ET), either ignore or poorly treat the contribution of ET _cnight to the daily total ET. Neglecting ET _cnight can lead to errors in determining the daily or the sum-of-hourly ET _c (i.e., ET _cSOH) and can also cause cumulative errors when making long-term water balance analyses. In this paper, the magnitudes, trends, and contribution to the nocturnal surface energy balance of various microclimatic variables (air temperature, T _a; vapor pressure deficit, VPD; relative humidity, RH; and wind speed at 3 m, u ₃) and surface energy fluxes (ET _cnight; soil heat flux, G; sensible heat flux, H; and net radiation, R _n); were quantified and interpreted for a nonstressed and subsurface-drip-irrigated maize canopy. The effect of microclimatic variables and surface energy flux components on the Bowen ratio energy balance system (BREBS)-measured ET _cnight and daytime evaporative loss, ETcday , were investigated in the growing season of 2005 (i.e., April 22-September 30) and 2006 (May 12-September 27). The nighttime evaporative losses were high early in the season during partial canopy closure because of increased surface soil evaporation and were also high later in the season during and after leaf aging, physiological maturity, and leaf senescence. The seasonal average nighttime evaporative losses for 2005 and 2006 were 0.19 and 0:11 mm=night, respectively. Losses of 0.50 mm or more occurred in 2005 and 2006 on eight and seven nights, respectively. The seasonal total ETcnight , ET _cday , and ET _cSOH in 2005 were 31, 612, and 642 mm, respectively. The ET _c values in 2006 were 16, 533, and 547 mm, respectively. In both years, the percent ratio of ET _cday to ET _cSOH usually was more than 80-85%. ET _cnight was affected primarily by u ₃, VPD, and T _a. A strong relationship between ET _cnight and nighttime sensible heat was observed. Some of the largest ratios of ET _cnight to ET _cSOH occurred on rainy nights with strong winds. Because of strong winds, the ET _cnight was high owing to the clear coupling among all energy exchanges within and above the canopy as a result of the mixing of the lower boundary layer of the microclimate. The results of this study showed that the ET _cnight can be up to 5% of the ET _cSOH , even for a subsurface-drip-irrigated maize canopy in which the soil surface is usually dry, thus, less evaporative losses potential compared with the surface or sprinkler-irrigated surfaces in which ET _cnight would be expected to be considerably higher because of wetter surface conditions. ET _cnight needs to be quantified for different vegetation surfaces and management practices, surface wetting, and climatic conditions to better account for nighttime water losses and better understand nighttime energy balance mechanisms.