Direct and indirect effects of atmospheric conditions and soil moisture on surface energy partitioning revealed by a prolonged drought at a temperate forest site

Abstract

[1] The purpose of this paper is to examine the mechanism that controls the variation of surface energy partitioning between latent and sensible heat fluxes at a temperate deciduous forest site in central Missouri, USA. Taking advantage of multiple micrometeorological and ecophysiological measurements and a prolonged drought in the middle of the 2005 growing season at this site, we studied how soil moisture, atmospheric vapor pressure deficit (VPD), and net radiation affected surface energy partitioning. We stratified these factors to minimize potential confounding effects of correlation among them. We found that all three factors had direct effects on surface energy partitioning, but more important, all three factors also had crucial indirect effects. The direct effect of soil moisture was characterized by a rapid decrease in Bowen ratio with increasing soil moisture when the soil was dry and by insensitivity of Bowen ratio to variations in soil moisture when the soil was wet. However, the rate of decrease in Bowen ratio when the soil was dry and the level of soil moisture above which Bowen ratio became insensitive to changes in soil moisture depended on atmospheric conditions. The direct effect of increased net radiation was to increase Bowen ratio. The direct effect of VPD was very nonlinear: Increased VPD decreased Bowen ratio at low VPD but increased Bowen ratio at high VPD. The indirect effects were much more complicated. Reduced soil moisture weakened the influence of VPD but enhanced the influence of net radiation on surface energy partitioning. Soil moisture also controlled how net radiation influenced the relationship between surface energy partitioning and VPD and how VPD affected the relationship between surface energy partitioning and net radiation. Furthermore, both increased VPD and increased net radiation enhanced the sensitivity of Bowen ratio to changes in soil moisture and the effect of drought on surface energy partitioning. The direct and indirect effects of atmospheric conditions and soil moisture on surface energy partitioning identified in this paper provide a target for testing atmospheric general circulation models in their representation of land-atmosphere coupling.

Analysing surface energy balance closure and partitioning over a semi-arid savanna FLUXNET site in Skukuza, Kruger National Park, South Africa

“…Our study results are consistent with similar studies, for example Gu et al (2006), who examined how soil moisture, VPD, and net radiation control surface energy partitioning at a temperate deciduous forest site in central Missouri, USA. Both studies agree that with ample soil moisture, during the rainy season, latent heat flux dominates over sensible heat flux, and reduced soil water availability reversed the dominance of latent heat over sensible heat, because of its direct effect on stomatal conductance.…”

“…The monthly and seasonal trends of energy partitioning were assessed, and how each component is affected by vegetation dynamics at the site. Surface energy partitioning was also characterized as a direct function of VPD and Rn during the wet and dry seasons, following Gu et al (2006). 1912-2001 and 1960-1999 climate analysis for the same area as reported by Kruger et al (2002) and Scholes et al (2001), showing a mean annual total precipitation of 547.1 mm and air temperature of 21.9 • C. The low rainfall during 2000-2003 seasons was also reported by Kutch et al (2008), who were investigating the connection between water relations and carbon fluxes during the mentioned period.…”

“…In the grassland Mongolian Plateau, Li et al (2006) concluded that sensible heat flux dominated the energy partitioning, followed by ground heat flux, with the rainy season showing slight increase in latent heat flux. Gu et al (2006) used different ratios (Bowen ratio, G / Rn, H / Rn and LE / Rn) to investigate surface energy exchange in the Tibetan Plateau, and showed that during the vegetation growth period, LE was higher than H, and this was reversed during the post-growth period.…”

Changes in aboveground biomass following alternative harvesting in oak-hickory forests in the eastern USA

“…2), which is consistent with the previous reports of the Ozarks forests (Li et al 2007) and deciduous forests in North America (Brown et al 1997). However, the above figure was lower than that reported by Gu et al (2006) at the Missouri Ozarks Flux (MOFLUX) site located in the Baskett Research and Education Area, which is 200 km away from the MOFEP site, where stand composition and structure are similar to that of MOFEP site. The MOFLUX is the nearest flux tower with direct carbon flux measurements.…”

Aerosol trends as a potential driver of regional climate in the central United States: evidence from observations

“…Gu et al (2006) saw evidence of coupling between net radiation, heat fluxes, and soil moisture using Ameriflux observations during the 2005 growing season in Missouri. The model study of Mickley et al (2012) found that US aerosol reductions lead to enhanced latent heat fluxes in early summer, which transition to enhanced sensible heat fluxes by late summer-early fall.…”