Soil heat flux is important for surface energy balance (SEB), and inaccurate estimation of soil heat flux
often leads to surface energy imbalance. In this paper, by using observations of surface radiation fluxes
and soil temperature gradients at a semi-arid grassland in Xilingguole, Inner Mongolia, China from June
to September 2008, the characters of the SEB for the semi-arid grassland were analyzed. Firstly, monthly
averaged diurnal variations of SEB components were revealed. A 30-min forward phase displacement of soil
heat flux (G) observed by a fluxplate at the depth of 5-cm below the soil surface was conducted and its effect
on the SEB was studied. Secondly, the surface soil heat flux (Gs) was computed by using harmonic analysis
and the effect of the soil heat storage between the surface and the fluxplate on the SEB was examined.
The results show that with the 30-min forward phase displacement of observed G, the slope of the ordinary
linear regression (OLR) of turbulent fluxes (H+LE) against available energy (Rn–G) increased from 0.835
to 0.842, i.e., the closure ratio of SEB increased by 0.7%, yet energy imclosure of 15.8% still existed in the
SEB. When Gs, instead of G was used in the SEB equation, the slope of corresponding OLR of (H+LE)
against (Rn–Gs) reached 0.979, thereby the imclosure ratio of SEB was reduced to only 2.1%.