Energy partitioning and its biophysical controls above a grazing steppe in central Mongolia

The objective of this paper was to explore the seasonal development of how net all-wave radiation (Rn) above a typical steppe in central Mongolia is partitioned into the three components: sensible heat (H), latent heat (λE), and soil heat (G) flux. Seasonal variability of this partitioning in association with biotic and abiotic variables was addressed in detail using the evaporative fraction of Rn (EF) and the canopy surface conductance (gc), which was derived from an inversion of the Penman–Monteith equation. The surface energy partitioning of this steppe ecosystem showed the following characteristics: (1) H dominated the energy partitioning, followed by a relatively large G, although this pattern was temporally altered under conditions when the canopy surface was wet and the vegetation was fully developed; (2) the correlation of the energy partitioning with canopy development and soil moisture conditions explained up to 79% of the observed variance in EF and gc. Both factors exhibited linear correspondences to leaf area index (AL) or soil water content (θ); (3) the effect of atmospheric vapour pressure deficit (δe) on gc (R2 = 0.97) was curvilinear and strongly influenced the energy partitioning. Due to the limitation of water supply, both EF and gc decreased significantly with increasing δe; (4) the effect of short-wave solar radiation (Kd) on EF and gc depended strongly on soil moisture conditions. When soil moisture was high, EF still showed a decrease with increasing Kd while gc seemed insensitive to Kd; and (5) on the daily scale, close coupling of the canopy with the atmosphere was often observed in the afternoon as represented by the declining role played by gc in λE and EF. However, on the seasonal scale, variations in EF and gc closely followed the variation in AL and the precipitation events or the dry–wet cycles at the site.