Studies of energy balance that rely on eddy covariance(EC) are always challenged by energy balance closure, which is mainly caused by the underestimations of latent heat flux(LE) and sensible heat flux(Hs). The Bowen ratio(BR) and energy balance residual(ER) approaches are two widely-used methods to correct the LE. A comprehensive comparison of those two approaches in different land-use types is essential to accurately correcting the LE and thus improving the EC experiments. In this study, two energy balance approaches(i.e., BR and ER) were compared to correct the LE measured at six EC sites(i.e., three vegetated, one mixed and two non-vegetated sites) in an oasis-desert ecotone of the Heihe River Basin, China. The influences of meteorological factors on those two approaches were also quantitatively assessed. Our results demonstrated that the average energy closure ratio((LE+Hs)/(Rn–Gs); where Rn is the surface net radiation and Gs is the surface soil heat flux) was approximately close to 1.0 at wetland, maize and village sites, but far from 1.0 at orchard, Gobi and desert sites, indicating a significant energy imbalance at those three latter sites. After the corrections of BR and ER approaches that took into account of soil heat storage, the corrected LE was considerably larger than the EC-measured LE at five of six EC sites with an exception at Gobi site. The BR and ER approaches yielded approximately similar corrected LE at vegetated and mixed sites, but they generated dissimilar results at non-vegetated sites, especially at non-vegetated sites with low relative humidity, strong wind, and large surface-air temperature difference. Our findings provide insight into the applicability of BR and ER approaches to correcting EC-based LE measurements in different land-use types. We recommend that the BR-corrected and ER-corrected LE could be seriously reconsidered as validation references in dry and windy areas.
Grasslands and agro-ecosystems occupy one-third of the global terrestrial area.However,great uncertainty still exists about their contributions to the global carbon cycle.This study used various combinations of a simple ecosystem respiration model and a photosynthesis model to simulate the influence of different climate factors,specifically radiation,temperature,and moisture,on the ecosystem carbon exchange at two dissimilar study sites.Using a typical alpine meadow site in a cold region and a typical cropland site in an arid region as cases,we investigated the response characteristics of productivity of grasslands and croplands to different environmental factors,and analyzed the seasonal change patterns of different model parameters.Parameter estimations and uncertainty analyses were performed based on a Bayesian approach.Our results indicated that:(1) the net ecosystem exchange(NEE) of alpine meadow and seeded maize during the growing season presented obvious diurnal and seasonal variation patterns.On the whole,the alpine meadow and seeded maize ecosystems were both apparent sinks for atmospheric CO2;(2) in the daytime,the mean NEE of the two ecosystems had the largest values in July and the lowest values in October.However,overall carbon uptake in the cropland was greater than in the alpine meadow from June to September;(3) at the alpine meadow site,temperature was the main limiting factor influencing the ecosystem carbon exchange variations during the growing season,while the sensitivity to water limitation was relatively small since there is abundant of rainfall in this region;(4) at the cropland site,both temperature and moisture were the most important limiting factors for the variations of ecosystem carbon exchanges during the growing season; and(5) some parameters had an obvious characteristic of seasonal patterns,while others had only small seasonal variations.
