Energy partitioning over a semi-arid shrubland in northern China

Abstract

During the last decade, the widely distributed shrublands in northern China have shown significant signs of recovery from desertification, the result of widespread conservation practices. However, to support the current efforts in conservation, more knowledge is needed on surface energy partitioning and its biophysical controls. Using eddy-covariance measurements made over a semi-arid shrubland in northwest China in 2012, we examined how surface energy-balance components vary on diurnal and seasonal scales, and how biophysical factors control bulk surface parameters and energy exchange. Sensible heat flux (H) exceeded latent heat flux (λE) during most of the year, resulting in an annual Bowen ratio (β, i.e. H/λE) of 2.0. λE exceeded H only in mid-summer when frequent rainfall co-occurred with the seasonal peak in leaf area index (LAI). Evapotranspiration reached a daily maximum of 3.3 mm day À1 , and summed to 283 mm yr À1 . The evaporative fraction (EF, i.e. λE/R n ), Priestley-Taylor coefficient (α), surface conductance (g s ) and decoupling coefficient (Ω) were all positively correlated with soil water content (SWC) and LAI. The direct enhancement of λE by high vapour pressure deficit (VPD) was buffered by a concurrent suppression of g s . The g s played a direct role in controlling EF and α by mediating the effects of LAI, SWC and VPD. Our results highlight the importance of adaptive plant responses to water scarcity in regulating ecosystem energy partitioning, and suggest an important role for revegetation in the reversal of desertification in semi-arid areas.

Biophysical controls of water vapour and energy fluxes: Towards the development of biome scale predictive models of evapotranspiration in the Albany Thicket, South Africa

“…The poor relationship of ET with VPD ( Figures 8 and 9c) was possibly due to their complex relationship. It is well established that at higher VPD, plants tend to close their stomata to optimize water use, and ET becomes asymptotic as reported elsewhere (Jia et al, 2016). This result is also consistent with the feedback and feedforward explanations for stomatal response to VPD described by Schulze (1986) and Duursma et al (2014).…”

“…This system was different from the present study site with rainfall >800 mm. However, the result was similar to the 3.3 mm observed by Jia et al (2016) in northern China but different from the 4.5-5 mm reported by Dzikiti, Jovanovic, Bugan, Israel, and Le Maitre (2014) in the Fynbos Biome in South…”

“…The ET0 was higher than ET for most of the measurement time (Figure 7), and the evaporative index ET P , which is a measure of rain-use efficiency, was 0.97 over the measurement period. The evaporative index was close to unity, and it was similar to the summer value (>0.9) reported by Jia et al (2016) in China over a shrubland. The P ET0…”

“…Equations 10 and 11 need to be validated when more data are accumulated. It is well established that at higher VPD, plants tend to close their stomata to optimize water use, and ET becomes asymptotic as reported elsewhere (Jia et al, 2016). The poor relationship of ET with VPD ( Figures 8 and 9c) was possibly due to their complex relationship.…”

Dryland climate change: Recent progress and challenges

“…The partitioning of the surface contribution to atmospheric heating into sensible and latent heat depends on the nature of the surface [ Malone et al ., ; Zhu et al ., ; Jia et al ., ; Kool et al ., ]. Under increasing atmospheric water demand, the availability of surface moisture has a decisive influence on this partitioning because of the characteristic properties of water [ Koster et al ., ; Sherwood and Fu , ].…”