Energy partitioning and its controls over a heterogeneous semi‐arid shrubland ecosystem in the Lake Naivasha Basin, Kenya

Abstract

This paper reports on the energy balance and partitioning analysed from 3-year micrometeorological flux measurements of energy and water fluxes over a heterogeneous semi-arid shrubland ecosystem in Lake Naivasha basin. The measurements enabled the characterization of the seasonal and interannual variability of the energy and water fluxes of the ecosystem covering two wet years (2012 and 2013) and one drought year (2014). On an annual scale, more than 60% of net radiation (Rn) was partitioned as latent (LE) and sensible (H) energy flux, with the latter being the larger consumer of Rn (~34%) and dominant for most months. The transition from H to LE dominance occurred from early noon to late afternoon in the wettest months of April and May. The residual energy balance closure term (C) accounted for between 25 and 40% of the Rn with the imbalance tending to be highest during periods of high insolation. Annual evapotranspiration was estimated at 873 mm, 632 mm and 537 mm for year 2012, 2013 and 2014 respectively. This accounted for at least 80% of the annual precipitation received for the respective years. Surface conductance (g s ) scaled significantly with leaf expansion and Priestley-Taylor α coefficient, but was limited during periods of drought because of inadequate soil moisture suggesting that, change in LE was paralleled by changes in green leaf area index. Low values of decoupling coefficient (Ω) during these periods indicated that evapotranspiration was strongly controlled by g s and vapour pressure deficit (VPD). However, under non-limiting moisture conditions evapotranspiration was decoupled from the atmosphere. This suggested that during the dry seasons and periods of prolonged drought, a lack of moisture in combination with high VPD leads to significant decreases in stomatal conductance that eventually limits partitioning of available energy into LE in the semi-arid ecosystem part of the Lake Naivasha basin. However, during wet season these factors are non-limiting and radiation is the dominant control of energy partitioning into LE in the ecosystem. The coupling and decoupling pattern provide insights towards formulating models that quantify evapotranspiration for ecosystems that experience seasonal shifts in controlling factors.

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

“…Similar results were observed by Odongo et al (2016) who found that during the dry season, ET was more strongly linked to VPD and G s compared with R n . Similar results were observed by Odongo et al (2016) who found that during the dry season, ET was more strongly linked to VPD and G s compared with R n .…”

“…The lower values from this study are indicative of the degree of aridity because the average annual rainfall received was 296.4 mm whereas the Benin study area received rainfall >1,000 mm. Similar results were observed by Odongo et al (2016) who found that during the dry season, ET was more strongly linked to VPD and G s compared with R n . Essentially, the trends in the α and Ω were similar (Figure 10a,c).…”

“…For example,Odongo et al (2016) found values <0.6 and so didTong et al (2017). The 8-day average G s was 0.21 mm s −1 and average G a was 41.5 mm s −1 .…”

Understanding the Partitioning of the Available Energy over the Semi-Arid Areas of the Loess Plateau, China

“…Odongo et al [40] had similar results in the semi-arid ecosystem which suggested that during the dry seasons, drought conditions in combination with highs level of VPD lead to significant decreases in stomatal conductance that eventually limit partitioning of available energy into latent heat flux. However, during the wet season these factors are non-limiting and radiation is the dominant control of energy partitioning.…”

“…Therefore, higher VPD corresponds to larger latent heat flux, as a positive impact. On the other hand, the VPD will affect the stomatal conductance of vegetation such that a higher VPD can reduce the stomatal conductance and water vapor released from vegetation, resulting in less latent heat flux, as a negative impact [40]. As shown in Figure 6a, even under relatively wet conditions in the semi-arid region, the positive effect of VPD initially dominates.…”