A remote sensing-based three-source energy balance model to improve global estimations of evapotranspiration in semi-arid tree-grass ecosystems

It is well documented that energy balance and other remote sensing-based evapotranspiration (ET) models face greater uncertainty over water-limited tree-grass ecosystems (TGEs), representing nearly 1/6th of the global land surface. Their dual vegetation strata, the grass-dominated understory and tree-dominated overstory, make for distinct structural, physiological and phenological characteristics, which challenge models compared to more homogeneous and energy-limited ecosystems. Along with this, the contribution of grasses and trees to total transpiration (T), along with their different climatic drivers, is still largely unknown nor quantified in TGEs. This study proposes a thermal-based three-source energy balance (3SEB) model, accommodating an additional vegetation source within the well-known two-source energy balance (TSEB) model. The model was implemented at both tower and continental scales using eddy-covariance (EC) TGE sites, with variable tree canopy cover and rainfall (P) regimes and Meteosat Second Generation (MSG) images. 3SEB robustly simulated latent heat (LE) and related energy fluxes in all sites (Tower: LE RMSD ~60 W/m2; MSG: LE RMSD ~90 W/m2), improving over both TSEB and seasonally changing TSEB (TSEB-2S) models. In addition, 3SEB inherently partitions water fluxes between the tree, grass and soil sources. The modelled T correlated well with EC T estimates (r > .76), derived from a machine learning ET partitioning method. The T/ET was found positively related to both P and leaf area index, especially compared to the decomposed grass understory T/ET. However, trees and grasses had contrasting relations with respect to monthly P. These results demonstrate the importance in decomposing total ET into the different vegetation sources, as they have distinct climatic drivers, and hence, different relations to seasonal water availability. These promising results improved ET and energy flux estimations over complex TGEs, which may contribute to enhance global drought monitoring and understanding, and their responses to climate change feedbacks.The research received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie TRuStEE project (grant agreement No 721995). It was also funded by Ministerio de Economía y Competitividad through SynerTGE CGL2015-G9095-R funded by MCIN/ AEI /10.13039/501100011033/ FEDER ‘a way of making Europe’. The study also benefitted from the DIVERSPEC-TGA project, funded by the Ministerio de Ciencia e Innovación of Spain MCIN/ AEI /10.13039/501100011033. The infrastructure at ES-LM1 was partly funded through the Alexander von Humboldt Foundation, ELEMENTAL (CGL 2017-83538-C3-3-R, MINECO-FEDER) and IMAGINA (PROMETEU 2019; Generalitat Valenciana). Funding for the US-Ton AmeriFlux site was provided by the U.S. Department of Energy's Office of Science. This research was also supported by the NASA Ecostress project. We thank Siyan Ma for contributing to the collection and processing of US-Ton’s in situ data. USDA is an equal opportunity provider and employer.Peer reviewe

Plant-soil interactions and acclimation to temperature of microbial-mediated soil respiration may affect predictions of soil CO2 efflux

12 páginas, 5 figuras, 2 tablas.It is well known that microbial-mediated soil respiration, the major source of CO2 from terrestrial ecosystems, is sensitive to temperature. Here, we hypothesize that some mechanisms, such as acclimation of microbial respiration to temperature and/or regulation by plant fresh C inputs of the temperature sensitivity of decomposition of soil organic matter (SOM), should be taken into account to predict soil respiration correctly. Specifically, two hypotheses were tested: (1) under warm conditions, temperature sensitivity (Q10 ) and basal rates of microbial-mediated soil respiration (Bs20, respiration at a given temperature) would be primarily subjected to presence/absence of plant fresh C inputs; and (2) under cold conditions, where labile C depletion occurred more slowly, microbial-mediated soil respiration could adjust its optimal temperatures to colder temperatures (acclimation), resulting in a net increase of respiration rates for a given temperature (Bs20). For this purpose, intact soil cores from an oak savanna ecosystem were incubated with sufficient water supply at two contrasting temperatures (10 and 30 C) during 140 days. To study temperature sensitivity of soil respiration, short-term temperature cycles (from 5 to 40 C at 8 h steps) were applied periodically to the soils. Our results confirmed both hypotheses. Under warm conditions ANCOVA and likelihood ratio tests confirmed that both Q10 and Bs20 decreased signifi- cantly during the incubation. Further addition of glucose at the end of the incubation period increased Bs20 and Q10 to initial values. The observed decrease in temperature sensitivity (Q10) in absence of labile C disagrees with the broadly accepted fact that temperature sensitivity of the process increases as quality of the substrate decreases. Our experiment also shows that after 2 months of incubation cold-incubated soils doubled the rates of respiration at cold temperatures causing a strong increase in basal respiration rates (Bs20). This suggest that microbial community may have up-regulated their metabolism at cold conditions (cold-acclimation), which also disagrees with most observations to date. The manuscript discusses those two apparent contradictions: the decrease in temperature sensitivity in absence of labile C and the increase in microbial-mediated soil respiration rates at cold temperatures. While this is only a case study, the trends observed could open the controversy over the validity of current soil respiration models.This research was supported by the Kearney Soil Science Foundation and the US Department of Energy’s Terrestrial Carbon Program, grant No. DE-FG03-00ER63013. These sites are members of the AmeriFlux and Fluxnet networks. J Curiel Yuste received a Marie Curie Intra-European Fellowship (EIF) from de European Union for project MICROCARB (FP6- 2005-Mobility-5 # 041409-MICROCARB) while conducting this research.Peer reviewe

A comparison of new and existing equations for estimating sensible heat flux density using surface renewal and similarity concepts

The definitive version is available at: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1944-7973This paper describes two approaches for estimating sensible heat flux, using surface renewal and similarity concepts. One approach depends on a temperature structure function parameter and is valid in the inertial sub-layer. The other approach depends on the temperature standard deviation and operates when measurements are made above the canopy top, either in the roughness or inertial sub-layer. The approaches were tested over turf grass, rangeland grass, wheat, grape vineyard and nectarine and olive orchards. It is shown that the free convection limit expression for the standard deviation method holds for slightly unstable conditions. When surface homogeneity and fetch requirements are not fully met in the field, the results show that the equations based on surface renewal principles are more robust and accurate than equations exclusively based on similarity backgrounds. It is likely that the two methods require no calibration unless the canopy is heterogeneous. Under unstable conditions, the free convection limit equation, which depends on the temperature standard deviation, can provide on-line sensible heat flux density estimates using affordable battery-powered data logger with temperature data as the only input. The approach performed well when measuring near or well above the canopy top, thus, suggesting that the method is useful for long term monitoring over growing vegetation.This work was supported by the Ministerio de Ciencia y Tecnología under the Spanish project REN2001-1630 CLI, the DURSI of the Generalitat of Catalunya and the University of Lleida. Data from the grassland was supported by grants from the US Dept of Energy and the California Agricultural Experiment Station.Peer reviewe

Leaf area distribution and radiative transfer in open-canopy forests: implications for mass and energy exchange

Leaf area and its spatial distribution are key canopy parameters needed to model the radiation regime within a forest and to compute the mass and energy exchange between a forest and the atmosphere. A much larger proportion of available net radiation is received at the forest floor in open-canopy forests than in closed-canopy forests. The proportion of ecosystem water vapor exchange (ëE) and sensible heat exchange from the forest floor is therefore expected to be larger in opencanopy forests than in closed-canopy forests. We used a combination of optical and canopy geometry measurements, and robust one- and three-dimensional models to evaluate the influence of canopy architecture and radiative transfer on estimates of carbon, water and energy exchange of a ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forest. Three-dimensional model simulations showed that the average probability of diffuse and direct radiation transmittance to the forest floor was greater than if a random distribution of foliage had been assumed. Direct and diffuse radiation transmittance to the forest floor was 28 and 39%, respectively, in the three-dimensional model simulations versus 23 and 31%, respectively, in the one-dimensional model simulations. The assumption of randomly distributed foliage versus inclusion of clumping factors in a one-dimensional, multi-layer biosphereatmosphere gas exchange model (CANVEG) had the greatest effect on simulated annual net ecosystem exchange (NEE) and soil evaporation. Assuming random distribution, NEE was 41% lower, net photosynthesis 3% lower, total ëE 10% lower, and soil evaporation 40% lower. The same comparisons at LAI 5 showed a similar effect on annual NEE estimates (37%) and ëE (12%), but a much larger effect on net photosynthesis (20%), suggesting that, at low LAI, canopies are mostly sunlit, so that redistribution of light has little effect on net photosynthesis, whereas the effect on net photosynthesis is much greater at high LAI

On the temporal upscaling of evapotranspiration from instantaneous remote sensing measurements to 8-day mean daily-sums

The regular monitoring of evapotranspiration from satellites has been limited because of discontinuous temporal coverage, resulting in snapshots at a particular point in space and time. We developed a temporal upscaling scheme using satellite-derived instantaneous estimates of evapotranspiration to produce a daily-sum evapotranspiration averaged over an 8-day interval. We tested this scheme against measured evapotranspiration data from 34 eddy covariance flux towers covering seven plant functional types from boreal to tropical climatic zones. We found that the ratio of a half-hourly-sum of potential solar radiation (extraterrestrial solar irradiance on a plane parallel to the Earth’s surface) between 10:00 hh and 14:00 hh to a daily-sum of potential solar radiation provides a robust scaling factor to convert a half-hourly measured evapotranspiration to an estimate of a daily-sum; the estimated and measured daily sum evapotranspiration showed strong linear relation (r2 = 0.92) and small bias (-2.7%). By comparison, assuming a constant evaporative fraction (the ratio of evapotranspiration to available energy) during the daytime, although commonly used for temporal upscaling, caused 13 underestimation of evapotranspiration on an annual scale. The proposed temporal upscaling scheme requires only latitude, longitude and time as input. Thus it will be useful for developing continuous evapotranspiration estimates in space and time, which will improve continuous monitoring of hydrological cycle from local to global scales