Partitioning overstory and understory evapotranspiration in a semiarid savanna woodland from the isotopic composition of water vapor (original) (raw)
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Geophysical Research Letters, 2010
In water-limited ecosystems, partitioning ecosystemscale evapotranspiration fluxes between plant transpiration and soil/canopy evaporation remains a theoretical and technical challenge. We used the Biosphere 2 glasshouse to assess partitioning of evapotranspiration across an experimentally manipulated gradient of woody plant cover using continuous measurements of near-surface variations in the stable isotopic composition of water vapor (d 2 H). Our technique employs a newly-developed laser-based isotope analyzer and the Keeling plot approach for surface flux partitioning. The applicability of the technique was verified by comparison to separate, simultaneous lysimeter and sap flow estimates of ET partitioning. The results showed an expected increase in fractional contribution of transpiration to evapotranspiration as woody cover increased-from T/ET = 0.61 at 25% woody cover to T/ET = 0.83 at 100% cover. Further development of this technique may enable field characterization of evapotranspiration partitioning across diverse woody cover gradients, a central issue in addressing dryland ecohydrological responses to land use and climate change.
Agricultural and Forest Meteorology, 2005
We describe a novel method for partitioning evapotranspiration (ET) from isotopic measurements of water vapor within large (4.86 m 3 ) plot-scale gas exchange chambers. Using this approach, the short-term (15-day) dynamics of transpiration (T) and evaporation (E) in experimental replicated stands of the invasive grass Eragrostis lehmanniana and the native Heteropogon contortus were assessed following a 39-mm irrigation event in semiarid grassland in southeastern Arizona, USA. Water vapor samples (20-40 mL each) were collected sequentially during a 6-min transient increase of vapor concentration inside the chambers and used to produce Keeling plots (isotope mixing relationships) for identification of the isotopic composition of ET and partitioning of component fluxes. The method worked well in plots free of grass cover and in the sparsely covered plots of E. lehmanniana. Keeling plot estimates of the isotopic composition of soil evaporation (d E ) in bare plots closely matched modeled values, lending strong support for the validity of the chamber approach. T/ET increased in stands of E. lehmanniana from 0.35 AE 0.07 on day 1 to 0.43 AE 0.08 on day 3 after the irrigation pulse, but decreased to 0.22 AE 0.05 by day 7 as the soil surface dried. Estimates of stand transpiration from the Keeling plot chamber method were positively correlated (Pearson's r = 0.76, p = 0.0004, n = 17) with independent estimates based on leaf-to-canopy scaling of stomatal conductance. We were unable to calculate T/ET on days 1 and 3 in plots of H. contortus because Keeling plot intercepts did not fall within the range of soil and canopy end-member isotope values. This likely occurred due to unaccounted effects of a wet litter layer on the estimation of d E . Our approach is useful for partitioning ET over a dynamic wetting event in semi-arid grassland at a scale relevant for experimental ecosystem studies, but requires further validation under a wide range of vegetation structures and environmental conditions. #
Hydrological Processes, 2010
In this study, we performed a partitioning of evapotranspiration (ET) under fully controlled conditions (climatic chamber) along growth of a tall fescue cover (Festuca arundinacea) into soil evaporation (Ev) and plant transpiration (Tr) by measuring their stable oxygen isotopic compositions (υ ET , υ Ev and υ Tr ). We showed that it was possible, under the chamber's particular conditions, to realize the partition without (1) making the hypothesis of steady state transpiration usually done in the field, nor (2) calculating υ Ev as a function of air relative humidity, soil water and atmospheric vapour isotopic compositions. The contribution of Ev to total ET decreased over the experiment from 100% (bare soil) to 94% [16 days after the seeding (DAS), 83% (28 DAS), 70% (36 DAS) and 5% (43 DAS)].
Evapotranspiration components determined by stable isotope, sap flow and eddy covariance techniques
Agricultural and Forest Meteorology, 2004
Understanding and modeling water exchange in arid and semiarid ecosystems is complicated by the very heterogeneous distribution of vegetation and moisture inputs, and the difficulty of measuring and validating component fluxes at a common scale. We combined eddy covariance (EC), sap flow, and stable isotope techniques to investigate the responses of transpiration and soil evaporation to an irrigation event in an olive (Olea europaea L.) orchard in Marrakech, Morocco. The primary goal was to evaluate the usefulness of stable isotope measurements of water vapor in the turbulent boundary layer for partitioning evapotranspiration under such dynamic conditions. The concentration and deuterium isotope composition (δ 2 H) of water vapor was collected from different heights within the ecosystem boundary layer of the olive canopy before and over several days following a 100 mm surface irrigation. 'Keeling plots' (isotope turbulent mixing relationships) were generated from these data to estimate the fractions of evaporation and transpiration contributing to the total evapotranspiration (ET) flux. Transpiration accounted for 100% of total ET prior to irrigation, but only 69-86% of ET during peak midday fluxes over the 5-day period following irrigation. The rate of soil evaporation and plant transpiration at the stand level was calculated from eddy covariance measurements and the evaporation and transpiration fractions from isotope measurements. Soil evaporation rate was positively correlated with daily atmospheric vapor pressure deficit (D), but transpiration was not. Component fluxes estimated from the isotope technique were then compared to those obtained from scaled sap flow measurements. Sap flow in multiple-stemmed trees increased following the irrigation, but large single-stemmed trees did not. We matched the source area for eddy covariance estimates of total ET fluxes with scaled sap flow estimates developed for the different tree types. Soil evaporation was determined from the difference between total ET and the scaled sap flow. Ecosystem-level transpiration and soil evaporation estimated by the isotope approach were within 4 and 15% of those estimated by scaled sap flow, respectively, for periods of peak fluxes at midday. Our data illustrate the utility of the isotope 'Keeling plot' approach for partitioning ET at the ecosystem scale on short time steps and the importance of accurate spatial representation of scaled sap flow for comparison with eddy covariance measurements of ET.
Examining evapotranspiration in a semi-arid region using stable isotopes of hydrogen and oxygen
Journal of Hydrology, 1990
Daily variation of the isotopic composition, both deuterium and oxygen-18, of water in foliage of a mixed Eucalyptus foecunda-Eucalyptus socialis, 'mallee' community was studied in relation to the isotopic composition of soil water and atmospheric water vapour, transpiration and evapotranspiration, and environmental conditions. The isotopic composition of water in foliage and the soil surface were enriched over adjacent stem and deeper soil water respectively, characteristic of water subject to evaporation. The composition of foliar water varied systematically over the day, being highest in the late afternoon, in response to lower relative humidity. The foliar water composition fitted a steady-state model of leaf water fractionation. It was found that the fraction of leaf water equilibrated with the atmosphere was at a minimum during the. middle of the day, fitting earlier observations of an inverse correlation of this fraction with transpiration rate. The equilibrated fraction was relatively high compared with previous transpiration studies, consistent with low transpiration rates. The soil water i s o t o p d e p t h profiles had not achieved steady state. Using existing models, we conclude that the foliage and soil compartments are exchanging water vapour, but that some complicated evaporation processes are occurring in the soil, probably as a result of daily temperature cycling. Examining the relationship of the two isotope tracers in the different compartments, the vapour transport fits a transport model in which it is strongly diffusion-limited, both within the soil and leaf. Because of some uncertainty as to the transport mechanisms involved, it is not clear what the relative contributions from soil and foliage 'are. The vegetation may be having a considerable influence on the total evaporation from the community, since the effects of the root system on the soil structure and soil water transport are not clear.
The use of stable isotopes to partition evapotranspiration fluxes into evaporation and transpiration
Acta Ecologica Sinica, 2010
It is crucial to partition evapotranspiration (ET) into evaporation (E) and transpiration (T) components for better understanding eco-hydrological processes and their underlying mechanisms, and improving the establishment and validation of hydrological models at the ecosystem scale. Traditional eddy covariance technique serves as a useful tool to estimate ET, but it encounters difficulties in quantifying the relative contribution of E and T to ET. Combining with eddy covariance technique, it is possible to partition ET based on the measurements of stable oxygen and hydrogen isotopes in liquid and vapor phases of water in the Soil-Plant-Atmosphere Continuum (SPAC) system. The key challenge is to precisely determine the oxygen-18 and deuterium isotopic compositions of ET (d ET), E (d E) and T (d T). d E can be estimated based on the Craig-Gordon model. d T is usually approximated by the d 18 O and dD of water in xylem or twig (d x), assuming d T equals d x under isotopic steady state (SSA). However, the SSA is only likely satisfied during midday in field conditions. The diurnal variations of d T is affected by isotopic composition of atmospheric water vapor (d v) and leaf water at the evaporating sites (d L,e), and relative humidity, resulting in the nonsteady-state behavior of d T at the sub-daily cycles. d ET can be estimated using the flux-gradient approach or the Keeling plot by measuring the vapor mixing ratio and d v at different heights in the surface layer. However, d v observations by the traditional cold trap/mass spectrometer method are limited to a coarse time resolution, leading to discrete time series of d ET. It is now possible to make in situ and high time resolution measurements of d v and to analyze a large number of plant and soil samples due to technical and instrumental advances in recent years. It provides an opportunity to improve the model prediction of d L,e , and more importantly, to calculate d T from d L,e without invoking the SSA. Combining with the flux-gradient approach or the Keeling plot technique, continuous d ET measurements can be made. It offers us a premise for accurate ET partitioning on diurnal time scale. In this review we introduced the recent advances, foci and challenges for studies on ET partitioning using the stable isotopes technique.
Frontiers in plant science, 2014
Semi-arid ecosystems contribute about 40% to global net primary production (GPP) even though water is a major factor limiting carbon uptake. Evapotranspiration (ET) accounts for up to 95% of the water loss and in addition, vegetation can also mitigate drought effects by altering soil water distribution. Hence, partitioning of carbon and water fluxes between the soil and vegetation components is crucial to gain mechanistic understanding of vegetation effects on carbon and water cycling. However, the possible impact of herbaceous vegetation in savanna type ecosystems is often overlooked. Therefore, we aimed at quantifying understory vegetation effects on the water balance and productivity of a Mediterranean oak savanna. ET and net ecosystem CO2 exchange (NEE) were partitioned based on flux and stable oxygen isotope measurements and also rain infiltration was estimated. The understory vegetation contributed importantly to total ecosystem ET and GPP with a maximum of 43 and 51%, respect...
Journal of Hydrology, 2013
Stable oxygen isotopes of water provide a valuable tracer for water movements within ecosystems and are used to estimate the contribution of transpiration to total ecosystem evapotranspiration (ft). We tested the Craig and Gordon equation against continuous field measurements of isotopic composition of evaporation and assessed the impact for partitioning evapotranspiration. Therefore, evaporation (E) and its isotopic signature (d 18 O E ) on bare soil plots, as well as evapotranspiration (ET) and its corresponding isotopic composition of (d 18 O ET ) of an herbaceous layer was measured with a cavity ring-down spectrometer connected to a soil chamber on a field site in central Portugal. We quantified the variation in d 18 O E arising from uncertainties in the determination of environmental input variables to the Craig and Gordon equation: the isotope signature (d 18 O e ) and the temperature at the evaporating site (T e ), and the kinetic fractionation factor (a k ). We could hence quantify ft based on measured d 18 O ET , modeled d 18 O E from observed soil water isotopic composition at the evaporating site (d 18 O e ), and modeled d 18 O of transpiration (d 18 O T ) from observed total soil water isotopic composition.
Contribution of transpiration to forest ambient vapour based on isotopic measurements
Global Change Biology, 1997
Using a simple isotope mixing model, we evaluated the relative proportion of water vapour generated by plant transpiration and by soil evaporation at two sites in the Amazon basin. Sampling was carried out at two different soil covers (forest and pasture), in a seasonal tropical rainforest at eastern Amazon where major deforestation is the result of land-use change, and compared to a less seasonal central Amazon forest. In both forests, vapour from transpiration was responsible for most, if not all, of the water vapour generated in the forest, while it could not be detected above the grassy pastures. Thus the canopy transpiration may be a major source of water vapour to the forest and perhaps to the atmosphere during the dry season. The results are discussed in relation to predictive models based on net radiation that usually are not able to distinguish between transpiration and evaporation.
Water
Partitioning evapotranspiration (ET) into its constituent fluxes (transpiration (T) and evaporation (E)) is important for understanding water use efficiency in forests and other ecosystems. Recent advancements in cavity ringdown spectrometers (CRDS) have made collecting high-resolution water isotope data possible in remote locations, but this technology has rarely been utilized for partitioning ET in forests and other natural systems. To understand how the CRDS can be integrated with more traditional techniques, we combined stable isotope, eddy covariance, and sap flux techniques to partition ET in an oak woodland using continuous water vapor CRDS measurements and monthly soil and twig samples processed using isotope ratio mass spectrometry (IRMS). Furthermore, we wanted to compare the efficacy of δ2H versus δ18O within the stable isotope method for partitioning ET. We determined that average daytime vapor pressure deficit and soil moisture could successfully predict the relative is...