Goulden Michael L.

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Michael L.

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Now showing 1 - 4 of 4
  • Article
    Combined measurement and modeling of the hydrological impact of hydraulic redistribution using CLM4.5 at eight AmeriFlux sites
    (Copernicus Publications on behalf of the European Geosciences Union, 2016-05-17) Fu, Congsheng ; Wang, Guiling ; Goulden, Michael L. ; Scott, Russell L. ; Bible, Kenneth ; Cardon, Zoe G.
    Effects of hydraulic redistribution (HR) on hydrological, biogeochemical, and ecological processes have been demonstrated in the field, but the current generation of standard earth system models does not include a representation of HR. Though recent studies have examined the effect of incorporating HR into land surface models, few (if any) have done cross-site comparisons for contrasting climate regimes and multiple vegetation types via the integration of measurement and modeling. Here, we incorporated the HR scheme of Ryel et al. (2002) into the NCAR Community Land Model Version 4.5 (CLM4.5), and examined the ability of the resulting hybrid model to capture the magnitude of HR flux and/or soil moisture dynamics from which HR can be directly inferred, to assess the impact of HR on land surface water and energy budgets, and to explore how the impact may depend on climate regimes and vegetation conditions. Eight AmeriFlux sites with contrasting climate regimes and multiple vegetation types were studied, including the Wind River Crane site in Washington State, the Santa Rita Mesquite savanna site in southern Arizona, and six sites along the Southern California Climate Gradient. HR flux, evapotranspiration (ET), and soil moisture were properly simulated in the present study, even in the face of various uncertainties. Our cross-ecosystem comparison showed that the timing, magnitude, and direction (upward or downward) of HR vary across ecosystems, and incorporation of HR into CLM4.5 improved the model-measurement matches of evapotranspiration, Bowen ratio, and soil moisture particularly during dry seasons. Our results also reveal that HR has important hydrological impact in ecosystems that have a pronounced dry season but are not overall so dry that sparse vegetation and very low soil moisture limit HR.
  • Article
    Drought legacies influence the long-term carbon balance of a freshwater marsh
    (American Geophysical Union, 2010-09-30) Rocha, Adrian V. ; Goulden, Michael L.
    Experimental manipulations provide a powerful tool for understanding an ecosystem's response to environmental perturbation. We combined paired eddy covariance towers with an experimental manipulation of water availability to determine the response of marsh carbon balance to drought. We monitored the Net Ecosystem Exchange of CO2 (NEE) in two ponds from 2004 to 2009 at the San Joaquin Freshwater Marsh (SJFM), and subjected one of the ponds to a yearlong drought treatment in 2007. The two ponds experienced similar flooding and environmental regimes before and after the drought, ensuring that differences between ponds were largely attributable to the 2007 drought. Drought substantially reduced surface greenness, as measured by the Enhanced Vegetation Index (EVI) and photosynthetic carbon sequestration, primarily by inhibiting leaf area development. Respiratory carbon losses were less influenced by drought than photosynthetic carbon gains. The effect of the drought lasted several years, with delayed leaf area development and peak carbon uptake rates during the subsequent year, and reduced leaf area for a couple of years. The combined effect of the drought and legacy effects created an overall loss of carbon that was equivalent to 4 years of the maximum annual carbon sequestration observed over a decade. Our results indicate that drought can have long-term impacts on ecosystem carbon balance and that future projected drought increases in Southern California will have a negative impact on marsh carbon sequestration.
  • Preprint
    The application of δ18O and δD for understanding water pools and fluxes in a Typha Marsh
    ( 2011-05) Bijoor, Neeta S. ; Pataki, Diane E. ; Rocha, Adrian V. ; Goulden, Michael L.
    The δ18O and δD composition of water pools (leaf, root, standing water, and soil water) and fluxes (transpiration, evaporation) were used to understand ecohydrological processes in a managed Typha latifolia L. freshwater marsh. We observed isotopic steady state transpiration and deep rooting in Typha. The isotopic mass balance of marsh standing water showed that evaporation accounted for 3% of the total water loss, transpiration accounted for 17%, and subsurface drainage accounted for the majority, 80%. There was a vertical gradient in water vapor content and isotopic composition within and above the canopy sufficient for constructing an isotopic mass balance of water vapor during some sampling periods. During these periods, the proportion of transpiration in evapotranspiration (T/ET) was between 56 ± 17% to 96 ± 67%, and the estimated error was relatively high (>37%) due to non-local, background sources in vapor. Independent estimates of T/ET using eddy covariance measurements yielded similar mean values during the Typha growing season. The various T/ET estimates agreed that transpiration was the dominant source of marsh vapor loss in the growing season. The isotopic mass balance of water vapor yielded reasonable results, but the mass balance of standing water provided more definitive estimates of water losses.
  • Preprint
    Reconciling carbon-cycle concepts, terminology, and methods
    ( 2006-01-06) Chapin, F. Stuart ; Woodwell, G. M. ; Randerson, James T. ; Rastetter, Edward B. ; Lovett, G. M. ; Baldocchi, Dennis D. ; Clark, D. A. ; Harmon, Mark E. ; Schimel, David S. ; Valentini, R. ; Wirth, C. ; Aber, J. D. ; Cole, Jonathan J. ; Goulden, Michael L. ; Harden, J. W. ; Heimann, M. ; Howarth, Robert W. ; Matson, P. A. ; McGuire, A. David ; Melillo, Jerry M. ; Mooney, H. A. ; Neff, Jason C. ; Houghton, Richard A. ; Pace, Michael L. ; Ryan, M. G. ; Running, Steven W. ; Sala, Osvaldo E. ; Schlesinger, William H. ; Schulze, E.-D.
    Recent patterns and projections of climatic change have focused increased scientific and public attention on patterns of carbon (C) cycling and its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric CO2. Net ecosystem production (NEP), a central concept in C-cycling research, has been used to represent two different concepts by C-cycling scientists. We propose that NEP be restricted to just one of its two original definitions—the imbalance between gross primary production (GPP) and ecosystem respiration (ER), and that a new term—net ecosystem carbon balance (NECB)—be applied to the net rate of C accumulation in (or loss from; negative sign) ecosystems. NECB differs from NEP when C fluxes other than C fixation and respiration occur or when inorganic C enters or leaves in dissolved form. These fluxes include leaching loss or lateral transfer of C from the ecosystem; emission of volatile organic C, methane, and carbon monoxide; and soot and CO2 from fire. C fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However, even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological advances have led to a diversity of approaches to measuring C fluxes at different temporal and spatial scales. These approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully specifying the fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we provide a less ambiguous framework for understanding and communicating recent changes in the global C cycle. Key words: Net ecosystem production, net ecosystem carbon balance, gross primary production, ecosystem respiration, autotrophic respiration, heterotrophic respiration, net ecosystem exchange, net biome production, net primary production.