Abstract

1. NEESPI Domain Fyodorovskoe Flux Towers Figure 1: VIC overview and the VIC lake and wetland algorithm schematic. Figure 3: Methane model of Walter et al (2001a). The model is forced by soil temperature and water table depth (which will come from the extended VIC model) and NPP (which will come from the BETHY and LPJ models). In Walter et al (20001a; b) global wetland extent was prescribed, however in this work it will be predicted by the VIC lake and wetlands model. Northern Eurasian wetlands and the carbon cycle: Model estimates of carbon storage and methane emissions Theodore J. Bohn1, KrishnaVeni Sathulur2, Erika Podest3, Dennis P. Lettenmaier1, Laura C. Bowling2, Kyle McDonald3 1University of Washington, Seattle, Washington, USA; 2Purdue University, Lafayette, Indiana, USA; 3JPL-NASA, Pasadena, California, USA American Geophysical Union Fall Meeting, San Francisco, CA, USA, Dec 10-15 2006 Abstract The Eurasian Arctic drainage constitutes over ten percent of the global land area, and stores a substantial fraction of the terrestrial carbon pool in its soils and boreal forests. Specifically, boreal forests in this region constitute an estimated carbon sink of 0.5 Pg/y. However, assessments of carbon storage and fluxes in this region, and their role in climate change, vary considerably due to large uncertainties in the extent of wetlands, which both store carbon as peat and emit carbon as methane. Accurate estimates of wetland extent have been confounded by insufficient resolution of satellite imagery and poor coverage of in situ observations. In this study we refine these estimates of wetland extent, carbon storage, and methane emissions using a system of linked large-scale models of hydrology, terrestrial carbon dynamics, and methane emissions. Large-scale hydrology comes from the Variable Infiltration Capacity (VIC) hydrological model, which includes an updated lake/wetland parameterization that estimates the water table depth as a function of both lake level and wetland soil moisture. Fast ecosystem processes such as photosynthesis and respiration are simulated via the Biosphere Energy-Transfer Hydrology (BETHY) terrestrial carbon model. Methane emissions in areas of open water or saturated soil are simulated with the Walter-Heimann (WHM) methane model. We validate this modeling system with respect to in situ observations of soil moisture and temperature, and fluxes of CO2 and methane at flux towers at Fyodorovskoe, Russia, over the period 1998-1999. 2. Model Validation: Fyodorovskoe Flux Towers c. CO2 Flux components Figures 2.c.1 and 2.c.2 show simulated, observed, and inferred 5-day average carbon fluxes for the two sites, respectively. Since the flux towers measure only net CO2 flux from the atmosphere (net ecosystem exchange, or NEE), we inferred the actual respiration and NPP by assuming that night-time NEE is representative of the average soil respiration rate throughout the day, and subtracting this from day-time NEE to obtain NPP. Simulated and results agree with observations in the general shape of the seasonal cycle. Several patterns are evident at the two sites: first, observed NEE exhibits considerable scatter during the growing season, despite having been aggregated to 5-day averages. Both our inferred respiration and NPP exhibit this scatter, but examination of the original half-hourly record shows that night-time NEE is considerably more variable than day-time NEE, implying that our inferring daily respiration from night-time NEE may be subject to large errors. These fluctuations may arise from advection via turbulent fluxes (Alexander Oltchev, pers. comm.), or alternatively they might occur in response to precipitation events, in which infiltrating water forces accumulated CO2 out of soil pore spaces (Eric Wood, pers. comm.). Second, BETHY appears to be under-simulating respiration at the forest site and over-simulating respiration at the bog site (this is more clearly expressed in scatter plots 2.c.3 and 2.c.4, for the forest and bog sites, respectively). This may be a matter of incorrect vegetation or soil parameters. 2.c.1. 2.c.2. Located in the Central Forest Biosphere Reserve in Russia’s upper Volga basin, the Fyodorovskoe flux towers have been in operation since 1998. Meteorological and eddy flux variables have been recorded at both bog and forest sites. Here we present the results of point tests in which observed meteorological forcings over the period 1998-1999 (with a 3-year spin-up) drove both VIC and a stand-alone version of BETHY. VIC’s daily estimates of soil temperature and water table position, and BETHY’s daily estimates of net primary productivity (NPP), were used as inputs to the Walter-Heimann methane model (WHM). The results are shown below. 1. Modeling Approach • Land Surface Hydrology Model • Variable Infiltration Capacity (VIC) Model (Liang et al. 1994) • water and energy balance closure • macroscale • spatially-distributed • land cover classification sub-grid variability • recent additions for cold land processes (Cherkauer et al. 2003) • implemented in arctic regions by Bowling et al. (2000) and Bowling et al. (2003) • lake energy balance component builds on work of Hostetler and Bartlien (1990) and Hostetler (1991) • lake/wetland model (Bowling, 2002) handles changes in lake extent 2.c.3. 2.c.4. a. Soil Temperature 2.d.1: Annual Carbon Fluxes Figures 2.a.1 and 2.a.2 show simulated and observed soil temperature at 15, 50, and 100 cm depths, for the forest and bog sites, respectively. The results agree quite well with observations at shallower depths, but at deeper depths VIC appears to have a larger seasonal cycle than the observed temperatures. This may result from inaccurate soil parameters; we are still optimizing the calibration here. 2.a.1. 2.a.2. d. Annual Carbon Fluxes Annual total fluxes were estimated for each site, as shown in table 2.d.1. If we neglect the export of DOC leached from the soil, we can assess whether the systems are sinks or sources of atmospheric carbon. Our simulations indicate that the old forest site is a net sink of both atmospheric CO2 (459 g C/m2y) and methane (1250 mg C/m2y), while the bog site is a net source for both CO2 (134 g C/m2y) and methane (433 mg C/m2y). The CO2 fluxes run counter to our expectations, but are consistent with BETHY’s under-simulating respiration at the forest site and over-simulating respiration at the bog site. However, the methane fluxes, calculated by WHM, are consistent with our expectation that the shallow water table during the growing season can lead to stronger methane emissions. Figure 2: Model framework used in this study. • Model Framework • based on framework of Joint Simulation of Biosphere Atmosphere Coupling (JSBACH) at Max Planck Institute, Hamburg • represents feedbacks between the physical climate system and land surface processes • modular framework: allows components of land surface model to be run offline (this project) or online • fast vegetation processes: BETHY • slow vegetation processes: LPJ • combination of land surface, photosynthesis and plant respiration schemes (VIC+BETHY) forms the basic coupled model; LPJ describes slow changes in the distribution of vegetation - - Conclusions / Future Work • Future Work: • Continue development of the parameterization of spatial variation of the water table in VIC • Finish the linking of VIC, BETHY, LPJ, and the Walter-Heimann methane model • Add simulations of DOC leaching and aquatic NPP • Validate these models against historical observations • Validate landcover classifications against in situ observations • Use climate model outputs to drive predictions of future lake/wetland extent and carbon cycling in Northern Eurasia over the next century • While this is a work in progress, we can make the following conclusions: • Although further refinement is needed, we can make reasonable predictions of carbon fluxes in forests and bogs. • Using NEE alone to validate a carbon budget can be somewhat imprecise, because it is the difference between two terms with large variances. Simultaneous measurements of several flux terms (e.g NEE, NPP, Rh, DOC export, etc.) are essential for constraining errors in carbon budgets. b. Water Table and Methane Figures 2.b.1 and 2.b.2 show water table position and methane emissions for the two sites. While both sites have relatively shallow water tables, the bog site’s water table remains shallow for a greater portion of the beginning and end of the growing season, resulting in larger spikes in the methane emissions curve. 2.b.1. 2.b.2. • Methane Model • Walter and Heimann(2000) with modifications described in Walter et al (2001a ) • soil methane production, and transport of methane by diffusion, ebullition, and through plants modeled explicitly • methane production occurs in the anoxic soil: bottom of the soil column to the water table • methane production rate controlled by soil temperature and NPP • time evolution of soil temperature will come from VIC REFERENCES Bowling, L.C., D.P. Lettenmaier and B.V. Matheussen, 2000. 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