# Presentation: 203848

## Measurement of Storage Change and Mean Advective Flux of CO2 at Niwot Ridge, CO.

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_**Contact Information:**_
**U.S. Geological Survey M.S.413, Federal Center, Denver, CO. 80225    Phone: 303-236-5691****  Email:deander@usgs.gov****             **

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**Under low wind speed conditions and radiational cooling at night, a shallow thermal inversion develops near the surface and deepens with time (Stull, 1988). If terrain is not level, a katabatic wind (drainage flow) will develop. In a forest ecosystem, above canopy air will become decoupled from the flow just above and within canopy. In this case, drainage flow will carry respiration from foliage, boles, and soil downslope. Eddy covariance sensors above canopy  will not detect this flux and subcanopy sensors are likely to grossly underestimate it. **
**It was recently pointed out that U.S. forests situated in complex terrain (comprising about 1/3 of North America) are responsible for about 1⁄4 of the national carbon uptake (Schimel et al. 2002).This environment may be too complex for reliable eddy covariance measurements and a variety of other techniques, perhaps in some combination, will need to be employed to measure or model carbon  flux in these areas.**
**For the Niwot Ridge Ameriflux site, we propose the use of the combination of eddy covariance and mean profile measurements to estimate net ecosystem exchange (NEE) of CO****2****.**

- Measurement of Storage Change and Mean Advective Flux of CO2 at Niwot Ridge, CO.
-                                               D.E. Anderson1, A.A. Turnipseed2, R. Monson2, J.Sparks2, D.Stannard1., and E.P. Weeks1
-                                                                          1 U.S. Geological Survey, Denver, CO. and 2 University of Colorado, E.P.O. Biology

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**Figure 1: Aerial view of the Niwot Ridge, Ameriflux site (towers near center of view). The Continental Divide is in the background.**

**Introduction**

**Methods**
-     **Initial experimental design included two main tall towers (33m and 25m) and a subcanopy tower (8m). These towers were outfitted with sampling tubes (1/4" I.D. Teflon) and horizontal wind sensing sonic anemometers (Handar). Sampling tube inlets were positioned at 1,3,6,10,and 15m, and, at the top level on the tall towers. Intervals at the short tower were 1 & 6m. Tower positions formed an equilateral triangle (150m on a side) and was oriented such that the main towers were aligned with the average (westerly) wind direction. A fourth tower (8m tall with ports at 1 & 6m) was installed on the north side of the array in 2002 to help reduce noise in flux estimates due to spatial heterogeneity of CO****2****. With its inclusion, the array now has a square shape, with diagonals of 150m. **
**    ****NEE of CO****2**** for the forest may be computed from: **
**where, C is CO****2**** concentration, u and w are velocities in the x (horizontal) and z (vertical) distances; bars indicate mean, and (‘) is deviation from the mean. In this presentation, we discuss storage change flux and advective flux estimates. These fluxes will later be combined with turbulent flux measured with eddy covariance instruments for long term NEE estimates for a range of meteorological conditions. Storage and mean advection flux terms are computed from data gathered in the control volume shown in figure 2 (representing the 2001 configuration).**
**    ****An instrumentation and plumbing schematic is shown in figure 3. Air is continuously drawn through all lines at 1.5 SLPM. Inlets are outfitted with flow restrictors (0.6mm orifice diameter), to reduce vapor pressure and thereby avoid condensation in lines. 4L buffer volumes reduce random noise in sampling by introducing a 2-3 min. time constant. Air is diverted sequentially from each line to an infrared gas analyzer (Li6262 in 2001 and Li7000 in 2002) run in the absolute mode. Air passing through the analyzer is draw by a separate, low volume, rotary vane pump (~1 SLPM). The analyzer is automatically spanned and zeroed every 2 hrs. Sample air is brought to a constant temperature and is at a pressure of 150mb subambient. Each line is sampled at 1Hz for 30s, however, to ensure that air from the previous sample has been purged from the system, only the last 15s are recorded for computing an average concentration.  It takes 7 min. to sample all lines. Half-hour averages are calculated from these data.**

**Results**
-     **Continuous measurements covered most of the growing seasons of 2001 and 2002. Fluxes were found to be below measurement detection limits during the winter and will not be reported here.**
**    ****Total cumulative flux (storage + mean advection) is depicted in figure 4, for both 2001 and 2002. Due to breaks in the data and a late start in 2001, cumulative totals are likely higher than that depicted. We estimate that for 2002, cumulative respiratory emission detected by this method is above 25 mol m****-2 **** and greater than 15 mol m****-2 ****in 2001. This compares with an NEE of 6 mol m****-2  ****measured with eddy covariance instruments in 2001. It is not known why cumulative rates differ in the later part of the growing season between years. The difference may not be due to soil temperatures (also depicted in the figure) which appear rather similar. **
**    ****The dependence of flux on soil temperature is shown in figure 5. Absence of a stronger relation may indicate the influence of other factors such as intermittent turbulence which can break down the inversion and transport mass vertically (eddy covariance should detect these episodes).**
**    ****A dependence of flux on wind direction is depicted in figure 6. Wind direction during drainage flow events is westerly. As the subcanopy inversion strengthens (indicated by the potential temperature gradient), wind direction becomes more north-westerly (figure 7).**

**Figure 4.**

**Figure 5.**

**Figure 6.**

**Figure 7.**

**Concluding Remarks **
-     **Storage change and mean advective flux were calculated for a subalpine forest at the Niwot Ridge, CO. Ameriflux site which often experiences subcanopy katabatic winds at night. Measurements were based on mean profiles of CO****2**** and wind speed gathered from 14 levels from 3-4 towers, over two growing seasons. As expected, there was some dependence of flux on soil temperature and with wind direction.  Measurements of flux described here will be combined with eddy covariance data to arrive at more reliable estimates of NEE of CO****2****  and can potentially provide a methodology for estimating national carbon sequestration derived from non-flat terrain environments, dominating forested lands.**

**Figure 2.**

**Figure 3.**

**Literature**
-    Schimel, D., T. Kittel, S.Running, R.Monson, A. Turnipseed, and D. Anderson. 2002. Carbon     sequestration studied in western U.S. mountains. EOS, Trans. AGU 83:445-
-    Stull, R. 1988. Boundary Layer Meteorology. Kluwer, Boston.
**Acknowledgements: **This research is sponsored by U.S. Geological Survey core research program and funds from the Department of Energy’s NIGEC and TCP programs; and with the cooperation of the U.S Forest Service.