WP4: Tower Flux measurements (Leader: Dr R. Harding + Lloyd, Moncrieff, Baxter, Williams)

We will make measurements of the fluxes of energy, water and CO2 from a small number of distinct arctic surface types using the Eddy Correlation (EC) method. These measurements provide estimates across a ‘footprint’ of a few hundred meters, a scale intermediate between the process measurements at the plant or plot level (WP3) and the aircraft measurements at the landscape scale (WP5). The EC measurements are essentially continuous, providing a description of the diurnal, seasonal and interannual variations. They should also capture transitional events, such as effluxes at snow melt, which may be missed by the intermittent measurements. These measurements are an essential component to hypothesis H7, landscape scaling, and H3, the importance of seasonality to the overall C budget and energy balance.

We will take measurements over the two main study areas, Abisko and Kevo. In each study area there will be a main tower, sampling mesic/dry tundra heath, and a secondary tower, sampling the second predominate land cover type (birch forest at Abisko and mire at Kevo). At the main masts we will use the Mk4 Hydra eddy correlation systems. This is a system designed by CEH and consists of fast response open-path, infra-red gas analyser, measuring water vapour and CO2 concentrations, that has been integrated into a Solent three-axis sonic anemometer. The raw data are stored by a low-power data logger and custom software has been developed to handle the data and make the flux calculations. At the Abisko birch forest mast we will also use a Mk4 Hydra, at the Kevo wetland site we will use an equivalent Licor 7500/Solent system. The systems will be mounted on extendable masts at 3m for the low vegetation and up to 9m for the birch forest. At each main site there will meteorological and soil climate measurements: air temperature, wind speed, humidity, radiation (4-components), PAR, snow depth, soil temperature (down to 1m), near surface soil moisture and water table. At the secondary sites there will be net radiation and soil climate measurements. Power supply is via solar panels and a wind turbine.

Deliverables
A. Continuous all year measurements of fluxes of heat, water vapour and CO2 from dry tundra heath at Abisko and Kevo study areas for 2 full years with associated footprint models (FM) (note for Abisko this extends the existing 2 yr record). (A paper in Glob. Ch. Biol.).
B. Continuous all year measurements of fluxes over the Abisko birch forest for 2 years, plus FM.
C. Continuous flux data over 2 active seasons (spring and summer) over the wetland site in Kevo. Associated continuous above and below ground climate measurements and FM.
D. Inter-comparison of fluxes both between and within study areas (including data from partners). (A paper in J.Geophys.Res.).
E. Analysis of drivers of temporal variability of patch scale fluxes. (A paper in Glob. Ch. Biol.).

Tasks
1. Identify suitable sites in study areas (in collaboration with WP3). July 2006.
2. Design and source meteorological measurements. Summer 2006.
3. Install climate measurements at sites. September/October 2006.
4. Install flux towers at Abisko (March 2007) and Kevo (April 2007).
5. Footprint modelling to estimate location of contributing surfaces.
6. Maintain flux towers (ongoing but with specific visits in autumn 2007 and 8 and spring 2008).
7. Decommission sites spring/summer 2009.
8. QC and gap fill data. Place on ACCESS and website data base. (six monthly).
9. Compare with chamber and aircraft measurements (in collaboration with WP7).