Experimental site and methods

238 R. Leuning et al. Agricultural and Forest Meteorology 104 2000 233–249 the best least-squares fit and sensitivity of the solution to errors in the matrix operator D D D . Using the criterion α= 0.01, six is the maximum number of source lay- ers that can be obtained from the eight concentration measurement heights used in this study. However, the number of layers finally adopted was reduced to five because errors in the measured scalar concentrations also contributed to instability of the inversion analysis.

3. Experimental site and methods

3.1. Study site Measurements were made in a rice paddy located at the agricultural station of Okayama University 34 ◦ 32 ′ N, 133 ◦ 56 ′ E from 6 to 13 August 1996. The field measured ∼300 m×300 m and was surrounded by similar rice fields in all directions, providing a fetch of 400 m in the prevailing SE wind direction. Average plant height was 0.72 m during the obser- vation period, row spacing was 0.29 m and leaf area index measured using a canopy analyser LAI-2000 LiCor Inc., Lincoln, NE, USA was 3.1±0.3 S.D.. The normal weekly irrigation cycle consisted of flood- ing for 4 days followed by a drained period of 3 days. The field was drained on the afternoon of 6 August until midday on 9 August, and the mean water depth during flooding thereafter was 0.08–0.10 m. 3.2. Measurement of temperature, water vapour, CO 2 and CH 4 profiles Temperature and relative humidity profiles were measured using Humitter sensors Vaisala Oy, Helsinki, Finland housed in double-walled radi- ation shields ventilated at 3 m s − 1 . Air inlets to the radiation shields were located at eight heights above the water, 0.12, 0.24, 0.36, 0.48, 0.60, 0.72, 1.10 and 2.40 m The sensors were calibrated using a high-precision temperaturehumidity chamber to a precision of 0.05 ◦ C in both temperature and dewpoint. Before the experiment, the sensors were also placed at the same height to provide an inter-calibration of the instruments. Concentrations of CO 2 and CH 4 were measured using non-dispersive infrared analysers, an LI-6251 LiCor Inc., Lincoln, NE USA for CO 2 , and a GA- 360E Horiba, Kyoto, Japan for CH 4 Miyata et al., 2000. The latter instrument was equipped with an air pre-conditioner to minimise the interference of non- methane hydrocarbons and water vapour in methane analysis Harazono et al., 1998. The methane analyser was calibrated between 09:00–10:00 and 17:00–18:00 hours each day using a reference cylinder of high grade air containing 1.7 ppmv CH 4 Takachiho, Tokyo, Japan. The CO 2 analyser was calibrated at the same time using two reference cylinders with 350 and 400 ppmv CO 2 in N 2 Takachiho, Tokyo, Japan. Air was sampled for gas analysis at the same eight heights as the temperature sensors, plus an extra line for reference air at 2.50 m. For each line, a diaphragm pump sampled air continuously through 10 mm i.d. nylon tubing, through an ice-trap to reduce moisture content, into a cylindrical PVC buffer 70 dm 3 in vol- ume, and then to a T-junction. A tube connected to the second arm of the T junction was immersed in a water tank 600 mm deep to control the air pressure and flow rate in the sampling line, and to vent excess air when not required for gas analysis. The third arm of each T-junction was connected to a solenoid valve and man- ifold to permit selection of each sampling line in turn for gas analysis. Air from a selected line was passed through flow meters, dried to a dew point of 2 ◦ C us- ing a Peltier cooler condenser, before being split and passed through the CO 2 and CH 4 analysers. For CO 2 analysis, reference and sample air lines were brought to the same pressure before the analyser by venting excess air through water bubblers 60 mm deep. For CH 4 , reference and sample air were passed to the pre-conditioning unit. A full sampling sequence was completed in 30 min. The standard error of concen- trations measured by the CH 4 analyser was 2.5 ppbv when it was used in fast-response mode and 1.0 ppbv in slow-response mode. The corresponding figure for the CO 2 analyser is 0.2 ppmv. These error estimates are more relevant than the absolute errors induced by uncertainties in the calibration gases because the anal- ysis of sourcesink distributions utilises profiles of the difference in concentration between each level and a reference level Eq. 6. 3.3. Turbulence measurements A miniature 3-D sonic anemometer with a 50 mm path length Kaijo Denki, DAT 395 was placed at R. Leuning et al. Agricultural and Forest Meteorology 104 2000 233–249 239 various heights within the canopy to measure σ w . Friction velocity, u ∗ , was measured using another 3-D sonic anemometer with path length of 0.15 m and installed at a height of 2.2 m Solent 1021R, Gill Instruments Ltd., Lymigton, UK. The two sets of measurements were combined to develop a composite profile of σ w u ∗ . as a function of zh c , where z is measurement height above water when the paddy was flooded or above ground when it was drained. Fluxes of sensible heat, H, water vapour, E, and CO 2 , F CO 2 at 2.2 m above the ground were mea- sured using the eddy covariance method Miyata et al., 2000. Fluctuations of the three wind velocity components and of air temperature were measured with a Solent RS3A sonic anemometer. Sonic virtual temperature fluctuations were corrected for variation in the speed of sound with air density according to Hignett 1992. A fast response, open-path infrared gas analyser with a 0.20 m span E009, Advanet Inc., Okayama, Japan was installed at the same height as the sonic anemometer with a horizontal separation of 0.17 m to measure the fluctuations in the CO 2 and water vapour concentrations. Miyata et al. 2000 provide details of corrections to account for the high-frequency losses in eddy covariance measurements resulting from path-averaging and instrument separation Moore, 1986; Leuning and Moncrieff, 1990. Corrections to eddy fluxes aris- ing from density fluctuations due to H and E Webb et al., 1980, and for the cross-sensitivity of the CO 2 gas analyser to water vapour Leuning and Mon- crieff, 1990; Leuning and Judd, 1996 were also applied. Methane fluxes above the canopy were estimated using classical flux-gradient relationships as de- scribed in detail by Miyata et al. 2000. In that paper, both the friction velocity and CO 2 were used as ‘tracers’ to evaluate the eddy diffusivity, K, required in F CH 4 = − ρ a K M CH 4 M a ds CH 4 dz 14 where s CH 4 is the mixing ratio of CH 4 relative to dry air and ρ a is the density of dry air, and M a and M CH 4 are the molecular masses of dry air and CH 4 , respec- tively.

4. Results