PhD Thesis, 2007 - University College Cork

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Chapter 5 CO 2 fluxes The time series of day and night fluxes were divided into twelve bimonthly bins over the two-year dataset and filtered for predetermined realistic threshold values for the season. Approximately 15% of the data in 2003 and 2004 were rejected during rainfall periods and within one hour after rain. After post-processing and filtering, 43.4% of the CO 2 flux data for 2003 (~60% day and 30% night data) and 50.6% for 2004 (~69% day and 35% night data) was good and suitable for further analysis. Most night data that was rejected was outside the growing season. The gaps in the time series were filled with non-linear regression equations defined using the Curve Fitting Function of MATLAB 6.5 (MathWorks Inc., USA) software for monthly (May and June 2003, March and April 2004) or bimonthly periods. For day data, the best gap filling functions were rational functions of polynomials in one variable of different orders that relate the 30-minute CO 2 flux either with air temperature (January-February and May 2003, April 2004) or with Q (all other periods). For night data an exponential Q 10 gap filling function was defined for the full two-year period and related the half hour flux with the 20 cm deep soil temperature. In 2003, between Julian day 145 and 155 (25 th May to 4 th June) no data were logged due to an electricity outage. The missing meteorological data were replaced with the last 5 good days of data before the outage and the first 5 days of good data after the outage except for precipitation. Precipitation data were obtained from a comparison with the nearby Valentia weather station (~30 km west of the site, 51° 56' N, 10° 14' W) records. The EC system suffered a break, in 2003 between Julian day 141 and 177 (21 st May to 26 th June). The 36 days gap in the EC CO 2 flux was filled using gap filling equations based on meteorological measurements. Notably the missing May fluxes were determined with a function based on air temperature, defined for the first 21 good days of the month. The 26 missing flux days in June were calculated with a rational function of polynomials of degree one based on Q, derived from the averaging of the same type of function for May and July-August 2003. 73
Chapter 5 CO 2 fluxes 5.5 Results The photosynthetically active radiation (Figure 5.1a) followed the expected pattern, with the highest monthly average values (for the day hours) in June and the lowest in December. The daytime annual Q average was 482 (2003) and 458 µmol m -2 s -1 (2004). Precipitation was abundant and frequent throughout the year (Figure 5.1b). The total annual precipitation was 2510 mm (2003) and 2356 mm (2004). The monthly precipitation ranged between a low of 40 mm in August 2003 to a high of 540 mm in November 2003. In 2004 the monthly variation was less extreme than in 2003 with no month experiencing rainfall less than 95 mm. At the Valentia weather station the rainfall measured in both 2003 and 2004 was slightly below the 30-year average (1961-1990). Figure 5.1 (a) Monthly Q, averaged for the daytime and (b) monthly precipitation during 2003 and 2004. Daily air and soil temperatures (Figures 5.2a and 5.2b) confirmed a very mild climate with little variation throughout the year. The annual average air temperature was 10.51 (2003) and 10.40 ºC (2004). T a average during the May to September period was 13.60 (2003) and 13.59 ºC (2004); in November-February it was 7.40 (2003) and 7.84 ºC (2004). The daily soil temperature at 20 cm depth was between 5 and 18 ºC. The WT (Figure 5.2c) remained close to the surface throughout the year. The range of WT was from 2 cm above the LL to about 15 cm below the LL surface. 74
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Department of Civil and Environment
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Declaration I declare that this the
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A comparison between the four years
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Table of contents DECLARATION I ABS
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4.3.2 Vegetation survey 43 4.3.3 En
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References Campbell, D. R. & Rochef
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References Friborg, T., Soegaard, H
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References Horst, T. W. & Weil, J.
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References Lafleur, P. M., Hember,
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References Malmer, N. 1986. Vegetat
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References Økland, R. H., Økland,
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References Shimoyama, K., Hiyama, T
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References Tahvanainen, T. & Tuomal
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References Vourlitis, G. L. & Oeche
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Appendix 1 Four-year CO 2 fluxes Da
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Appendix 1 Four-year CO 2 fluxes 5
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PhD Thesis, 2007 - University College Cork

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