Determination of CO2 and H2O fluxes of a vegetation surface by a micrometeorological method

It is a common fact that global climate change is caused by increasing greenhouse gases concentration in the atmosphere. There are lots of gas flux measurements over surfaces such as forest, soil, croplands etc. 10 % of global green house gas budget is caused by agricultural sector (FAO, 2012). In the world, however; studies about greenhouse gas fluxes resulting from agricultural sector are insufficient when compared with the forest lands. Each countries should calculate their national greenhouse gas budget by considering IPCC recommendations, which consist of three approaches. One of them (Tier 1) can be used by developing countries which do not have special emmision and storage factors for the calculation of GHG budget for agricultural activities until now. However, countries have different climate, soil and plant varieties. For this reason, together with the results obtained from studies by defined circumstances and developed models, the second (Tier 2) and third (Tier 3) alternatives of greenhouse gas calculation, which are recommended by IPCC, should be used. Therefore, countries are responsible to determine the greenhouse gas amount given to the atmosphere and taken back from the atmosphere according to their conditions and plant varieties. In our country, the number of studies conducted by internationally accepted methods and technology about greenhouse gas emissions and sink caused by agricultural crops are unfortuanetly insufficient. That's why, gas and energy flux changes between atmosphere and biosphere must be determined by measurements, which represent internationally accepted methods, instruments and standards. In this study, carbon dioxide and water vapor flux exchanges between the vegetative surface and atmosphere are measured by an internationally accepted micrometeorological method, namely the Eddy Covariance method (EC) during one growing period of watermelon in 2012 at Kırklareli Atatürk Soil, Water and Agricultural Meteorology Research Institue, in the Kirklareli city of Turkey. Main goal of the study is to determine greenhouse gas exchange arising from agricultural field for domestic crop conditions. Turkey is the second major watermelon producer with 20% production rate in the world after China, which has a 23% production rate (FAO, 2011). This product ratio has a big economical role in Turkey, although no study exists in Turkey or in the rest of the world about CO2 and H2O above watermelon vegetation, using the EC method. In this contex, carbon and water vapor fluxes above watermelon canopy were measured using EC method between plantation and harvest periods in the world for the first time. The EC method is widely used to measure gas fluxes within the atmoshperic boundary layer. After thechnological developements in 1990s, this technique has become one of the most used flux measurement methods. Corresponding technological developements let the wind velocity and gas concentration to be measured in 10 Hz interval. Covariance between the vertical wind velocity and gas concentration is the main principle of the method. Major assumptions of method can be expressed as the measurements are conducted inside the boundary layer, fetch/footprint is adequate, flux is fully turbulent, as well as the terrain is horizontal and uniform. Moreover, the instruments are able to detect very small variations. The method requires two main instruments, namely the 3D sonic anemometer and infrared gas analyzer. These intruments need to be mounted according to the fetch length/footprint, which are indicators whether the measured fluxes belong to the interested field. Generally, the fetch length can be determined as at least 100 times of plant height with respect to the horiontal distance between measurement point and the edge of the field. To analyze the flux data, additional meteorological variables such as precipitation, global solar radiation, photosynthetic photon flux density (PPFD), net radiation, soil water content, soil temperature etc. should be measured at the same experiment field. The flux can be obtained after the covariance calculation between the measured vertical wind velocity and gas concentration. Time series of the flux can have defective measuremets. This type of incorrect data measured by the open path infrared gas analyzer can be resulted from frequency responses, precipitation, wind direction and calm wind. Once these erroneous fluxes have been detected, they should be removed from the related time series. Then, corresponding corrections are to be applied to the flux data such as the tilt correction, rotation correction and Webb, Pearman, Leuning (WPL) correction. Finally; gaps in data should be filled. After this last process net ecosystem exchange (NEE) and latent heat flux (LE) data can be obtained definitevely. The above mentioned process is the same for the carbon and water vapour fluxes. For the carbon fluxes, however, an additional step is to be taken to obtain the ecosystem respiraiton (Reco). The NEE should be divided into Reco and gross primary production (GPP). The method is mathematically complex, and requires a lot of caution for setting up and processing data. Also, the necesarry instruments for the application of this method is costly. Although there are such disadvantages, the EC is one of the most directly and widely used methods in order to measure gas fluxes in the world. After the measurements and analysis of CO2 fluxes above watermelon surface for given period, the accumulated gross primary production (GPP), cumulative total ecosystem respiration (Reco) and cumulative net ecosystem exchange (NEE) were determined as 1160.2 g C m-2 (4246.3 g CO2 m-2), 846.35 g C m-2 (3097.6 g CO2 m-2) and 299.03 g C m-2 (-1094.45 g CO2 m-2); respectively. Daily mean GPP, NEE and Reco values were calculated as 8.99, -2.31 and 6.56 g C m-2, respectively. During the growing period, daily maximum GPP was determined as 17.64 g C m-2 and daily minimum GPP was found as 1.27 g C m-2. Maximum and minimum carbon amounts emmited to the atmosphere from watermelon surface by respiration activity were calculated as 10.08 and 2.91 g C m-2. Maximum carbon amount sinked from the atmosphere by the crop was measured as -9.09 g C m-2. Emmision factor for watermelon was determined as 6.56 g C m-2 d-1 (24 g CO2 m-2d-1). Additionally, daily avarage carbon sink was estimated as 2.31 g C m-2 (8.45 g CO2 m-2). In addition to the CO2 flux measurements in the growing period of crop, the water vapor amount given to the atmosphere (by transpiration and evaporation), which was determined by EC method and the actual evapotranspiration value was measured as 422.01 mm. Concordantly; daily total minimum and maximum ETa during the growing period were determined as 1.01 and 7.26 mm, respectively. Daily mean total ETa was calculated as 3.27 mm in the developing period of the watermelon plant. As a result, it is obvious that this type of measurements should be carried out for additional growing periods and the flux studies above all agricultural crops should be increased, especially in the developing countries.