Evaluation of land surface characteristics, CO2 flux and surface energy partitioning during Rice Paddy growing season and Fallow period in Central Taiwan

Fluxes of turbulent surface energy, CO₂ and land surface characteristics are estimated during rice paddy growing season and subsequent fallow period at central Taiwan in order to understand and to quantify the differences between these two different land use systems. As a direct method to estimate the required parameters an eddy covariance system (EC) consists of Li-7500 open path gas analyzer for estimating the fluxes of water vapor and CO₂ and a CSAT3 (Campbell Scientific) sonic anemo-thermometer to estimate the three dimensional wind characters are used with a measurement frequency of 20 Hz. The radiation fluxes viz., incoming and outgoing shortwave and long wave radiations wee measured through solar infrared radiation station (SIRS) system. Ground heat flux was estimated through soil heat flux plates. All the signal data were logged in CR5000 data logger (Campbell Scientific) and averaged over 10 minutes interval. During rice paddy the period of measurement starts with maximum tillering stage of the rice paddy crop and last up to maturity stage. The corrections imposed on the observed EC data during rice paddy growing periods were; coordinate rotation, WPL correction, canopy heat storage correction, advected heat flux and photosynthetic heat flux correction as a post processing measure to ensure data quality and energy balance closure to the maximum possible extent. After imposing the relevant correction the energy closure gap improved significantly. The partitioning of available energy into turbulent flux shows that during rice paddy majority of the available energy is converted into latent heat than sensible heat. This is due to the presence of the flood water and reduced crop canopy resistance for the movement of water vapor as influence by the actively opening of stomata during day time. Where as during fallow period there is overlap with respect to the partitioning between latent and sensible heat flux. The corresponding Bowen ratio for the two observational periods are 0.12 and 0.57 respectively which is significantly different than the default value adopted for AIRMOD wherein the value is 0.3. With regards to the flux of CO₂ rice paddy behave as a CO₂ sink, where as fallow land behaves as a net CO₂ source throughout the period. The fluxes exerted a clear diurnal pattern throughout the measurement periods. The albedo measure during rice paddy is found to be 0.09 and 0.13 for rice paddy and fallow period. The measured albedo highly different from the values employed in meteorological models wherein they adopts a constant value of 0.14 in AIRMOD and 0.18 used in MM5 and WRF. Similarly the calculated surface emissivity during both the period are 0.86 and 0.93, where as the value employed in MM5 and WRF is 0.92. With regards to the resistance to evaporation, fallow period exerts more surface resistance to water vapor due to the presence of dry compact soil surface and limited soil moisture than rice paddy environment.