Atmospheric Exchanges of Riparian Vegetation in a Semi-Arid Environment

The relationship between photosynthesis and transpiration from riparian vegetation in a semi-arid region is the primary focus of interest in this study. An eddy covariance system was used to measure fluxes of energy, momentum, and carbon over an extended period which included a monsoon season and a subsequent prolonged dry period. The photosynthetic portion of the measured CO2 flux was obtained by subtracting the mean value of nighttime CO2 flux (respiration) from the daytime CO2 flux, and the resulting photosynthesis estimates were then compared with transpiration and other variables. Because the data revealed a somewhat complex relationship between photosynthesis and transpiration on both an hourly and a daily basis, a further analysis was made involving both the calculation of the canopy conductance and a 'stand alone' version of the Simple Biosphere (SiB2) Model. The results showed the assumed relationship between canopy photosynthesis and canopy conductance used in SiB2 is inconsistent with observations, however Monteith's suggestion that the ratio of the CO2 concentration inside leaves to that outside leaves is approximately constant, which leads to a linear relationship between canopy conductance and photosynthesis, is consistent with observations.Digitized from paper copies provided by the Department of Hydrology & Atmospheric Sciences