REGRESSION-MODELS FOR CALCULATING GAS FLUXES MEASURED WITH A CLOSED-CHAMBER

Portable closed chambers provide a valuable tool for measuring crop ph otosynthesis and evapotranspiration. Typically, the fates of change of CO2 and water vapor concentration are assumed to be constant in the s hort time required to make the closed-chamber measurement, and a linea r regression model is used to estimate the C0(2) and H2O fluxes, Howev er, due to the physical and physiological effects the measurement syst em has on the measured process, assuming a constant rate and using a l inear model may underestimate the flux. Our objective was to provide a model that estimates the CO2 and H2O exchange rates at the time of ch amber closure, We compared the linear regression model with a quadrati c regression model using field measurements from two studies, Generall y, 60 to 100% of all chamber measurement data sets were significantly nonlinear, causing the quadratic model to yield fluxes 10 to 40% great er than those calculated with the linear regression model, The frequen cy and degree of nonlinearity were related to the measured rate and ch amber volume, Closed-chamber data should be tested for nonlinearity an d an appropriate model used to calculate flux, The quadratic model pro vides users of well-mixed closed chambers an alternative to a simple l inear model for data sets with significant nonlinearity.