EVALUATION OF THE PORTABLE CHAMBER TECHNIQUE FOR MEASURING CANOPY GAS-EXCHANGE BY CROPS

An experiment was conducted to evaluate the portable chamber technique for concurrent measurement of canopy evapotranspiration (ET) and carbon exchange rate (CER) and to validate the ET measurements using weighing lysimeters. Hourly gas exchange measurements were made on a half-day diurnal basis over a wide range of leaf area indices (0.2-3.8 m2 m-2) on peanut (Arachis hypogaea L.) for both irrigated and dry soil conditions. The chamber technique used the LI-COR LI-6200 portable photosynthesis system with an open leaf chamber inside the canopy chamber. Measurement of vapor pressure was achieved with the leaf chamber humidity sensor and thermocouple, while carbon dioxide was cycled through the- system infra-red gas analyzer. Full-sun and dark CER measurements were made to compute gross photosynthesis (PG). Based on replicated measurements, the average coefficients of variation for PG and ET were 15% and 17%, respectively. The field and lysimeter sites gave comparable values of ET and PG. The PG responses to varying light and LAI conditions were consistent with previously cited values. Computed chamber ET must be extrapolated back to the time the chamber is closed to obviate underestimation of the ET rate (1% s-1) owing to the increase in vapor pressure in the chamber. A correction in ET for reduced light owing to radiation transmission losses appears to be unnecessary. Under varying light conditions where measurements were intentionally made during the sunny periods, the chamber overestimated the instantaneous lysimeter ET. Under clear sky conditions, chamber ET compared well with both instantaneous and hourly lysimeter ET. In the latter comparison, errors were less than 0.13 mm h-1 with and R2 of 0.90. This good agreement was consistent over the morning hours, across the range of leaf area indices, and for both wet and dry soil conditions.