REMOTE AND REAL-TIME SENSING OF CANOPY TRANSPIRATION AND CONDUCTANCE - COMPARISON OF REMOTE AND STEM-FLOW GAUGE METHODS IN SOYBEAN CANOPIES AS AFFECTED BY SOIL-WATER STATUS

A newly developed remote and real-time method for estimating transpiration rates and conductance of crop canopies is examined in comparison with concurrent measurements by the stem flow gauge method. The remote method is based on the energy balance of a plant canopy, which uses the net radiation absorbed by the canopy and the remotely sensed canopy temperature as key inputs. Measurements were made on drought-stressed, waterlogged, and periodically-irrigated soybean canopies. Canopy transpiration values used for verification of the remote method were calculated from the mean transpiration rates per unit leaf area measured by the hand-made stem flow gauges, and the leaf area index obtained by destructive sampling. The raw measurements by the gauge show that the transpiration values for individual plants were well-synchronized and responded very well to the change of environmental factors such as solar radiation and windspeed. Although the plant-to-plant difference was dominated by the difference in the leaf area, it was larger under clear sky conditions than under cloudy conditions. Canopy transpiration values derived from the remote method are compared with those by the stem flow gauge method from three different aspects: 1) diurnal time course for a particular canopy, 2) one-to -one comparison of ten-minute mean values for all soil water conditions, and 3) daily total values for all soil and meteorological conditions. The results indicated that the remote method provides reasonable estimates of canopy transpiration and conductance over a wide range of soil water status and micrometerological conditions on an instantaneous or daily basis. The method allows continuous measurements of these canopy parameters under natural conditions with neither plant sampling nor disturbing the micrometeorological environment of the canopy.