A simplified method for remote sensing of daily canopy transpiration - A case study with direct measurements of canopy transpiration in soybean canopies

Daily values of actual canopy transpiration were obtained by stemflow heat balance method (SFG) in drought-stressed, waterlogged, and periodically-irrigated soybean canopies (Glycine max Merr.). Spectral and thermal infrared measurements and basic meteorological factors were obtained over the canopies under a wide range of sky conditions. The potential canopy transpiration (Tr(p)) was found to be linearly correlated to the product of an instantaneous measurement of spectral vegetation index (SAVI) and daily sum of incoming solar radiation (R(s)), as Tr(p) = alpha SAVI R(s). The proportionality constant alpha was 0.28 for soybean canopies. On the basis of this simplified relation, a simple method was proposed to estimate the daily values of actual canopy transpiration (Tr(a)). The method utilizes either instantaneous measurements of plant and air temperature differential (t(plant) - t(air)) or CWSI (Crop Water Stress Index) around mid-day as a major input. Both t(plant) - t(air) and CWSI can be estimated by thermal remote sensing. Values of Tr(a) estimated by models using the instantaneous measurements of either t(plant) - t(air) or CWSI were found to be well correlated to those measured by SFG. Results showed that the simple model based on a combined use of spectral and thermal data would provide a basis for estimation of potential and actual daily transpiration with a few easily-available inputs.