Soil Moisture Inferences from Thermal-Infrared Measurements of Vegetation Temperatures

Remote sensing methods for the estimation of soil moisture yield direct information only for the topmost layers of soil. Reflected solar, thermal-infrared (IR), and microwave techniques are sensitive to the surface skin, from the surface to about 5 cm, and from the surface to about 10 cm, respectively. When the growth of vegetation is of major interest, soil moisture needs to be infrared at least to the depth of rooting of the plants. Since remote measurement of soil moisture is depth limited, it has been suggested that plant measurements, specifically plant temperatures, may yield information about soil moisture within the root zone. To examine this possibility, three plots of wheat, initially treated similarly, and later irrigated differently, were monitored for vegetation temperature (by infrazed thermometry) and for soil-water content (thrice weekly neutron moisture meter measurements). Vegetation temperatures were converted to a crop water stress index (CWSI). The CWSI was found to be a nonunique function of extractable water. The nonuniqueness was probably caused by inability to adequately specify the root zone and by the fact that plants require a recovery period (five to six days for this experiment) after being stressed before normal water uptake and transpiration proceeds.