Using geodetic GPS receivers to measure vegetation water content

A GPS-based method is presented to measure vegetation water content. Commercially available geodetic-quality GPS receivers and antennas are used. The method is tested using GPS measurements collected over three field seasons. The GPS data are compared with in situ data for three plant types: desert grass, wheat, and alfalfa. The GPS retrievals of vegetation water content are based on the GPS signal-to-noise ratio (SNR) data. Instrumental issues that affect the SNR data are discussed, particularly satellite transmit power variations, footprint variations, and temperature effects. The amplitudes of the SNR data show a nearly linear relationship to the water content in grasses (0–0.5 kg/m²) and wheat crops (0–0.9 kg/m²). As predicted by theory, this simple linear relationship breaks down in vegetation with heavy water content, such as alfalfa. The field results are consistent with forward model predictions, whose effect restricts the use of this simple linear model for vegetation to water content of less than ~1 kg/m².