Using Geodetic GPS Receivers to Measure Vegetation Water Content

22 A GPS-based method is presented to measure vegetation water content. Commercially available 23 geodetic-quality GPS receivers and antennas are used. The method is tested using GPS 24 measurements collected over three field seasons. The GPS data are compared with in situ data 25 for three plant types: desert grass, wheat and alfalfa. The GPS retrievals or vegetation water 26 content are based on the GPS signal-to-noise ratio (SNR) data. Instrumental issues that affect the 27 SNR data are discussed, particularly satellite transmit power variations, footprint variations, and 28 temperature effects. The amplitudes of the SNR data show a nearly linear relationship to the 29 water content in grasses (0-0.5 kg/m) and wheat crops (0-0.9 kg/m). As predicted by theory, 30 this simple linear relationship breaks down in vegetation with heavy water content, such as 31 alfalfa. The field results are consistent with forward model predictions, which effect restricts the 32 use of this simple linear model for vegetation to water content of less than ~1 kg/m. 33 34