Inversion of Wheat Volumetric Water Content Using GNSS Refractometry

Objectives Wheat volumetric water content is one of the important variables in indication of wheat growth status, and it is essentially affecting the wheat yield. However, the traditional measuring techniques, such as manual sampling method and satellite remote sensing techniques, are difficult for monitoring wheat volumetric water content accurately and continuously. To solve this issue, we propose a new wheat volumetric water content estimating method with global navigation satellite system (GNSS) refractometry.

Methods First,we use two pairs of GNSS receivers and antennas to collect the refracted signal in the wheat crop layer and the direct signal in the air respectively. Amplitude attenuation degree of the refracted signal is quantified as the amplitude ratio of the refracted GNSS signal to the direct one, and the ratio can be derived from signal-to-noise ratio (SNR) observations collected by the two receivers. Mathematic relationship between the ratio, wheat volumetric water content, wheat height, and air temperature is described using the linear functions. Then, the inversion model of wheat volumetric water content, which takes the SNR at GNSS elevation angle of 55°, wheat height, and temperature as the inputs, is developed. Finally, the model is validated through a data set collected in an experimental campaign over 120 days.

Results The results show that a good agreement exiting between the model calculated wheat volumetric water content estimations and manual sampling ones in the wheat stem extension, heading, and ripening stage, The mean, standard deviation and root mean square of model calculated wheat volumetric water content estimation error are 0.058 kg/m³, 0.396 kg/m³, 0.399 kg/m³ at 1 to 7 kg/m³.

Conclusions The proposed method can be used to monitor the volumetric water content of wheat accurately in a cost-effective way.