Inter-Comparison of Proximal Near-Surface Soil Moisture Measurement Techniques

Precision agriculture is experiencing substantial development through the improved availability of cost-effective instruments for data collection. This includes ground-based proximal sensing technologies that are able to compete with satellite and aircraft observation systems, due to low operational costs, high operational flexibility, and high spatial resolution. This article was therefore designed to compare the performance of multiple sensing systems mounted on a smart buggy platform. A number of proximal sensing technologies were then evaluated and intercompared for their accuracy in retrieving high resolution near-surface soil moisture. The sensors tested included an
L
-band microwave radiometer (ELBARA III), a global navigation satellite system reflectometer sensor (LARGO), and an electromagnetic induction sensor (EM38). Data were collected during the fifth Soil Moisture Active Passive Experiment (SMAPEx-5) in Yanco, NSW, Australia, in September 2015. Observations from each sensor were converted to surface soil moisture values which were in turn evaluated against reference measurements obtained by
in situ
soil moisture measurements. The sensing technologies tested here have been individually assessed by many other studies, but within different regions and environments including surface condition, local weather, observing height, size of footprint, etc. Consequently, this article has used a single platform to intercompare the different sensors to be evaluated concurrently. Results from this article indicated that the
L
-band microwave radiometer achieved the best performance in retrieving surface soil moisture. The average RMSE and
R
were found to be 0.055 cm
³
/cm
³
and 0.68 for ELBARA III, 0.084 cm
³
/cm
³
and 0.51 for LARGO, and 0.090 cm
³
/cm
³
and 0.32 for the EM38.