  Proximal soil sensors are taking hold in the understanding of soil
hydrogeological processes involved in precision agriculture. In this context,
permanently installed gamma ray spectroscopy stations represent one of the best
space-time trade off methods at field scale. This study proved the feasibility
and reliability of soil water content monitoring through a seven-month
continuous acquisition of terrestrial gamma radiation in a tomato test field.
By employing a 1 L sodium iodide detector placed at a height of 2.25 m, we
investigated the gamma signal coming from an area having a ~25 m radius and
from a depth of approximately 30 cm. Experimental values, inferred after a
calibration measurement and corrected for the presence of biomass, were
corroborated with gravimetric data acquired under different soil moisture
conditions, giving an average absolute discrepancy of about 2%. A quantitative
comparison was carried out with data simulated by AquaCrop, CRITeRIA, and
IRRINET soil-crop system models. The different goodness of fit obtained in bare
soil condition and during the vegetated period highlighted that CRITeRIA showed
the best agreement with the experimental data over the entire data-taking
period while, in presence of the tomato crop, IRRINET provided the best
results.
