  Proximal gamma-ray spectroscopy supported by adequate calibration and
correction for growing biomass is an effective field scale technique for a
continuous monitoring of top soil water content dynamics to be potentially
employed as a decision support tool for automatic irrigation scheduling. This
study demonstrates that this approach has the potential to be one of the best
space-time trade-off methods, representing a joining link between punctual and
satellite fields of view. The inverse proportionality between soil moisture and
gamma signal is theoretically derived taking into account a non-constant
correction due to the presence of growing vegetation beneath the detector
position. The gamma signal attenuation due to biomass is modelled with a Monte
Carlo-based approach in terms of an equivalent water layer which thickness
varies in time as the crop evolves during its life-cycle. The reliability and
effectiveness of this approach is proved through a 7 months continuous
acquisition of terrestrial gamma radiation in a 0.4 ha tomato (Solanum
lycopersicum) test field. We demonstrate that a permanent gamma station
installed at an agricultural field can reliably probe the water content of the
top soil only if systematic effects due to the biomass shielding are properly
accounted for. Biomass corrected experimental values of soil water content
inferred from radiometric measurements are compared with gravimetric data
acquired under different soil moisture levels, resulting in an average
percentage relative discrepancy of about 3% in bare soil condition and of 4%
during the vegetated period. The temporal evolution of corrected soil water
content values exhibits a dynamic range coherent with the soil hydraulic
properties in terms of wilting point, field capacity and saturation.
