Evaluation of the volume of influence of four tubular capacitive probes

Efficient water management is a critical challenge in agriculture, particularly in regions such as Navarra, Spain, where irrigation accounts for up to 87% of total freshwater consumption. Capacitive soil moisture probes are widely adopted in precision agriculture; however, a notable inconsistency persists between the sensing ranges claimed by manufacturers (typically 5–15 cm) and those reported in the scientific literature (generally <6 cm). This discrepancy arises largely from the absence of standardized criteria to define the effective sensing volume of these sensors.

This study presents a replicable empirical methodology to characterize the volume of influence of four commercially available capacitive probes: AquaCheck, EnviroPro, Gerbil, and Sentek. Controlled laboratory experiments were conducted under air and water conditions, using 0.2 mm paper layers to incrementally simulate increasing distances from the moisture source. Sensor outputs were normalized to enable direct comparison across heterogeneous measurement units, including Volumetric Water Content (VWC%) and Scaled Frequency Units (SFU%).

All probes exhibited a logarithmic decrease in signal intensity with increasing distance from the water source. By modeling the sensing domain as a cylindrical volume with a 10 cm height and defining its effective extent at the 99.5th percentile of cumulative signal response, substantial differences among probes were observed. The estimated sensing volumes ranked as follows: Gerbil (710.59 cm³), EnviroPro, AquaCheck, and Sentek (236.71 cm³).

The results demonstrate that sensing volumes vary considerably among manufacturers and are strongly dependent on the percentile threshold used to define the effective volume of influence. These findings confirm the lack of uniformity in probe sensing behavior and underscore the need for technical standardization. Although derived from controlled laboratory conditions and therefore comparative in nature, the results provide critical insight for interpreting soil moisture measurements and offer a more reliable technical basis for informed decision-making in irrigation management.