One of the most important factors to evaluate when selecting a soil moisture sensor is the volume of soil that the sensor integrates into the volumetric water content (VWC) measurement. For some applications, a sensor with a small measurement volume is desirable (e.g., greenhouse applications, near-surface measurements). However, in many field situations, significant small-scale heterogeneity is present in the soil, meaning that a small volume measurement may not adequately reflect the average volumetric water content at the measurement location. For many field applications, a larger measurement volume minimizes these issues and yields a more representative measurement of the true VWC. This application note describes the results of tests that were conducted to quantify the measurement volume of METER VWC sensors.
The tests used to evaluate the measurement volume of the METER sensors were based on those in Sakaki et al. 2008 and are described here briefly. Each VWC sensor was suspended in air in a stationary position using a ring stand and clamp. A plane of water (water-filled, flat-sided plastic container) was then incrementally brought horizontally toward the sensor while recording sensor output. The outer edge of the measurement volume was identified as the distance where the presence of the water changed the sensor output appreciably. We repeated this process at six different sensor orientations to obtain a three-dimensional representation of the sensor measurement volume.
Results and discussion
The sensor measurement volumes are encompassed by envelopes shown in Figures 1 through 7 below. If the measurement volume for each sensor is approximated as an ellipsoidal cylinder with the dimensions measured experimentally, as Figures 1 to 7 demonstrate, then you can calculate the total measurement volumes in Table 1.
It is well known that the electric field distribution inside the measurement volume is strongly weighted toward the sensor surfaces. Care should still be taken to ensure good soil-sensor contact to avoid air gaps at the sensor surface where it is most sensitive. It is also likely that electromagnetic field lines propagate further through air than through higher dielectric material (i.e., soil), so the dimensions in Figures 1 through 7 and the volumes in Table 1 should be taken as the maximum possible for each sensor. They should be good guidelines for installing the sensors near the soil surface or near a foreign object in the soil.
Sakaki, Toshihiro, et al. “Empirical two‐point α‐mixing model for calibrating the ECH2O EC‐5 soil moisture sensor in sands.” Water resources research 44.4 (2008). Article link.