Real‐time Soil Water Dynamics Using Multisensor Capacitance Probes: Laboratory Calibration

There is a continued need for better methods to perform accurate, real‐time, nearly continuous soil water measurements at specific depth intervals, with minimal soil disturbance, and covering field‐scale areas. The objectives of this research were to assess the characteristics of a newly developed multisensor capacitance probe and to calibrate the sensors against a Mattapex silt loam (fine‐silty, mixed, mesic Aquic Hapludult) soil. The soil was uniformly packed in small increments, with the aid of a large hydraulic press, in a wooden box (35.5 by 35.5 cm, 40.5 cm deep). Probe installation in the boxed soil mimicked that required for correct field installation. A highly significant (r2 = 0.992 for n = 15, and RMSE = 0.009 cm3 cm‐3 water), nonlinear (θv = 0.490 SF2.1674) relationship was found between the soil volumetric water content (θv, cm3 cm‐3) and the scaled frequency [SF = (Fa − Fs)(Fa − Fw)‐1]. The SF represents the ratio of individual sensor's frequency (inside PVC pipe) response in soil (Fs) compared with sensor responses in air (Fa) and in nonsaline water (Fw) at room temperature (≈22°C). Axial and radial sensitivity studies showed that these capacitance sensors give integrated readings over a primary depth interval of 10 cm and a radial capacitance fringe within 10 cm of the wall of the access pipe. Temperature effects of air and water were measured, and the calculated effects on assessment of θv were less than the RMSE for a temperature range of 10 to 30°C. Our calibration studies indicate that these multisensor capacitance probes can be used to accurately measure volumetric soil water contents in a soil water monitoring system.