Laboratory Calibration of a Water Content Reflectometer.

Accurate soil water content measurement is important for many reasons, such as determination of crop water use, moisture effects on runoff, and cropping system effects on water availability. Water content reflectometers can be a useful tool for automated measurement of volumetric water content. However, measurements can be influenced by bulk density, clay content, and temperature. Therefore, the probes may need to be calibrated. The objective of this study was to determine if Campbell Scientific water content reflectometer probes (CS655) require soil-specific calibration prior to field use. The soil used for calibration was from the surface horizon of a Smolan silty clay loam with a clay percentage of roughly 35%. The soil was air-dried, ground through a 2mm sieve, and wetted to the appropriate water contents by known gravimetric measurements. After each water content was achieved, the soil was packed in 3-centimeter layers six times into 20.3-cm columns of PVC pipe to take a reading and collect data. Multiple regression analysis was used to determine if CS655 output is a good predictor of known soil water content and determine if the relationship between measured and known water content was dependent on the probe used. CS655 probes under-predicted the volumetric water content by up to 4% at known water contents less than 24%. The probes over-predicted the volumetric water content by up to 7% at known water contents greater than 24%. The relationship between the water content measured by the CS655 and the known water content was best described by a 2nd-order polynomial as determined by stepwise regression (p<0.001). There was not any significant difference between the probes. Through this study, it is clear that calibration of CS655 probes is necessary for accuracy less than +7% and -4%.