Soil Moisture Measurements: Comparison of Instrumentation Performances

It has long been recognised that reliable, robust and automated instrumentations for the measurement of soil moisture content can be extremely useful, if not essential, in hydrological, environmental and agricultural applications. A number of automated techniques for point measurement of soil water content have been developed to operational level over the past few decades. While each of those techniques have been individually evaluated by the thermogravimetric (oven drying and weighing) method, typically under laboratory conditions, there have been few studies which have made a direct comparison between the various techniques, particularly under field conditions. This paper compares ECH2O probes, EC-5 (both Decagon Devices) and Time Domain Reflectometer sensors (CS616 Campbell Scientific Water Content Reflectometer), with gravimetric data and between themselves. Data were collected during two experiments set with different soils and a wide range of soil moisture resulting from an irrigation/drying cycle. Results show that all the tested probes give acceptable results after being calibrated in the field. The capacitive sensors can be used with the same calibration equation at both the selected depths, with RMSE ranging between 2.5% and 3.6%. TDR probes showed a dependence from depth but a lower RMSE (1.6%).