Soil water sensor performance

Numerous sensors are currently available to measure soil water content. Although several studies have compared relative sensor performance in the field, there have been no reports of sensor comparisons with carefully controlled soil water contents. Weighed soil columns were used to compare 23 soil water sensors representing eight sensor types. Included in the study were: a Troxler neutron gage, a Troxler Sentry 200-AP capacitance probe, Aqua-Tel capacitance sensors, time domain reflectometry (TDR) with two- and four-rod waveguides, gypsum blocks, Watermark electrical resistance blocks, and Agwatronics heat dissipation blocks. Measurement errors of the volumetric water content of the soil were determined for each sensor over a range of water contents from the maximum water holding capacity to below 5%. A loam and a sandy loam soil were wetted to the maximum water holding capacity and subsequently drained through four cycles. Sensors were calibrated using data from the first cycle and measurement errors for each sensor were determined using those calibrations in three additional cycles. Measurements outside the range of 0 to 50% volumetric soil water content were discarded. Of 64 possible readings in the test, only the neutron gage and the Aqua-Tel capacitance sensor gave 64 viable readings. The Sentry capacitance probe had the lowest measurement error and yielded 62 of 64 viable readings. Watermark sensors had measurement errors similar to the electrical capacitance sensors, but averaged 57 of 64 viable readings. In order of decreasing performance, the Aqua-tel electrical capacitance sensor, the Sentry electrical capacitance probe, the neutron gage, and the Watermark sensors performed best in this study when accuracy, reliability, durability, and installation factors were considered.