Time-domain and Frequency-domain Reflectometry Type Soil Moisture Sensor Performance and Soil Temperature Effects in Fine- and Coarse-textured Soils

Abstract

The performances of six time-domain reflectometry (TDR) and frequency-domain reflectometry (FDR) type soil moisture sensors were investigated for measuring volumetric soil-water content (θ_v) in two different soil types. Soil-specific calibration equations were developed for each sensor using calibrated neutron probe-measured θ_v. Sensors were also investigated for their performance response in measuring θ_v to changes in soil temperature. The performance of all sensors was significantly different (P<0.05) than the neutron probe-measured θ_v, with the same sensor also exhibiting variation between soils. In the silt loam soil, the 5TE sensor had the lowest root mean squared error (RMSE) of 0.041 m³/m³, indicating the best performance among all sensors investigated. The performance ranking of the other sensors from high performance to low was: TDR300 (High Clay Mode), CS616 (H) and 10HS, SM150, TDR300 (Standard Mode), and CS616 (V) (H: horizontal installation and V: vertical installation). In the loamy sand, the CS616 (H) performed best with an RMSE of 0.014 m³/m³ and the performance ranking of other sensors was: 5TE, CS616 (V), TDR300 (S), SM150, and 10HS. When θ_v was near or above field capacity, the performance error of most sensors increased. Most sensors exhibited a linear response to increase in soil temperature. Most sensors exhibited substantial sensitivity to changes in soil temperature and the θ_v response of the same sensor to high vs. normal soil temperatures differed significantly between the soils. All sensors underestimated θ_v in high temperature range in both soils. The ranking order of the magnitude of change in θ_v in response to 1°C increase in soil temperature (from the lowest to the greatest impact of soil temperature on sensor performance) in silt loam soil was: SM150, 5TE, TDR300 (S), 10HS, CS620, CS616 (H), and CS616 (V). The ranking order from lower to higher sensitivity to soil temperature changes in loamy sand was: 10HS, CS616 (H), 5TE, CS616 (V), SM150, and TDR300 (S). When the data from all sensors and soils are pooled, the overall average of change in θ_v for a 1°C increase in soil temperature was 0.21 m³/m³ in silt loam soil and -0.052 m³/m³ in loamy sand. When all TDR- and FDR-type sensors were pooled separately for both soils, the average change in θ_v for a 1°C increase in soil temperature for the TDR- and FDR-type sensors was 0.1918 and -0.0273 m³/m³, respectively, indicating that overall TDR-type sensors are more sensitive to soil temperature changes than FDR-type sensors when measuring θ_v.