Field calibrations of soil moisture sensors in a forested watershed

Abstract

Spatially variable soil properties influence the performance of soil water content monitoring sensors. The objectives of this research were to: (i) study the spatial variability of bulk density (ρ(b)), total porosity (θ(t)), clay content (CC), electrical conductivity (EC), and pH in the upper Mākaha Valley watershed soils; (ii) explore the effect of variations in ρ(b) and θ(t) on soil water content dynamics, and (iii) establish field calibration equations for EC-20 (Decagon Devices, Inc), ML2x (Delta-T-Devices), and SM200 (Delta-T-Devices) sensors to mitigate the effect of soil spatial variability on their performance. The studied soil properties except pH varied significantly (P < 0.05) across the soil water content monitoring depths (20 and 80 cm) and six locations. There was a linear positive and a linear inverse correlation between the soil water content at sampling and ρ(b), and between the soil water content at sampling and θ(t), respectively. Values of laboratory measured actual θ(t) correlated (r = 0.75) with those estimated from the relationship θ(t) = 1 - ρ(b)/ρ(s), where ρ(s) is the particle density. Variations in the studied soil properties affected the performance of the default equations of the three tested sensors; they showed substantial under-estimations of the actual water content. The individual and the watershed-scale field calibrations were more accurate than their corresponding default calibrations. In conclusion, the sensors used in this study need site-specific calibrations in order to mitigate the effects of varying properties of the highly weathered tropical soils.

Keywords: sensor field calibration; soil water content; tropical soils; variable soil properties.

Figure 1.

A map showing the upper Mākaha Valley sub-watershed and the six monitoring locations of the field calibration from which soil samples were collected and used to determine bulk density, total porosity, clay content, electrical conductivity and pH.

Figure 2.

Correlation between the actual total porosity at the two depths (20 and 80 cm) and that estimated from the relationship θ_t = 1 − ρ_b/ρ_s, where θ_t is total porosity, ρ_b is bulk density, and ρ_s is the particle density (2.65 g cm⁻³).

Figure 3.

Effect of spatial variability of soil bulk density (figures on left) and actual total porosity (figures on right) on field measured soil water content.

Figure 4.

Field measured actual volumetric water content versus sensors’ readings plotted with default and site-specific field calibration models for (A) EC-20; (B) ML2x; and (C) SM200.