Abstract
Soil water sensors are commonly used to monitor water content and matric potential in order to study hydrological processes such as evaporation. Finding a proper sensor is sometimes difficult, especially for measurements in topsoil, where changes of temperature and soil water dynamics occur generally with greater intensity compared to deeper soil layers. We assessed the perfor-mance of Hydra Probe water content sensors and MPS-1 matric potential sensors in topsoil in the laboratory and in the field. A common soil-specific calibration function was determined for the Hydra Probes. Measurement accuracy and sensor-to-sensor variation were within the manufacturer specification of ±0.03 m3·m-3. Hydra Probes can operate from dry to saturated conditions. Sensor-specific calibrations from a previous study were used to reduce sensor-to-sensor variation of MPS-1. Measurement accuracy can be expressed by a mean relative error of 10%. According to the manufacturer, the application range of matric potential readings is from -10 kPa to -500 kPa. MPS-1 delivered also values beyond this range, but they were not reliable. Sensor electronics of the MPS-1 were sensitive to ambient temperature changes. Beyond instrument effects, field measurements showed substantial temperature-driven fluctuations of soil water content and matric potential, which complicated data interpretation.
Highlights
The water cycle is a complex dynamical system that includes hydrological processes in atmosphere, vadose zone, and groundwater environment [1]
Since default calibration has proven to be improper, sensor-specific calibrations are recommended for the MPS-1 [19] [20]
Using a default calibration such a difference would result in a different water content of ±0.03 m3·m−3 in the dry range, which is of the same magnitude as the measurement accuracy specified by the manufacturer (Table 3)
Summary
The water cycle is a complex dynamical system that includes hydrological processes in atmosphere, vadose zone, and groundwater environment [1]. Researchers interested in evaporation and related quantities typically use sensors to measure soil temperature and either water content [2] [4] or matric potential [5], or both quantities [6]. Knowing sensor performance is a prerequisite when selecting a device for a certain application. Substantial properties describing this attribute are measuring range, precision (closeness of repeated measurements under unchanged conditions), accuracy (closeness of measurements of a quantity to that quantity’s true value), and temperature range and sensitivity. Finding a proper sensor is sometimes difficult, especially for measurements in topsoil, where temporal changes of temperature and soil water content are greater than that in deeper soil layers [7]. Since sensors are typically tested and evaluated in the lab, another question is how a certain device performs under field conditions
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