Playing in the sandbox: An experimental set-up for comparison of soil moisture profile sensors
Creators: name: Nieberding, Felix
name: Huisman, Johan Alexander
name: Huebner, Christof
name: Weuthen, Ansgar
name: Schilling, Bernd
name: Bogena, Heye
Publisher: name: Forschungzentrum Jülich
Year: 2023

description: 2nd TERENO-OZCAR Conference 2023, Bonn, Germany, 25 Sep 2023 - 28 Sep 2023
description: For an efficient and economical use of limited water resources, sensing techniques for the determination of soil moisture in the root zone are becoming increasingly important. Soil moisture profile sensors (SMPS) have a high potential for climate-smart agriculture due to their easy handling and ability to perform simultaneous measurements at different depths. However, determining soil moisture with reasonable accuracy is a complex task. Especially bound water content, salinity and temperature influence the soil dielectric permittivity and might thus affect the electromagnetic soil moisture measurement of the SMPS.To date, an accurate and easy-to-use method for the evaluation of long SMPS is not available. To this end, we designed a laboratory and a field experiment to better discriminate between changes in soil dielectric permittivity and sensor variability due to environmental effects. The tested SMPS are the SoilVUE10 (50 cm) from Campbell Scientific, the Drill&amp;Drop (60 cm) from Sentek, as well as the SMT500 (50 cm), which is an early prototype SMPS developed by TRUEBNER GmbH. The following questions were addressed: (1) How high is the measurement variability of the vertical measurement sections of an SMPS? (2) How strong is the sensor response influenced by changes in temperature? (3) How accurate can SMPS determine soil moisture dynamics in the root zone and how large is the sensor-to-sensor variability of SMPS? We addressed questions 1 and 2 by placing the SMPS into a container filled with well-characterized fine sand (type F36, Quarzwerke Frechen). The sand was water saturated and the temperature of the container was stepwise increased from 5 to 40 °C using a water cooling/heating. Question 3 was addressed by setting up a 2 x 2 x 1.5 m sandbox, also filled with F36 sand at a field site. The water level inside the sandbox can be controlled by pumping water in or out using piezometer tubes. The SMPS were installed into the sandbox and the measurements were compared against reference measurements made using CS610 TDR probes connected to a TDR100 system (Campbell Scientific) and against SMT100 (TRUEBNER) TDT measurements.The Drill&amp;Drop sensor showed the lowest intra- and inter-sensor variability but exhibited a stronger temperature dependence than the other two SMPS. In the sandbox, the well-sieved, fine sand provided a challenging environment for the SMPS evaluation. While it is well suited to produce a homogeneous testbed, the soil hydraulic conditions changed rapidly from residual to saturated water content. Overall, the three SMPS performed at a similar accuracy level but exhibited a curvilinear response, with higher RMSEs at intermediate soil moisture contents. Our experimental setup proved to be useful for the evaluation and characterization of soil moisture profile sensors with respect to temperature stability, inter- and intra-sensor variability and measurement accuracy.