Abstract

A three-dimensional intermediate test system with the ability to control boundary conditions and soil moisture variations was developed. The setup had the advantage of being able to accurately characterize the heterogeneity through packing with test soils with well-defined properties and to control the boundary and initial conditions that are not possible in field settings. A distributed soil moisture sensor system was tested under controlled conditions in the test facility before field deployment. The developed 3-D tank has dimensions of L=4.87 m, W= 2.44 m packed to a depth of 0.40 m. The tank was packed with a heterogeneous configuration using five uniform silica sand with the effective sieve numbers #70, #16, #8, #12/30, and #20/30 (Accusands Unimin Corp Ottawa, MN), respectively. Soil moisture variations were monitored using 30 soil moisture sensors (ECH2O EC-5 and 5TE, TEROS12). The testing focused on observing and recording soil moisture patterns and the performance of the sensors under various imbibition and drainage scenarios expected in the field. The sensors were able to successfully capture the complex spatial and temporal variations of the soil moisture in the tank. Each sensor was individually calibrated for each type of the test sands used to provide unique fitting parameters relating to the sensor’s measured voltage to known water content. During the experiments, the head at one of the boundaries was kept constant, resulting in full saturation at this boundary that was captured by the sensors. Based on the time-series data, the variations in the specific properties of the sand in the packing led to different saturations. The varying hydraulic properties of the packed sand affected the water flow and soil moisture dynamics that were captured by the sensors. Even under such highly controlled test conditions in laboratory settings, heterogeneities resulting from packing imperfections and compaction introduced some uncertainties in the measurements. These observations suggest the importance of incorporating any available information on the natural heterogeneity when designing sensor deployment strategies in the field.

Highlights

  • Accurate soil water content measurements are critical for estimating energy and water balances and understanding chemical and biological processes in the vadose zone, plant root zones, and groundwater

  • Robinson et al [1] presented a review of dielectric and electrical conductivity measurements in soils using time domain reflectometry (TDR)

  • The experiments focused on observing soil moisture patterns in the sandy soil and investigate the performance of soil moisture sensors

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Summary

Introduction

Accurate soil water content measurements are critical for estimating energy and water balances and understanding chemical and biological processes in the vadose zone, plant root zones, and groundwater. Before field deployment, soil water monitoring devices require a rigorous laboratory evaluation and calibration for a range of soil types. An evaluation of various ECH2O probe models for soil moisture monitoring showed the sensitivity of sensor readings to supply voltage, temperature, and bulk soil electrical conductivity [5]. Sakaki et al [6,7] provided evidence that sensor-to-sensor differences in the readings were relatively small but not negligible They proposed a two-point -mixing model to convert sensor readings into soil water content [6]. Recent developments and improvements of ECH2O soil moisture sensors (Decagon Devices, Inc.) allow for detailed monitoring of soil water content at a relatively low cost both in the laboratory and field. The goal is to provide some guidance to design the sensor deployment strategies under naturally occurring conditions of soil heterogeneity encountered in the field

Experimental Setup
Design of Experiments
Results
Data Validation
Statistical Analysis
Conclusions

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