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United States Department of Agriculture

Agricultural Research Service

Title: Measuring Soil Hydrologic Properties Using Dielectric Sensors

Authors
item Heathman, Gary
item McAfee, Scott

Submitted to: International Symposium and Workshop on Time Domain Reflectometry for Innovative Soils Applications
Publication Type: Proceedings
Publication Acceptance Date: August 29, 2006
Publication Date: September 17, 2006
Citation: Heathman, G.C., Mcafee, S.J. 2006. Measuring soil hydrologic properties using dielectric sensors. TDR 2006: 3rd International Symposium and Workshop on Time Domain Reflectometry for Innovative Soils Applications, September 17-28, 2006, Purdue University, West Lafayette, IN. Paper No. 20.

Interpretive Summary: It is important to have an understanding of soil hydrologic properties in many aspects of environmental research. Soil hydrologic or hydraulic properties govern the transport of water and chemicals through the soil medium. However, soil hydrologic properties determined from laboratory analyses often are non-representative of field conditions. In this work, we compare soil hydrologic properties determined in the field with values obtained from soil core laboratory analysis and determine the differences associated with the use of simple methods to estimate these properties using either field or laboratory data. In this preliminary study, three types of sensors for measuring soil dielectric properties were used to determine volumetric soil water content and ultimately, used to estimate soil hydraulic properties. The objective of this work was to determine in situ soil hydraulic properties using dielectric measurements of soil water content ('v) and compare the field measured values with those estimated by simpler methods based on soil-core laboratory analysis. Results of the study show that the use of three types of dielectric sensors proved adequate to their purpose in measuring the change in 'v during field infiltration and drainage experiments and in estimating soil hydraulic properties.

Technical Abstract: Knowledge of soil hydrologic properties is essential to many aspects of environmental research. Soil hydrologic properties determined from laboratory analyses often are non-representative of field conditions. In this work, we compare soil hydrologic properties determined in the field with values obtained from soil core laboratory analysis and determine the differences associated with the use of pedotransfer functions to estimate these properties using either field or laboratory data. Double-ring infiltrometers, in combination with tensiometric measurements at three depths, were used to determine soil hydrologic properties in-situ. The studies were conducted on two small watersheds in northeastern Indiana to obtain soil hydraulic conductivity (K) as a function of soil water content and soil moisture as a function of matric potential. After reaching steady-state flow conditions, TDR probes were used to measure the changes in profile soil water content over time during vertical drainage. Soil matric potential was measured with tensiometers placed at 15, 30 and 60 cm depths, concurrent with TDR measurements at 15cm intervals to a depth of 60cm. Coincident with these measurements; we measured 0-5cm soil moisture daily in the inner ring with the Theta-probe to capture changes in near-surface soil moisture to simulate remotely sensed surface soil moisture observations. At the end of the drainage experiment soil cores were collected to determine the relationship between soil moisture and matric potential using the laboratory pressure-plate procedure. The data were used to:1) compare field and laboratory measurements of water retention and estimates of K, 2) to evaluate the use of pedotransfer functions using either field or lab data, and 3) investigate the relationship between the 0-5cm soil moisture dynamics and profile soil moisture to a depth of 60cm. The use of TDR and Theta probes eliminated the need to take numerous gravimetric soil samples normally required for these types of field studies.

Last Modified: 9/23/2014
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