TY - JOUR
T1 - Quantifying and correcting for clay content effects on soil water measurement by reflectometers
AU - Singh, J.
AU - Lo, T.
AU - Rudnick, D. R.
AU - Irmak, S.
AU - Blanco-Canqui, H.
N1 - Funding Information:
This study is based upon work that was jointly supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture (USDA-NIFA) , under award number 2016-68007-25066 , “Sustaining agriculture through adaptive management to preserve the Ogallala aquifer under a changing climate”; USDA-NIFA Hatch Project number 1015698, “Integrating Hydrological Modeling and Characterization Approaches across Scales to Understand the Effects of Efficient Irrigation Management on Groundwater/Surface Water Systems”; United States Geological Survey Section 104B under award number G16AP00068; and the Daugherty Water for Food Global Institute. The authors are grateful to Nebraska producers Phil Christenson and Tim Varilek as well as university farm managers Stuart Hoff and Toby Spiehs for authorizing soil collection. The authors thank Turner Dorr, Jacob Nickel, and Alan Boldt for assisting the authors on this project. The authors especially appreciate both the editor who gave the original manuscript a chance to be revised and the reviewer whose technical expertise and constructive criticism propelled remarkable improvements to the manuscript.
Publisher Copyright:
© 2019 Elsevier B.V.
PY - 2019/5/1
Y1 - 2019/5/1
N2 - The presence of clay particles increases the specific surface area of a soil and can affect the calibration of electromagnetic soil water sensors including reflectometers. To quantify and correct for this effect in two relatively new reflectometers, three TDR315 and three CS655 sensors were installed in each of five soils with clay content ranging from 5 to 49%. As the soils were dried in a temperature controlled room, sensor reported soil volumetric water content (θ v ) according to the factory calibration was compared against reference θ v determined by weighing the soils. Sensor reported θ v was similar to reference θ v in the sand soil (root mean square difference (RMSD) < 0.02 m 3 m −3 ), but the discrepancy was larger for the clayey soils. An increase in clay content tended to cause TDR315 to underestimate low θ v and tended to cause CS655 to overestimate θ v in a concave down pattern. At the levels evaluated in this experiment, differences in clay content produced a larger effect than differences in temperature (24 versus 35 °C) and salinity (0 versus 3.09 g/L CaCl 2 ) on factory calibration accuracy of both sensors. Soil specific empirical calibrations developed using a square root mixing model fitted the experimental data very closely (R 2 > 0.93) for both sensors. By estimating calibration coefficients based on clay content alone to recalculate sensor θ v from sensor reported apparent relative permittivity, RMSD from reference θ v was reduced by approximately 36% for both sensors as compared with using the factory calibration. Applying the same procedure to independent literature data tended to improve θ v accuracy of both sensors increasingly as clay content increased above 20%. The findings suggest that a simple, user-friendly correction for clay content effects may provide initial practical improvement over the factory calibration of a reflectometer in clayey soils.
AB - The presence of clay particles increases the specific surface area of a soil and can affect the calibration of electromagnetic soil water sensors including reflectometers. To quantify and correct for this effect in two relatively new reflectometers, three TDR315 and three CS655 sensors were installed in each of five soils with clay content ranging from 5 to 49%. As the soils were dried in a temperature controlled room, sensor reported soil volumetric water content (θ v ) according to the factory calibration was compared against reference θ v determined by weighing the soils. Sensor reported θ v was similar to reference θ v in the sand soil (root mean square difference (RMSD) < 0.02 m 3 m −3 ), but the discrepancy was larger for the clayey soils. An increase in clay content tended to cause TDR315 to underestimate low θ v and tended to cause CS655 to overestimate θ v in a concave down pattern. At the levels evaluated in this experiment, differences in clay content produced a larger effect than differences in temperature (24 versus 35 °C) and salinity (0 versus 3.09 g/L CaCl 2 ) on factory calibration accuracy of both sensors. Soil specific empirical calibrations developed using a square root mixing model fitted the experimental data very closely (R 2 > 0.93) for both sensors. By estimating calibration coefficients based on clay content alone to recalculate sensor θ v from sensor reported apparent relative permittivity, RMSD from reference θ v was reduced by approximately 36% for both sensors as compared with using the factory calibration. Applying the same procedure to independent literature data tended to improve θ v accuracy of both sensors increasingly as clay content increased above 20%. The findings suggest that a simple, user-friendly correction for clay content effects may provide initial practical improvement over the factory calibration of a reflectometer in clayey soils.
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U2 - 10.1016/j.agwat.2019.02.024
DO - 10.1016/j.agwat.2019.02.024
M3 - Article
AN - SCOPUS:85061785005
VL - 216
SP - 390
EP - 399
JO - Agricultural Water Management
JF - Agricultural Water Management
SN - 0378-3774
ER -