Content Reflectometer Erin L. Bush Gerard J. Kluitenberg Nathan O. - PowerPoint PPT Presentation

Lab Calibration of a Water Content Reflectometer Erin L. Bush Gerard J. Kluitenberg Nathan O. Nelson

Introduction  Importance of accurate soil moisture measurements  Monitor water use by cropping systems  Assist with management decisions  Kansas Agricultural Watershed Research Site  Smolan silty clay loam: 35% clay  Confirm calibration with field measurements  Calibration necessary due to changes in soil properties  Texture differentiation  Ranges of salinity  Bulk density

Objective Determine if CS655 water content reflectometers require soil-specific calibration prior to field use

Methods  Soil Preparation  Air dried, 2mm sieve  Gravimetric water content measured  Wetting Process  Soil wetted to target gravimetric water content  0.0005 M CaSO 4  Bulk EC < 0.9 ds/m  Water content achieved

Methods  Soil Packing  4 - 20.3 cm (h) PVC columns  6 layers, 3 cm each  Target bulk density: 1.25 g cm -3  Porosity at target bulk density: 0.53  8 water contents: 0.03-0.40 cm 3 cm -3

Methods  Taking Readings  4 probes  One probe per column  Volumetric water content, EC, degrees Celsius, permittivity  Check permittivity  Measured gravimetric water content

Results 0.48 0.1 Content 0.46 0.08 Water� (cm 3 /cm 3 ) 0.44 0.06 Volumetric� 0.42 0.04 0.40 0.02 0.38 0.38 0 Probe& 1 Probe& 2 Probe& 3 Probe& 4

Calibration Correction  Linear Correction : Volumetric water content comparison as a linear function  Permittivity Calibration 1 : Permittivity vs known volumetric water content as a quadratic function  Permittivity Calibration 2 : Known volumetric water content vs square root of permittivity

Linear Correction 0.5 y� =� 1.27x� - 0.042 R ² � =� 0.99 Content 0.4 Water� 0.3 (cm 3 /cm 3 ) Volumetric� Probe� 1 0.2 Probe� 2 Probe� 3 CS655� Probe� 4 0.1 1:1� line 0.0 0.0 0.1 0.2 0.3 0.4 0.5 Known� Volumetric� Water� Content� (cm 3 /cm 3 )�

Permittivity Calibration 1 0.5 -0.0003x 2 +� Content� y� =� 0.02x R ² � =� 0.99 0.4 Water� 0.3 (cm 3 /cm 3 ) Volumetric� 0.2 All� Data Topp� Equation Known� 0.1 0.0 0.0 10.0 20.0 30.0 40.0 Permittivity

Permittivity Calibration 2 0.5 Content� 0.4 y� =� 0.09x� - 0.09 R ² � =� 0.99 Water� 0.3 (cm 3 /cm 3 ) Volumetric� 0.2 Known� 0.1 All� data 0.0 0 2 4 6 Square� Root� of� Permittivity

Statistical Analysis Root Mean Square Error Correction Method RMSE CS655 Topp Equation 0.0330 Linear Correction 0.0109 Volumetric Water Content vs. Permittivity Calibration 0.0224 Volumetric Water Content vs. Square Root of Permittivity 0.0081 Calibration

Conclusion  No significant differences between the probes  Calibration necessary for accuracy:  - 4% to + 6%  Depends on soil moisture  Future research: Evaluate effects of soil properties on calibration  Bulk density  Salinity  Different textures

Acknowledgements  4-R Research Fund  Kansas State Research and Extension  Kansas Soybean Commission  Kansas Corn Commission