AquaCheck Calibration Equations Summary (11071) [PDF]

Zitiervorschau

AquaCheck (Pty) Ltd Soil Calibration 2014 South Africa

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AquaCheck Soil Calibration by Angelique Zeelie (MSc Soil Physics) | 21.08.2014

AquaCheck Soil Calibration Introduction: in situ methodology There are two known methods regarding calibration of capacitance probes: 1) laboratory confined calibrations and 2) field calibrations. Laboratory calibrations are favored in practice above the field calibration, as field calibrations are more labor intensive and time consuming. The methodology for in situ soil calibration employs a destructive sampling procedure, which means that replication or multiple sampling for the same sensor is not possible. The samples must be taken, within the sphere of influence of the multisensory device (i.e. probe). This is not always possible because a lot of samples must be taken over time. However, field calibration has a more realistic implication for the commercial farmer, especially when conducted within the farming vicinity along with growing crops. AquaCheck recommend the following methodology for addressing the calibration procedure for different soil textural classes. For calibration purposes AquaCheck has recorded a total of four field (in situ) field calibrations. The field trials focused on gravimetric soil moisture determination, sampling over a wide range of soil water contents and then correlating these measurements to known scaled frequency (SF) readings. Core samplers where used and therefore bulk density (𝝆b) could be calculated along with the gravimetric water content. Subsequently the volumetric water content was separately derived from the data collected.

Soil types: Selected soil chemical- and physical properties The following soil textural classes were evaluated: sand, clay and silt-loam. The field sites were investigated with respect to their soil physical -and chemical properties. Soil data from selected chemical -and physical soil properties shown in, Tables 1 & 2.

T ABLE 1 S OIL CHEMICAL PROPERTIES Soil Type Sand Clay Silt-loam

CEC (meq+/100g) 2.00 11.27 5.30

EC (dS/m) 0.16 0.26 0.11

pH (1:1 H2O) 6.36 6.21 5.30

C (%) 0.12 0.56 0.91

*CEC: cation exchange capacity; EC: electrical conductivity; C: total carbon

T ABLE 2 S OIL PHYSICAL PROPERTIES Soil Type

𝝆b (g/cm3)

Clay + Silt (%)

Sand (%)

Gravel (v/v %)

Sand Clay Silt-loam

1.44 1.47 1.36

8 68 38

92 32 62

0 0 ±30

*𝜌b: bulk density

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AquaCheck Soil Calibration by Angelique Zeelie (MSc Soil Physics) | 21.08.2014

Calibration equations: gravimetric- & volumetric regression analysis The rundowns below describe the calibration equation for each individual soil type with respect to gravimetric- and volumetric regression analyses, Tables 3 & 4. Graphic illustrations for each soil type along with a generic soil regression analysis follows, see Figures 1 & 2. All regression analyses where derived as linear relationships, therefore the Y-variables are expressed as the mean response to the variables of X. The mathematical representation of the regression equations are:

For the purpose of this calibration equation the Y-variable represents either the gravimetric water content (GWC) or the volumetric water content (VWC) and the X-variable represents the Scaled Frequency (SF). T

β define the relationship between the SF and the GWC as well as the VWC as

outlined in Tables 3 & 4 respectively. These values were derived for each of the soil types described. A generic soil regression analysis was also conducted.

T ABLE 3 CALIBRATION COEFFICIENTS FOR GWC REGRESSION ANALYSIS Soil Type Sand Clay Silt-loam Generic

𝜶 -6.004 -4.0527 -2.0514 -5.7405

𝜷 0.3624 0.4213 0.323 0.4002

R2 0.777 0.783 0.717 0.763

n 56 49 26 134

*R2: measures the portion of variability in Y (response variable) explained by the regression; n: observations

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AquaCheck Soil Calibration by Angelique Zeelie (MSc Soil Physics) | 21.08.2014

F IGURE 1 GRAVIMETRIC WATER CONTENT (%)

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AquaCheck Soil Calibration by Angelique Zeelie (MSc Soil Physics) | 21.08.2014

T ABLE 4 CALIBRATION COEFFICIENT FOR VWC REGRESSION ANALYSIS Soil Type Sand Clay Silt-loam Generic

𝜶 -8.6463 -5.9575 -2.79 -7.4347

𝜷 0.5219 0.6193 0.4392 0.5564

R2 0.777 0.783 0.717 0.728

n 56 49 26 134

F IGURE 2 VOLUMETRIC WATER CONTENT (%)

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AquaCheck Soil Calibration by Angelique Zeelie (MSc Soil Physics) | 21.08.2014

Example: interpreting calibration equations After the foregoing introduction to the different calibration equations, an example follows to see how these equations may be applied in practice. The generic equation’s values will be used for this example. Generic equation interpretation: The regression equation for gravimetric water content (GWC in %), GWC = -5.7405 + 0.4002*SF-reading & The regression equation for volumetric water content (VWC in %), VWC = -7.4347 + 0.5564*SF-reading For example, if the SF-reading (independent variable) measures a value of 50, the equations will derive the following values for the GWC and VWC equations (response variable): GWC = -5.7405 + 0.4002*50 = 14.3 % VWC = -7.4347 + 0.5564*50 = 20.4 % This generic equation was derived from a total of 134 observations (as given in Table 1 & 2), which is the combined data for the three soil types and has an R2 –value of 0.73 for the regression equation relating to the VWC.

Conclusion: The preceding soil calibration equations derived for the AquaCheck (Pty) Ltd sensor is unique to the product and should by no means be used for calibrating additional capacitance products. The reported equations were conducted for determining both gravimetric- and volumetric soil water content per soil type, as several current patrons have requested different soil water measuring units. It is strongly recommended when selecting an equation that it is based on the soil particle size distribution, textural class, of the soil closest to that which the AquaCheck probes are installed in. If in doubt with regards to the soil textural class, select the generic soil calibration equation. AquaCheck wants to highlight the importance of soil-and-site specific calibrations, as there are many external factors which have an influence on each of the installations and therefore the calibration equations derived for this exercise must never be deemed as absolute.

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AquaCheck Soil Calibration by Angelique Zeelie (MSc Soil Physics) | 21.08.2014

Information & Queries For additional information contact AquaCheck in South Africa: Mr. Hannes le Roux Chief Executive Tel: +27 (0)21 970 5140 Fax: +27 (0)21 975 1254 E-mail: [email protected] Web: www.aquacheck.co.za

Acknowledgements We at AquaCheck would like to acknowledge the following participants from their various organizations for reading and/ or given their valued inputs with the planning and completion of this project: Name

Institution

Mr. Tienie du Preez

Private Consultant, South Africa

Dr. J.E. Hoffman

Stellenbosch University, South Africa

Ms. Nordely Wright

Oro Agri SA (Pty) Ltd., South Africa

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