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A heat pulse technique for measuring water flow in soil Tyson Ochsner USDA-ARS Soil and Water Management Research Unit St. Paul, MN

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Application of the heat pulse technique Upstream sensor Heater Downstream sensor 1 cm The heat transfer occurs by conduction and convection.

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Heat transfer equations where J w is the water flux. The governing heat transfer equation is The solution of this governing equation leads to a simple relationship between water flux and the temperature increase ratio.

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We tested the technique in the lab using packed columns of sand, sandy loam, and silt loam soil. Laboratory experiments

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Heat pulse signals converted to T d /T u The temperature increase ratio increases with flow rate. Sand Flux (cm h -1 )

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Resulting water flux estimates Strong linear relationship between flux measured by the sensor and that measured at the column outlet.

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Soil type Slope (S) Intercept Std. Err. r2r2 cm hr -1 Sand0.7650.6050.3900.998 Sandy loam0.2320.4520.1250.998 Silt loam0.4002.880.0710.999 Linear regression results Regression results indicate good precision and linearity, but the slopes are less than one.

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Linking theory and measurements Choosing a = 1/S and b = 1 leads to an enhanced conduction model. Choosing a = 1 and b = S leads to a reduced convection model.

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Results The smallest flux detected with the heat pulse technique was 0.12 cm hr -1. The relationship between heat pulse flux estimates and the outflow flux was linear up to 40 cm hr -1. The standard heat transfer model over- predicted the sensitivity of the heat pulse sensor to water flux. A reduced convection model accounted for the discrepancies between the measured data and the standard model.

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Closing statements Uncorrected heat pulse measurements are fairly accurate in sand. Theoretical over-predictions of instrument response to convective heat transfer are common. A reliable procedure for correcting heat pulse measurements is needed. The heat pulse approach for measuring soil water flux warrants further development.

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Acknowledgements Collaborators are Robert Horton, Gerard J. Kluitenberg, and Quanjiu Wang. This work was conducted at Iowa State University.

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