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Soil-water Determination Using Fiber Optics¹

ABSTRACT

Light transmission through a 1.0-mm diam quartz rod was found to be consistently related to the water status of the surrounding soil. Increased soil water resulted in decreased light transmission. The measurements were made using a modified Tempe cell containing glass beads and a very fine sandy loam soil material. The major components of the measurement system were a quartz rod, a lightemitting diode, a phototransistor, fiber optic cable, and connectors. The rod was in the shape of a loop centered in the cell and separated from direct contact with the surrounding soil by a small pocket of glass beads. Increasing levels of air pressure were applied at the Tempe cell inlet; drainage volume and instrument voltage were recorded at equilibrium. These data were analyzed separately with pressure and volumetric water content as independent variables. The voltage had a highly significant response to either independent variable and there was no significant difference between slopes or intercepts of regression lines across repeated desorption cycles. The range of volumetric water content measured in the experiment was 0.10 to 0.40. It was concluded that this technique of soil water measurement is feasible and deserves further research.

¹ Contribution from the Dep. of Soil Science, North Dakota State Univ., Fargo, ND 58105.

² Graduate Research Assistant and Associate Professor of Soil Science, respectively, North Dakota State Univ., Fargo, ND 58105.

Received for publication May 20, 1985.

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