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Surface Soil Water Content Measurement Using Pulsed Nuclear Magnetic Resonance Techniques¹

ABSTRACT

A tractor-mounted pulsed nuclear magnetic resonance (NMR) moisture measurement instrument has been developed and fabricated for use in collecting ground truth data for calibrating and evaluating remotely sensed determinations. The measurement approach is based upon NMR which depends on the interaction between hydrogen nuclear magnetic moments and a magnetic field. The sensor unit, consisting of an electromagnet, detection coil, and tuning capacitor, is towed behind the tractor and provides continuous readout of the volumetric soil water content at selected depths to 63 mm. Preliminary laboratory measurements with this instrument on two soils, a clay and a loamy fine sand, correlate linearly with volumetric soil water content. Discrimination of water NMR signals from signals from other hydrogen-bearing material is based on nuclear relaxation times, which reflect the molecular environment and structural bonding characteristics of the water molecules in the soil.

¹ Contribution from USDA-ARS, Hydrology Laboratory, Plant Physiology Institute, Beltsville, MD 20705.

² Soil Scientist, USDA-SCS, Hydrology Laboratory, Beltsville, MD, and Senior Research Scientists, Southwest Research Institute, San Antonio, TX.

Received for publication May 26, 1984. Accepted for publication January 16, 1985.