Emerging methods for noninvasive sensing of soil moisture dynamics from field to catchment scale: a review

Soil moisture is an important state variable in the terrestrial system because it controls the exchange of water and energy between the land surface and the atmosphere. In this study, we review recent advances in noninvasive techniques that allow continuous noninvasive and contactless measurements of soil moisture dynamics at the field to basin scale. In particular, we report on (1) cosmic-ray neutron probes, (2) Global Navigation Satellite System reflectometry, (3) ground-based microwave radiometry, (4) gamma-ray monitoring, (5) terrestrial gravimetry, and (6) low-frequency electromagnetic surface waves. Each method is described in terms of its basic principle, measurement scales, calibration issues, measurement accuracy, and applications. We hope that this review will further stimulate the community to invest in the continued development of novel soil moisture sensing methods that address the need for large-scale soil water content measurements with sufficiently high temporal resolution. (C) 2015 Wiley Periodicals, Inc.