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

Abstract

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 paper, we review recent advances in non-invasive techniques that allow continuous non-invasive and contactless measurements of soil moisture dynamics at the field to basin scale. In particular, we report on 1) cosmic-ray neutron probes, 2) GNSS reflectrometry, 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

Soil Moisture for Hydrological Applications: Open Questions and New Opportunities

“…Notwithstanding soil moisture is largely recognized as a fundamental physical variable, e.g., soil moisture is included at the 2nd place among the Essential Climate Variable by the Global Climate Observing System (GCOS), its monitoring over large areas is still an open research activity [23,29]. Indeed, differently from precipitation, in situ monitoring networks for soil moisture are much less developed and only in few countries (e.g., United States) a good coverage of in situ stations is present (see Figure 1).…”

“…The emergence of wireless sensor networks made it possible to cover larger areas with low-budget soil moisture sensors [35], but still this technology is only used for scientific studies. In the last decade, novel technologies were developed for providing soil moisture measurements over larger areas [29]. Currently, cosmic ray neutron sensors, Global Positioning System (GPS), and geophysical measurements (e.g., electrical resistivity and electromagnetic induction) are the most promising techniques [33,[36][37][38].…”

“…Moreover, the maintenance of in situ soil moisture networks requires significant economic and human resources (even though lower than climatic stations) and, frequently, it is a non-trivial task to make a network operational for several (e.g., >5) years [34]. On this basis, it is evident that the development of established techniques providing measurements at 0.1-1 km spatial scale is vital [29].…”

Validation of Spaceborne and Modelled Surface Soil Moisture Products with Cosmic-Ray Neutron Probes

“…Being a relatively new soil moisture monitoring method, the accuracy of CRNP soil moisture data is still a topic of discussions [51,59,109]. As mentioned earlier, CRNP observes a footprint of several hundreds of metres; therefore a direct validation using data from single point in situ sensors (e.g., TDR) is unfeasible given the difference in spatial scale.…”

Advancing NASA’s AirMOSS P-Band Radar Root Zone Soil Moisture Retrieval Algorithm via Incorporation of Richards’ Equation

“…Soil moisture impacts the water and energy cycles and the exchange of trace gases, including carbon dioxide, between land and atmosphere [1]. A number of comprehensive review articles published in recent years [1][2][3][4][5] highlight the crucial importance of spatial and temporal soil moisture information at various scales for virtually all hydrologic, atmospheric, and ecological processes.…”