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

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. Soil moisture is highly variable in space and time with characteristic length scales ranging from a few centimeters up to several kilometers and characteristic time scales ranging from minutes up to years. Information on soil moisture dynamics is important for optimizing agricultural management, and to improve our understanding of biogeochemical processes, vadose zone processes, and atmospheric processes. Despite a wealth of available soil moisture measurement techniques, there still exists a gap in measurement capability of classical soil moisture monitoring technologies that makes large-scale soil moisture characterization with high temporal resolution highly challenging. In this presentation, we will review emerging soil moisture sensing techniques that are particularly well suited to overcome this space–time scale gap in measurement capability. In particular, we focus on recent advances in noninvasive techniques that allow continuous noninvasive and contactless measurements of soil moisture dynamics at the field to basin scale, e.g. cosmic-ray neutron probes (CRNP), GNSS reflectometry, ground-based microwave radiometry, gamma-ray monitoring, terrestrial gravimetry, and low-frequency electromagnetic surface waves (LFEMW). These methods have the following important advantages compared to other soil moisture sensing methods: (1) soil structure remains undisturbed by the measurements, (2) the measurement device can be operated continuously (e.g., also during tilling operations), and (3) the soil moisture measurements integrate large areas (i.e., larger than 100m²). We will present basic principles with a particular focus on measurement scale and accuracy, and will also provide example applications for selected methods.