Remotely Sensed Soil Moisture Can Capture Dynamics Relevant to Plant Water Uptake

• Published in: Water resources research Vol. 59; no. 2
• Main Authors: Feldman, Andrew F., Dong, Jianzhi, Akbar, Ruzbeh, Crow, Wade T., McColl, Kaighin A., Konings, Alexandra G., Tumber-Dávila, Shersingh Joseph, Holbrook, Noel M., Rockwell, Fulton E., Scott, Russell L., Reichle, Rolf Helmut, Chatterjee, Abhishek, Joiner, Joanna, Poulter, Benjamin, Entekhabi, Dara
• Format: Journal Article
• Language: English
• Published: Goddard Space Flight Center Wiley 01-02-2023; John Wiley & Sons, Inc
• Subjects: Agricultural land; Biosphere; Depth; Earth Resources and Remote Sensing; Estimates; Evaporation; Grasslands; Hydrosphere; isotope; Isotopic tracers; L‐band satellite; microwave remote sensing; Moisture effects; Plant water; plant water uptake; Remote sensing; Representations; Satellite observation; Satellite soil moisture estimates; Satellites; Shrubs; Soil; Soil dynamics; Soil layers; Soil moisture; soil sensing depth; Soil water; Spatial variations; Surface boundary layer; Surface layers; Terrestrial ecosystems; Tracers; Uptake; Vegetation; Water; Water uptake
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2022WR033814

A frequently expressed viewpoint across the Earth science community is that global soil moisture estimates from satellite L-band (1.4 GHz) measurements represent moisture only in a shallow surface layer (0–5 cm) and consequently are of limited value for studying global terrestrial ecosystems because plants use water from deeper rootzones. Using this argumentation, many observation-based land surface studies avoid satellite-observed soil moisture. Here, based on peer-reviewed literature across several fields, we argue that such a viewpoint is overly limiting for two reasons. First, microwave soil emission depth considerations and statistical considerations of vertically correlated soil moisture information together indicate that L-band measurements carry information about soil moisture extending below the commonly referenced 5 cm in many conditions. However, spatial variations of effective depths of representation remain uncertain. Second, in reviewing isotopic tracer field studies of plant water uptake, we find a prevalence of vegetation that primarily draws moisture from these upper soil layers. This is especially true for grasslands and croplands covering more than a third of global vegetated surfaces. Even some deeper-rooted species (i.e., shrubs and trees) preferentially or seasonally draw water from the upper soil layers. Therefore, L-band satellite soil moisture estimates are more relevant to global vegetation water uptake than commonly appreciated (i.e., relevant beyond only shallow soil processes like soil evaporation). Our commentary encourages the application of satellite soil moisture across a broader range of terrestrial hydrosphere and biosphere studies while urging more rigorous estimates of its effective depth of representation.