Vegetation Optical Depth and Soil Moisture Retrieved from L-Band Radiometry over the Growth Cycle of a Winter Wheat

Abstract

L-band radiometer measurements were performed at the Selhausen remote sensing field laboratory (Germany) over the entire growing season of a winter wheat stand. L-band microwave observations were collected over two different footprints within a homogenous winter wheat stand in order to disentangle the emissions originating from the soil and from the vegetation. Based on brightness temperature (T B ) measurements performed over an area consisting of a soil surface covered by a reflector (i.e., to block the radiation from the soil surface), vegetation optical depth (τ) information was retrieved using the tau-omega (τ-ω) radiative transfer model. The retrieved τ appeared to be clearly polarization dependent, with lower values for horizontal (H) and higher values for vertical (V) polarization. Additionally, a strong dependency of τ on incidence angle for the V polarization was observed. Furthermore, τ indicated a bell-shaped temporal evolution, with lowest values during the tillering and senescence stages, and highest values during flowering of the wheat plants. The latter corresponded to the highest amounts of vegetation water content (VWC) and largest leaf area index (LAI). To show that the time, polarization, and angle dependence is also highly dependent on the observed vegetation species, white mustard was grown during a short experiment, and radiometer measurements were performed using the same experimental setup. These results showed that the mustard canopy is more isotropic compared to the wheat vegetation (i.e., the τ parameter is less dependent on incidence angle and polarization). In a next step, the relationship between τ and in situ measured vegetation properties (VWC, LAI, total of aboveground vegetation biomass, and vegetation height) was investigated, showing a strong correlation between τ over the entire growing season and the VWC as well as between τ and LAI. Finally, the soil moisture was retrieved from T B observations over a second plot without a reflector on the ground. The retrievals were significantly improved compared to in situ measurements by using the time, polarization, and angle dependent τ as a priori information. This improvement can be explained by the better representation of the vegetation layer effect on the measured T B .

Estimating Gravimetric Water Content of a Winter Wheat Field from L-Band Vegetation Optical Depth

“…Tower-based passive microwave measurements over a winter wheat (Triticum aestivum L.) field were carried out using the ELBARA-II radiometer during summer 2017 [1]. The measurements were performed at the Selhausen remote sensing field laboratory (Germany) [21].…”

“…The collected brightness temperature (T B ) data showed that the T BV measurements had a higher temporal variability with generally higher values over the entire growing period, and also a larger increase and dynamic range during crop development, compared to T BH (e.g., a STD of 18.4 K for T BH and 53 K for T BV using a 40 • incidence angle). This can be explained by the stronger vertical orientation of the wheat canopy leading to a higher emission contribution in the V polarization [1]. In addition, weekly in situ measurements of vegetation properties were performed for: leaf area index (LAI), total above ground biomass (TOB), vegetation water content (VWC), growing stages (BBCH), and vegetation height (d).…”

“…The vegetation layer changed significantly during the phenological cycle of the winter wheat plants with a leaf-dominated vegetation structure between DOY 100 and 120, followed by a period of stalk-dominated vegetation structure (DOY 120-180) where the highest VWC was reached during flowering, and a period where the vegetation turned into senescence (DOY ≥ 200) and the heads as well as the stems of the wheat began to bend towards the soil surface. For a more detailed description of the field setup and the collected measurement datasets (i.e., in situ and radiometer) see Meyer et al [1]. In the current study, we used the polarization (p) dependent vegetation optical depth (τ) parameter, which was retrieved from the 40 • incidence angle brightness temperature measurements using the τ-ω model [32].…”

“…In this regard, our research aims to develop and validate at the field scale a new approach to retrieve the gravimetric vegetation water content (m g ) from the vegetation attenuation of L-band microwave radiation (τ parameter). Tower-based L-band (1.4 GHz) microwave measurements above a winter wheat field were acquired over an entire growing cycle together with in situ measurements of vegetation properties [1]. The proposed attenuation-based approach to retrieve m g is built on the τ model of Schmugge and Jackson [2] and on the Debye-Cole dual dispersion dielectric model of Ulaby and El-Rayes [3].…”

Soil Moisture Remote Sensing across Scales

“…They showed that models dealing with more complex physical phenomena, such as multiple scattering and reflections, can provide better retrievals for dense (and strongly-scattering) vegetation while giving the same results as the "Tau-Omega" model for sparse and low vegetation. Furthermore, using a passive radiometer over wheat and mustard fields, Meyer et al [10] showed the importance of using a time-, polarization-, and angle-dependent optical depth (τ) parameter at the field scale for accurate soil moisture retrieval. Although the importance of those parameters is probably less important at a large scale, these effects should probably be taken into account for multi-incidence angle radiometers such as the SMOS follow-on, SMOS-HR (with an increased resolution of 10 km [11]).…”