Temporal Characteristics of P-Band Tomographic Radar Backscatter of a Boreal Forest

Monteith, Albert R.; Ulander, Lars M. H.

doi:10.1109/jstars.2021.3050611

Cited by 17 publications

(6 citation statements)

References 62 publications

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“…This behavior is opposite to the findings by Monteith and Ulander [58], who observed a decrease of backscatter intensity in the spring and summer months and an increase of backscatter values in autumn and winter. These behaviors could be explained by the different wavelengths, as Monteith and Ulander [58] used a longer wavelength P-band microwave sensor for their study, where more influence of the ground (soil moisture) and the tree trunks instead of the canopy should be more prominent. Additionally, more sparse boreal forests were studied compared to the temperate forests in this study.…”

Section: Sentinel-1 Backscatter Time Seriescontrasting

confidence: 99%

“…Moreover, while the Sentinel-1 acquisitions are either early in the morning or evening, the in situ evapotranspiration is based on the daily averages. Compared to the Sentinel-1 acquisition times, the other product acquisition times fall, therefore, within different domains of the diurnal cycle of biophysical activity, which might explain a decrease in the correlation for some data combinations compared to others [58,65,68]. A noticeable decrease in correlation for the ascending overpass direction can be observed at the AOI "Oberweissbach", where the correlation values are systematically lower compared to the other stations.…”

Section: Evapotranspiration Time Seriesmentioning

confidence: 94%

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Sentinel-1 Backscatter Time Series for Characterization of Evapotranspiration Dynamics over Temperate Coniferous Forests

et al. 2022

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Forests’ ecosystems are an essential part of the global carbon cycle with vast carbon storage potential. These systems are currently under external pressures showing increasing change due to climate change. A better understanding of the biophysical properties of forests is, therefore, of paramount importance for research and monitoring purposes. While there are many biophysical properties, the focus of this study is on the in-depth analysis of the connection between the C-band Copernicus Sentinel-1 SAR backscatter and evapotranspiration (ET) estimates based on in situ meteorological data and the FAO-based Penman–Monteith equation as well as the well-established global terrestrial ET product from the Terra and Aqua MODIS sensors. The analysis was performed in the Free State of Thuringia, central Germany, over coniferous forests within an area of 2452 km2, considering a 5-year time series (June 2016–July 2021) of 6- to 12-day Sentinel-1 backscatter acquisitions/observations, daily in situ meteorological measurements of four weather stations as well as an 8-day composite of ET products of the MODIS sensors. Correlation analyses of the three datasets were implemented independently for each of the microwave sensor’s acquisition parameters, ascending and descending overpass direction and co- or cross-polarization, investigating different time series seasonality filters. The Sentinel-1 backscatter and both ET time series datasets show a similar multiannual seasonally fluctuating behavior with increasing values in the spring, peaks in the summer, decreases in the autumn and troughs in the winter months. The backscatter difference between summer and winter reaches over 1.5 dB, while the evapotranspiration difference reaches 8 mm/day for the in situ measurements and 300 kg/m2/8-day for the MODIS product. The best correlation between the Sentinel-1 backscatter and both ET products is achieved in the ascending overpass direction, with datasets acquired in the late afternoon, and reaches an R2-value of over 0.8. The correlation for the descending overpass direction reaches values of up to 0.6. These results suggest that the SAR backscatter signal of coniferous forests is sensitive to the biophysical property evapotranspiration under some scenarios.

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Section: Sentinel-1 Backscatter Time Seriescontrasting

confidence: 99%

Section: Evapotranspiration Time Seriesmentioning

confidence: 94%

Sentinel-1 Backscatter Time Series for Characterization of Evapotranspiration Dynamics over Temperate Coniferous Forests

et al. 2022

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“…Clear diurnal cycles were found in tower-based observations from forest stands (e.g. Hamadi et al, 2014;Monteith and Ulander, 2021) and agricultural cropland (e.g. Vermunt et al, 2020), as well as in aggregated satellite observations from larger forested areas (Paget et al, 2016;Emmerik et al, 2017;Konings et al, 2017).…”

Section: Introductionmentioning

confidence: 89%

Extrapolating continuous vegetation water content to understand sub-daily backscatter variations

Vermunt

Steele‐Dunne

Khabbazan

et al. 2022

Hydrol. Earth Syst. Sci.

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Abstract. Microwave observations are sensitive to vegetation water content (VWC). Consequently, the increasing temporal and spatial resolution of spaceborne microwave observations creates a unique opportunity to study vegetation water dynamics and its role in the diurnal water cycle. However, we currently have a limited understanding of sub-daily variations in the VWC and how they affect microwave observations. This is partly due to the challenges associated with measuring internal VWC for validation, particularly non-destructively, and at timescales of less than a day. In this study, we aimed to (1) use field sensors to reconstruct diurnal and continuous records of internal VWC of corn and (2) use these records to interpret the sub-daily behaviour of a 10 d time series of polarimetric L-band backscatter with high temporal resolution. Sub-daily variations in internal VWC were calculated based on the cumulative difference between estimated transpiration and sap flow rates at the base of the stems. Destructive samples were used to constrain the estimates and for validation. The inclusion of continuous surface canopy water estimates (dew or interception) and surface soil moisture allowed us to attribute hour-to-hour backscatter dynamics either to internal VWC, surface canopy water, or soil moisture variations. Our results showed that internal VWC varied by 10 %–20 % during the day in non-stressed conditions, and the effect on backscatter was significant. Diurnal variations in internal VWC and nocturnal dew formation affected vertically polarized backscatter most. Moreover, multiple linear regression suggested that the diurnal cycle of VWC on a typical dry day leads to a 2 (HH, horizontally, and cross-polarized) to almost 4 (VV, vertically, polarized) times higher diurnal backscatter variation than the soil moisture drydown does. These results demonstrate that radar observations have the potential to provide unprecedented insight into the role of vegetation water dynamics in land–atmosphere interactions at sub-daily timescales.

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“…That is why some previous studies also revealed saturation problems, especially for high-frequency SAR data gathered in areas with high-level forest AGB [12]. The geometry partly determines the scattering direction, scattering distribution, and scattering strength of forest structure reflections [16]. Since forest backscatter is governed partly by forest structure geometry, including tree boles, branches, and leaves, relating SAR backscatter coefficients to biomass components, such as the biomass of tree stems, bark, branches, and needles or leaves, may improve the previously established thresholds of saturation problems [13].…”

Section: Introductionmentioning

confidence: 99%

Forest Total and Component Above-Ground Biomass (AGB) Estimation through C- and L-band Polarimetric SAR Data

Zeng

Zhang

et al. 2022

Forests

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Forest biomass plays an essential role in forest carbon reservoir studies, biodiversity protection, forest management, and climate change mitigation actions. Synthetic Aperture Radar (SAR), especially the polarimetric SAR with the capability of identifying different aspects of forest structure, shows great potential in the accurate estimation of total and component forest above-ground biomass (AGB), including stem, bark, branch, and leaf biomass. This study aims to fully explore the potential of polarimetric parameters at the C- and L-bands to achieve high estimation accuracy and improve the estimation of AGB saturation levels. In this study, the backscattering coefficients at different polarimetric channels and polarimetric parameters extracted from Freeman2, Yamaguchi3, H-A-Alpha, and Target Scattering Vector Model (TSVM) decomposition methods were optimized by a random forest algorithm, first, and then inputted into linear regression models to estimate the total forest AGB and biomass components of two test sites in China. The results showed that polarimetric observations had great potential in total and component AGB estimation in the two test sites; the best performances were for leaves at test site I, with R2 = 0.637 and RMSE = 1.27 t/hm2. The estimation of biomass components at both test sites showed obvious saturation phenomenon estimation according to their scatter plots. The results obtained at both test sites demonstrated the potential of polarimetric parameters in total and component biomass estimation.

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Temporal Characteristics of P-Band Tomographic Radar Backscatter of a Boreal Forest

Cited by 17 publications

References 62 publications

Sentinel-1 Backscatter Time Series for Characterization of Evapotranspiration Dynamics over Temperate Coniferous Forests

Sentinel-1 Backscatter Time Series for Characterization of Evapotranspiration Dynamics over Temperate Coniferous Forests

Extrapolating continuous vegetation water content to understand sub-daily backscatter variations

Forest Total and Component Above-Ground Biomass (AGB) Estimation through C- and L-band Polarimetric SAR Data

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