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

Abstract

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.

On the Potential of Active and Passive Microwave Remote Sensing for Tracking Seasonal Dynamics of Evapotranspiration

“…The microwave-based indicators (backscatter & VOD) are proxies by their nature and serve as first-order indicators of relative dynamics, as found in [14]. The focus of this work is the assessment of whether they could allow identifying seasonal patterns of ET as well as their spatio-temporal anomalies along differing dry and wet years.…”

Earth-Observation-Based Monitoring of Forests in Germany—Recent Progress and Research Frontiers: A Review

“…From the publications included in this review, eight studies were classified as having a focus on or contributing to research on biomass/productivity [10,47,59,76,81,87,113,119]. Three of these studies are on the European scale [10,59,81].…”

“…Others cover central Europe [47] and Bavaria [76]. Three studies include individual sites: European-wide sites including four Integrated Carbon Observing System (ICOS)-affiliated forest ecosystem sites in Germany [87], eddy covariance data of temperate forest ecosystems located between 35 • N and 65 • N including five sites in Germany [113], and local study sites in Thuringia [119]. Most studies made use of spaceborne multispectral data, including MODIS, Landsat, OCO-2, and TROPOspheric Monitoring Instrument (TROPOMI).…”

“…Most studies made use of spaceborne multispectral data, including MODIS, Landsat, OCO-2, and TROPOspheric Monitoring Instrument (TROPOMI). One study used radar C-band data from Sentinel-1 [119].…”

“…Productivity increased during spring and autumn in nearly half of the investigated forest area but did not compensate for summer decrease in 25% of core forest areas. Mueller et al [119] characterized evapotranspiration dynamics over temperate coniferous forest sites in Thuringia based on Sentinel-1 backscatter time series. They found that SAR backscatter signal of coniferous forests is sensitive to evapotranspiration under some scenarios.…”