Monitoring of Agricultural Grasslands With Time Series of X-Band Repeat-Pass Interferometric SAR

Abstract

This study evaluates the potential of X-band interferometry for monitoring of agricultural grasslands. Time series of HH-polarization COSMO-SkyMed 1-day repeat-pass interferometric SAR (InSAR) pairs is analyzed in regard to detecting mowing events, and assessing vegetation height and biomass on grasslands. The time series of four InSAR pairs was analyzed in regard to the ground reference data collected during an extensive campaign covering 11 agricultural grasslands. The calculated temporal interferometric coherence was found to be inversely correlated to the vegetation height and wet above-ground biomass. It was found that grass removal increases the coherence magnitude indicating a potential use of this parameter for the detection of mowing. However, precipitation and farming activity between the acquisitions interfere with this effect. Temporal coherence was expressed as a function of the vegetation height through the random motion of scatterers in the vegetation layer. For vegetation height limited to the range between 0 and 1 m, a very strong correlation between the grass height and the linearised temporal coherence was found, with a coefficient r = 0.81. No significant correlation was found between the backscattering coefficient and the wet above-ground biomass as well as the height of grass. However, a strong negative correlation was found between the backscattering coefficient and the measured soil moisture.

Relating Sentinel-1 Interferometric Coherence to Mowing Events on Grasslands

“…Increase of coherence after a mowing event is in line with a similar previous study that was based on X-band data and 1-day interferometric pairs [27]. In contrast to that study, the use of C-band in this experiment might have ensured that the coherence values were statistically significantly higher for a period of up to 36 days after an event.…”

“…Precipitation up to 3-h before one or both of the image acquisitions changes the structure and dielectric constant of the vegetation on grassland, remarkably decreasing the coherence. Loss of coherence due to precipitation has also been reported by Zalite et al [27] and Ahmed et al [42]. At the same time, R58 and R80 geometries did not show any significant differences-the notches around the median that represent 95% confidence intervals are overlapping.…”

“…In the study carried out by Zalite et al [27], it was shown that precipitation can hinder the detection of mowing events when using a similar approach to that employed in this research. In order to investigate the effect precipitation has on temporal coherence, precipitation amounts before the images in the first interferometric pair after the mowing events were estimated using a DualPol weather radar located in Sürgavere, 50 km to the north-west of the study area.…”

“…One of the obligations of farmers is to maintain their grasslands by mowing or grazing on a yearly basis. National Paying Agencies (NPAs) are required to validate the mowing detection of mowing events on grasslands, has been studied by Zalite et al [26,27], where it was demonstrated how 1-day X-band interferometric coherence is much higher for mowed grasslands when compared to grasslands covered by vegetation. However, meteorological effects play an important role in the interpretation of interferometric coherence [28][29][30], including over bare ground [31].…”

Detection of Archaeological Residues in Vegetated Areas Using Satellite Synthetic Aperture Radar

“…Studies have shown inverse relationships between vegetation height and coherence at various SAR frequencies, including at X-band and with COSMO SkyMed [60,61]. However, with X-band, complete temporal decorrelation is soon reached with vegetation growth and short temporal baselines are necessary to derive quantitative information on crop height [60,61].…”