# Relating Sentinel-1 Interferometric Coherence to Moving Events on Grasslands
## 3 Results and Discussion  
Coherence values for each of the six measurement areas where vegetation parameters were recorded throughout the summer are given in Figures 2 and 3 for VH and VV polarisations, respectively. These time series illustrate the complexity of the relationship between moving events and coherence. First of all, a moving event was rarely characterised by the complete removal of vegetation, and grass with height of 0.1 to 0.2 m was often left on the field. Thus, very high coherence caused by coherent scattering from soil was rarely observed. Even when bare soil was present (G1 in May, and G3/G4 at the end of August), coherence values fluctuated. In the case of G1, low VV coherence (\(\gamma_{vv}\)) in the presence of bare soil was recorded in the middle of May. It was likely caused by farming activity visible on the field survey photos. For G3/G4, after the complete removal of vegetation in August, the grasslands were subjected to further farming activities, causing coherence to remain rather low. In the presence of vegetation, coherence values were also rather varied. However, some changes were apparent, and could be linked to moving events. For example, R160 \(\gamma_{vv}\) increased from 0.15 to 0.47 for G5 after a moving event on 6 June. Figure 4 illustrates the increase of coherence after this event. To a lesser extent, \(\gamma_{vv}\) increased from 0.18 to 0.34 for the same grassland after an event on 3 August. For the same events, R58 \(\gamma_{vv}\) changed from 0.16 to 0.27 and from 0.24 to 0.38. The increase of coherence was mostly not so apparent for other grasslands that were inspected weekly. In the case of G6, the responses to an event in 8 June and 9 June were weak: R160 \(\gamma_{vv}\) increased from 0.15 to 0.23 and R58 from 0.19 to 0.22. Response from R58 was probably influenced by rain--the 3-h precipitation estimate before the 18 June acquisition was 0.5 mm. The second moving on 17 July was followed by a slight increase in R160 and R58 \(\gamma_{vv}\). The R160 \(\gamma_{vv}\) value, however, might have been decreased by precipitation, as the 3-h precipitation estimate before the 21 July acquisition was 1.3 mm. R80's \(\gamma_{vv}\) value also increased, from 0.14 to 0.27 for G4 after an event in mid July, while R160 coherence was severely influenced by rain. Weaker responses could be observed in the cases of G1 and G2. An increase of \(\gamma_{vv}\) was recorded after an event in July when R160 increased from 0.17 to 0.23 (G1) and R58 increased from 0.14 to 0.24 (G2). For other events, precipitation might have mitigated the increase. Precipitation estimates indicated rain in the case of the 21 July/12 August pair for G1, and the 19 July/31 July and 31 July/ 12 August pairs for G2. For both fields, \(\gamma_{vv}\) remained low in the beginning of August, possibly because of plant growth. The 12 August/24 August coherence was low for G1 and G2, and field survey data indicated that grass had grown by about 30 cm on G1 and by 10 cm on G2 between the image pair acquisitions. This indicates that the 12-day repeat pass configuration is not temporally dense enough for the moving detection in the case of rapidly growing vegetation. Figure 2: The 2015 time series of recorded height and coherence values for grasslands where the field survey was conducted. Polarization is vertical transmit, horizontal receive (VH). Date of the first image of the coherence pair is given on the \(x\)-axis. Vertical blue lines indicate the sum of 3-h precipitation estimates (PE) in mm for the image pair. Figure 3: The 2015 time series of recorded height and coherence values for grasslands where the field survey was conducted. Polarization is vertical transmit, vertical receive (VV). Date of the first image of the coherence pair is given on the \(x\)-axis. Vertical blue lines indicate the sum of 3-h precipitation estimates (PE) in mm for the image pair. The stronger response to moving on G5 could be partially explained by the vegetation structure present on the grassland.