# Relating Sentinel-1 Interferometric Coherence to Moving Events on Grasslands
## 3 Results and Discussion
The soil had a rather large moisture content (>50% soil moisture vs. <30% for other grasslands), and the grass was sparse, with lower wet above-ground biomass (AGB) values (e.g., 2.1 for G1, 4.1 for G3, and 1.6 for G5 in kg/m\({}^{2}\) at the peak of the season). The sparseness of vegetation would cause more coherent backscatter from the soil, especially after partial removal of vegetation, resulting in higher coherence values. G5 was the only grassland where weak relation between soil moisture and coherence measurements was detected, with R\({}^{2}\) = 0.25 for \(\gamma_{vh}\) and R\({}^{2}\) = 0.56 for \(\gamma_{vv}\). After a moving event on G3 in the middle of July, the increase of coherence values was small. Low coherence values computed from the R58 24 July/5 August image pair could be caused by the specific moving method used. Grass was moved above \(\approx\)30 cm to harvest the seeds, and the remaining straws were pressed to the ground. The biomass that was left on the ground decreased the portion of backscatter coming from the soil and led to lower coherence values. Additionally, the positions of these long straws could be changed by the wind between the acquisitions, thereby decreasing the coherence. Furthermore, R160 acquisitions were most probably also influenced by rain. Conversely, several polarimetric studies have reported higher sensitivity to harvesting events when vegetation was left on the ground after an event [19; 20; 40]. Differences could also be observed between the results for VV and VH. The response of VH coherence (\(\gamma_{vh}\)) was rather different from that of \(\gamma_{vv}\) for G1 in May and the beginning of June--values of \(\gamma_{vh}\) were much smaller, by 0.2-0.3 units, ranging across all geometries. In the case of G2, R160 \(\gamma_{vh}\) reached >0.30 in July, while \(\gamma_{vv}\) remained under 0.20. Further differences in response could be observed for G5: \(\gamma_{vv}\) 0.27 vs. \(\gamma_{vh}\) 0.44 for R58 6 June/18 June image pair, and \(\gamma_{vv}\) 0.38 vs. \(\gamma_{vh}\) 0.27 for R58 5 August/17 August image pair, as well as for G6: \(\gamma_{vv}\) 0.23 vs. \(\gamma_{vh}\) 0.33 for R160 16 June/25 June image pair, and \(\gamma_{vv}\) 0.13 vs. \(\gamma_{vh}\) 0.28 for R80 26 July/7 August image pair. The divergences could have been caused by differences in scattering due to soil roughness and vegetation structure. For example,  
Figure 4: Sentinel 1A 12-day VH coherence before (**left**) and after (**right**) a moving event on G5 on 6 June 2015.  
the VV backscatter for G1 in May (bare soil) was on average 10 dB higher than VH, with VH values dropping as low as \(-\)24 dB, reaching close to the noise-equivalent sigma zero (NESZ) value of \(-\)28 dB. Easily observable differences existed between \(\gamma_{vv}\) and \(\gamma_{vh}\) values of different RONs. These could be explained by the difference in acquisition times, with conditions on the grasslands varying rapidly when the vegetation is growing. An additional source of decorrelation was likely the morning dew, as morning acquisitions (R80) produced consistently lower coherence values on average. The varying dew conditions can cause variation in the position of scatterers in a scene, thus decreasing coherence. The R58 geometry, on the other hand, might have resulted in lower coherence values on average, due to shallower incidence angles (for the middle of the study area, the incidence angles were 38.2\({}^{\circ}\) for R160, 44.8\({}^{\circ}\) for R58, and 39.3\({}^{\circ}\) for R80). This is in contrast to polarimetric approaches for harvest detection, where shallower incidence angles are often preferred [41]. The observed differences in coherence responses after moving events could be caused by several factors, such as soil roughness and moisture, species composition, and precipitation (or intercepted moisture). Additionally, time separation between a moving event and first acquisition of the InSAR pair has to be taken into account.