## 3 Results  
### Time Series Land Surface Phenologies of NDVI and VODs  
All three VOD time series were able to capture the seasonality of croplands (both bimodal patterns and unimodal patterns), similar to MODIS NDVI (Figure 4). The bimodality arises from the planting of winter grains in the fall, and growth and harvest in the subsequent early summer. After snow melts, croplands are primarily barren or contain crop residues (non-photosynthetic biomass) prior to tilling and planting. Green-up occurs after seed germination at emergence (initiation of visible aboveground growth) but prior to significant aboveground biomass accumulation. This pattern is evident in Figures 4 and 5, where the rise in VOD occurs later than NDVI. Cropland peak VODs, which are sensitive to aboveground vegetation water content and biomass, consistently lagged in time and space from the peak NDVI, which is sensitive to vegetation greenness and leaf area index (Figures 4 and 5). Furthermore, the VODs showed slower senescence, likely a consequence of residual biomass (crop residue) following harvest. VOD PH for sites with \(\geq\)10% tree cover were not significantly different from sites with \(\geq\)99% croplands at \(\alpha\) = 0.05 and \(p\) > 0.05 (Figures 4 and 5) [46,47]. Higher microwave frequencies exhibit higher dynamic range: the shorter wavelengths are more attenuated by vegetation canopy (Figures 4 and 5). Figure 4: VOD time series for the three microwave frequencies: green pluses = 18.7; purple circles = 10.65; orange triangles = 6.925 GHz; and blue diamonds = NDVI Selected series arranged from south to north of our study region: (**a**) Stavropol, RU, site 2; (**b**) Kirovohrad, UA, site 15; (**c**) Petropavlovsk 3, KZ, site 42; and (**d**) Kazan, RU, site 49; and (**e**) Mari El, RU, site 50 (MFO site included for contrast). Note that the VOD range for the MFO site is higher (0.2–1.9) than the range for the cropland sites (0.2–1.6). LSPs at the lower latitudes within the region exhibit longer and/or bimodal patterns arising from the cultivation of winter grains (Figure 4a,b). On the other hand, LSPs at the higher latitudes exhibit shorter unimodal growing season due to cultivation of spring grains (Figure 4c,d). At each study site, the VOD and NDVI exhibit similar seasonal patterns, amid interannual variation. For example, in Petropavlovsk 3, KZ (Figure 4c), in 2003 there was high and earlier peak LSPs in fewer AGDD, while in 2010 there was lower and flatter peak in more AGDD, due to the 2010 heatwave affecting European Russia [19,20]. Note the 2007 VOD in Kirovohrad, UA (Figure 4b) is almost flattened compared to other years, due to the devastation of crops by the 2007 heatwave in Ukraine [48]. The heatwave effect is more evident on the VODs compared to their counterpart NDVI (Figure 4b), due to the sensitivity of VODs to vegetation water content. The Mari El mixed forest site in Russia displayed a distinct LSP in magnitude and shape compared to the rest of the 49 cropland sites (Figure 4e). Both VOD and NDVI were larger in magnitude (note the y-scales) and flatter in shape. The VODs in particular showed a much-delayed senescence compared to the NDVI. The magnitude and peak timing of VOD were found sensitive to woody plant foliage, whereas NDVI seasonality was governed by the green herbaceous vegetation [49, 50, 51]. Figure 5: VODs in three microwave frequencies (green squares = 18.7, purple circles = 10.65, and orange triangles = 6.925 GHz) and NDVI (blue diamonds) interannual variability of a southern study site (Simferopol, UA, 45.6\({}^{\circ}\)N) for 2003–2010. The 6.925 GHz vegetation transmissivity was missing from the source dataset in 2004. Seasonal patterns changed from bimodal (2003–2004) to unimodal (2005–2009) and back to bimodal (2010). Figure 5 shows Time series of VODs and NDVIs in a southern study site (Simferopol, UA) that experienced alternate unimodal and bimodal seasonal patterns over the eight years.