## 3 Results
### Thermal Time to Peak and Peak Height NDVI and VOD Phenometrics
#### 3.2.2 The Maximum Value Approach for VOD Phenometrics  
For the land surface phenology of VOD, Figure 10 presents the PH\({}_{\text{VOD}}\) as a function of the corresponding TTP\({}_{\text{VOD}}\) for all sites. In the study sites dominated by croplands, PH\({}_{\text{VOD}}\) ranged between 0.85 and 1.74, while the PH\({}_{\text{NDVI}}\) (also calculated using the maximum value approach) ranged between 0.60 and 0.83. The ordering of the PH\({}_{\text{VOD}}\) magnitude from high to low was 18.7 GHz > 10.65 GHz > 6.925 GHz: the higher the frequency, the higher the attenuation by vegetation, and, thus, the larger the VOD magnitude. The PH\({}_{\text{VOD}}\) lags the PH\({}_{\text{NDVI}}\), because the PH\({}_{\text{VOD}}\) responds to vegetation water content and aboveground biomass, which both continue to increase after the apparent peak absorption of photosynthetically active radiation, which is captured by the PH\({}_{\text{NDVI}}\). Generally, the shorter the microwave wavelength, the later the PH\({}_{\text{VOD}}\) occurs in terms of AGDD. The lag between the PH\({}_{\text{VOD}}\) for 10.65 and 18.7 GHz was narrower than between the PH\({}_{\text{VOD}}\) for 6.925 GHz from the other two. We have presented a quantified lag graph (Figure 11b) among the maximum value determined PH VODs and the CxQ model derived PH NDVI in Section 4.1. Figure 9: Annual phenometrics of NAHT as a function of PH\({}_{\text{NDVI}}\): (**a**) for all study sites; and (**b**) selected study sites for 2003–2010. Sites are numbered according to their latitudinal position from south (1) to north (50) (Figure 1). AMSR-E VOD was found more highly correlated with MODIS VI's compared to anther passive microwave data, Soil Moisture and Ocean Salinity (SMOS) VOD did with the MODIS VI's [53]. Second, the covariate \(\mathrm{PH_{NDVI}}\) was correlated with response variable \(\mathrm{PH_{VOD}}\) as described in the previous paragraph and in Table 1 and Figure 11[45]. The interpretation from the fulfilment of these two ANCOVA assumptions is that peak greenness (\(\mathrm{PH_{NDVI}}\)) had a significant and positive effect on \(\mathrm{PH_{VOD}}\) and the effect is similar for all three microwave frequencies (the effect of \(\mathrm{PH_{NDVI}}\) on \(\mathrm{PH_{VOD}}\) did not depend on microwave frequency). Even though \(\mathrm{PH_{NDVI}}\) and \(\mathrm{PH_{VOD}}\) have significant positive correspondence, the latter lagged the former (Figure 12b). This behavior was expected since microwaves are differentially sensitive to aboveground biomass and canopy water content; in contrast, the VNIR is differentially sensitive to photosynthetically active radiation (PAR) absorption. The peak VOD lags the peak greenness by several weeks following delayed increases in vegetation water content and development of aboveground biomass. Average VOD lag for all study years and sites was about three weeks, but it varied in time and space, and by microwave frequency (Figure 12b). We have also presented the \(\mathrm{PH_{VOD}}\) and \(\mathrm{PH_{NDVI}}\) as a function of their respective TTP metric in Figure 12a for ease of interpretation of the lags in Figure 12b. The temporal and spatial average \(\mathrm{PH_{VOD}}\) lag increases as microwave frequency increases. The 6.925, 10.65, and 18.7 \(\mathrm{GHz}\)\(\mathrm{PH_{VOD}}\) average lags are 1.9, 2.2 and 2.7 weeks, respectively, relative to the peak greenness (Figure 12b and Table 1). \(\mathrm{PH_{NDVI}}\) lags \(\mathrm{PH_{VOD}}\) 6.925 GHz at three sites only. Few sites that showed \(\mathrm{PH_{NDVI}}\) temporal average lag to that of the \(\mathrm{PH_{VOD}}\)--three sites for the 6.925, one site for the 10.65 and one site for the 18.7 \(\mathrm{GHz}\)\(\mathrm{PH_{VOD}}\) over the study period (all lags less than a week). Across the 50 sites the average temporal lag of \(\mathrm{PH_{VOD}}\) relative to \(\mathrm{PH_{NDVI}}\) ranges from \(-0.8\) to +5.1 weeks for 6.925 GHz, from \(-0.1\) to 6.7 weeks for 10.65 GHz, and from \(-0.9\) to 7.2 weeks for 18.7 GHz.