To determine how tree canopy changes over the season, we conducted a temporal comparison of VO estimated fPAR for different varieties. Figure 3 shows the overall trend of VO estimated fPAR with individual almond variety and age over the growing season in 2019. In general, these different changing patterns might directly relate to the status of individual orchard management and practices. For example, canopy changes were similar for the trees in Orchards 2 and 3 that VO estimated fPAR first dropped a little from May to June or July and then bounced back in August, showing a "flat-U" shape for most of the trees excluding 6-year-old 'Nonpareil' trees continuously decreased during the season. All 11-year-old 'Nonpareil', 'Butte', and 'Carmel' varieties are from Orchard 3, where the trees might be under-irrigated during the midseason (June and July), causing a certain level of defoliation. Lampinen et al. reported continuous increases in lightbar estimated midday fPAR over time for both young (3-year-old) and mature (10-year-old) walnut orchards between April and November 2010. However, Rubke found that midday canopy fPAR increased only during the 2012 growing season, with data collected over 3 years (2012-2014). Overall, our results agreed with their findings because the majority (~50%) of our sampled trees are from Orchard 1 ('P16.013', 'P13.019', and 'Lonestar') with continuous increments of canopy fPAR over the season. The different changing patterns of the different varieties could also be attributed to their seasonal calendars. For example, as shown in Figure 2, the full bloom date and commercial harvest date for 'Nonpareil' (in Orchard 2) were on February 23 and August 20 in 2019, respectively. Most other varieties had the full bloom 1-6 days later and were harvested 2-4.5 weeks later than 'Nonpareil'.
We compared the correlations between VO and lightbar estimated fPAR over the entire growing season in 2019 at the per-row level. Table 3 shows the overall comparison of R2, RMSE (to the 1:1 line), and the regression line slopes when we force a zero intercept. R2 and RMSE are improved from May to June/July, while they remained the same level from the June/July to August (harvest). For example, they were 0.81 and 4.06%, respectively, when we tested all varieties on May 28. In June, R2 and RMSE improved to 0.91 and 2.94%, respectively, when the date of VO data collection was within a couple of days of lightbar data collection. When we tested 'Nonpareil' only, we observed similar patterns but with greater R2 (0.95-0.96) and smaller RMSE (2.06-2.27%). It can be inferred that both VO and lightbar methods measured canopy light interception similarly. So, we considered a hypothetical 1:1 correlation between the VO and lightbar measurements. All slopes of the regression lines were significantly different from the 1:1 line (95% confidence level) except for July 26, 'Nonpareil'. We then used different regression slopes for developing adjusted models for potential yield prediction using Equation 6 and summarized the results in Table 4. Comparing the regression line slopes to the 1:1 line in different days also indicated that the VO and lightbar measurements were better correlated when both data were collected within a few days. Therefore, we further calibrated and visualized the VO and lightbar estimated fPAR data in June.