## 3 Results
### Illumination Condition and Vegetation Indices
Colors represent low mean IC values (purple) to high mean IC values (yellow). Furthermore, the images used in this analysis correspond to different dates in different years. Because it is known from other studies that IC change at different dates [13; 14; 16; 17; 39], the images were chronologically ordered to investigate temporal trends on IC in Section 3.2. Time Series for IC, Enhanced Vegetation Index (EVI), and Normalized Difference Vegetation Index (NDVI) from 1984-2017  
Temporal trends for IC, EVI, and NDVI show positive and similar slopes and are highly significant (\(p<0.01\)) (Figure 13). NDVI shows the best adjustment (R\({}^{2}\) = 0.57, \(p<0.01\)) followed by EVI (R\({}^{2}\) = 0.54, \(p<0.01\)) and then by IC (R\({}^{2}\) = 0.28, \(p<0.01\)), the latter which showed the poorest adjustment due to the high variation as a result of known seasonal changes. For the 39 selected images, the minimum mean IC value (0.71) corresponds to the date 31 December 1989 with mean EVI and NDVI values of 0.48 and 0.84 respectively. The maximum mean IC value (0.88), corresponds to the date 4 September 2016 with mean EVI and NDVI values of 0.59 and 0.88 respectively. A comparison between two close images, both from the dry season but separated 28 years also depicts the effect of the increase of mean IC in both mean EVI and NDVI (Figure 14). Figure 12: Scatter plot of the mean illumination condition (IC) versus the mean NDVI value for each of the 39 selected images. Figure 13: Time-series plot of the mean IC, mean EVI and mean NDVI values for all the 39 selected images. The solid black line represents the mean IC calculated for all the images available (\(n=397\)). Observing an increase in IC, EVI, and NDVI over time despite having a limited number of images but still representative of different seasons and years to make this analysis would also indicate that there is a general improvement in IC and hence in EVI and NDVI from old to new Landsat images. The modeling of IC for all images in the period studied is shown in Figure 13. The mean IC value in the area shows an overall increase over time. This can be clearly observed comparing images close in date but separated in years, where the increase in IC occurs to a similar degree for every date and month compared (Figure 15). In more detail, the mean IC trend experiences an overall increase with a slight drop between the years 1989-1993 and then an increase again. It can also be observed that due to missing images in some dates between the years 1984-2012, the IC seasonality pattern cannot be as clearly seen as after the year 2012 to present when the coverage of images is more complete. Overall, EVI and NDVI show higher values in the recent period 2014-2017 than in the previous period 1988-2002 regardless of the seasonal effects. The lack of clear images between the period 2003-2013 is due partly to cloudiness and to the Scan Line Corrector (SLC) failure in Landsat 7 [29]. Monthly trends are shown in Figure 2.0 Clear seasonal trends can be observed. The lowest mean IC occurs in the dry season (December-February) and higher IC can take place in two different times of the year, at the beginning of the rainy season (April-May) and in the short dry period within the rainy season (August-September). Every month shows a similar and steady increase in IC over time from old to recently acquired images, although the strength of the increase varies between months. March shows the best adjustment in the increase of IC with time (R\({}^{2}\) = 0.71), whereas November showed the  
Figure 2.1 Barplot comparing mean values of IC, EVI, and NDVI for two close images in date but 28 years apart. Figure 2.2 Comparison of mean IC for images close in date but separated in years. The criteria of selection were first close dates, then the highest number of years in difference. lowest (R\({}^{2}\) = 0.18).