# Remote Monitoring of the Impact of Oil Spills on Vegetation in the Niger Delta, Nigeria
## 3 Results and Discussion
### Time Series Analysis of \(\Delta\)NDVI for SS and CS  
The graphs of the time series of \(\Delta\)NDVI for the pairs of sites in dense, sparse, and mangrove vegetation are shown in Figures 12-14. Figures 12-14 show the temporal changes in NDVI values of each spill site (SS) with its corresponding control site (CS) from the values extracted from Table 4. Figure 12a-h is the graph for dense vegetation having several years ranging from 0-4 years (Figure 12a,c,d), 0-6 years (Figure 12e,f) and 0-7 years (Figure 12b,g,f) with the lowest and highest oil spill volumes of 280 and 1500 bbl, respectively, and with 4 and 7 years being the lowest and the highest number of years. The range of years for sparse vegetation in Figure 13a-h are 0-4 years (Figure 13a), 0-5 years (Figure 13b,g), 0-7 years (Figure 13e,f), 0-8 years (Figure 13c) and 0-10 years (Figure 13d,h), with the lowest and highest oil spill volumes of 234 and 1500 bbl and with the lowest and highest numbers of years of 4 and 10 years, respectively. For mangrove vegetation in Figure 14a-f, the years range from 0-5 years (Figure 14a,d), 0-6 years (Figure 14b,f) and 0-7 years (Figure 14c,e), with the lowest and highest oil spill volumes of 264 and 2500 bbl and with both the lowest and highest numbers of years being 5 and 7 years, respectively. Figure 12: (**a–h**) Temporal changes in NDVI (\(\Delta\)NDVI) for SS and CS pairs in dense vegetation for different oil spill volumes. Figure 14: (**a––f**) Temporal changes in NDVI (\(\Delta\)NDVI) for SS and CS pairs in mangrove vegetation for different oil spill volumes. Figure 13: (**a–h**) Temporal changes in NDVI (\(\Delta\)NDVI) for SS and CS pairs in sparse vegetation for different oil spill volumes. For most sites, the SSs and CSs exhibit the same underlying trend in \(\Delta\)NDVI, likely due to the vegetation at those locations being exposed to the same inherent environmental conditions, such as water availability and soil type. However, the negative \(\Delta\)NDVIs over time are generally of greater magnitude at SSs than at CSs. For example, the negative \(\Delta\)NDVI values are all notably greater in magnitude at SSs than at CSs for dense vegetation at a spill volume of 280 bbl (Figure 12a), for sparse vegetation at spill volumes of 260 bbl (Figure 13c), 803 bbl (Figure 13f) and 1500 bbl (Figure 13h). Furthermore, for mangrove vegetation, negative \(\Delta\)NDVI values are most notable at spill sites with volumes of 1554 bbl (Figure 14e) and 2500 bbl (Figure 14f). Nonetheless, several CSs exhibit negative \(\Delta\)NDVI values despite not being exposed to an oil spill. In these cases, factors such as seasonal changes in rainfall and weather conditions could be responsible for affecting the condition of the vegetation. It is apparent that oil spills have clearly affected the recovery of some vegetation in the Niger Delta. Although the vegetation condition at some SSs appeared to recover after the oil spill (e.g., dense vegetation--Figure 12a; sparse vegetation--Figure 13c,e-h; mangrove vegetation--Figure 14b,d), it was not able to recover within the same period at other SSs (e.g., dense vegetation--Figure 12c; sparse vegetation--Figure 13b). Mangroves and rainforests affected by hydrocarbon have been shown to have reduced chlorophyll [1; 45]. However, the oil pollution impact on adult trees is weak [71]. Figure 15 shows \(\Delta\)NDVIs categorised according to spill volume for the different vegetation types. From the result, there is no apparent relationship between the health of vegetation at SSs and the volume of the oil spill. For instance, it can be observed that 100% of the \(\Delta\)NDVI values for dense vegetation (Figure 15a) were negative at spill volumes 280, 1000, and 1500 bbl, whereas more than 75% were greater than zero at 346 and 429 bbl, with the most degradation occurring at spill volumes of 1000 and 1500 bbl.