Comparison of the VI changes

Differences and similarities when assessing vegetation cover were compared among various spectral vegetation indices. All VI showed qualitative variations in

vegetation cover. However, there were significant differences in the values of the VI over desert, agricultural areas, and forests. The spatial and temporal distribution of the VI produced insights for assessing vegetation cover change in the study area. To explore these relationships, the research chose the highest negative value in the NDVI correlation image, and collected the values for the same pixel from the NDVI time-series image (11 values), to calculate the linear regression slope and R for this pixel. The research then collected the values of the same pixel from the SAVI and EVI time series images (see Table 6.2).

Table 6.2: Comparison of the highest negative values between the VI

Julian date Years NDVI SAVI EVI

250 1972 0.163 0.115 - 245 1978 0.15 0.124 - 279 1986 0.106 0.098 - 273 1987 0.125 0.073 0.075 298 1999 0.123 0.051 0.069 209 2001 0.165 0.101 0.081 253 2003 0.042 0.033 0.033 248 2006 0.021 0.03 - 235 2010 0.053 0.053 0.039 251 2013 0.046 0.02 0.018 234 2014 0.044 0.031 0.014

The table shows the value which had the highest negative value in different years in each VI. The Julian date calendar was used to indicate the date of the image within a particular year (collected between July and October) and to avoid seasonal

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Figure 6.27: The highest negative value in the NDVI time-series image.

Figure 6.28: The highest negative value in the SAVI time-series image.

Figure 6.29: The highest negative value in the EVI time-series image. The results obtained from all the indices are shown in Figure 6.27, depicting the trends for the values in different years and the differences in values among the VI. Although there were differences among the VI in values, all the VI showed areas which saw a decrease in the VI (vegetation cover) over the period of study. It is noted that the largest residual in all three cases was for 2001, on Julian day 209 y = -0.0029x + 5.9275 R² = 0.597 0 0.05 0.1 0.15 0.2 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 N DV I v alu es Years y = -0.0023x + 4.5909 R² = 0.7446 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 SA V I v alu es Years y = -0.0024x + 4.9116 R² = 0.6908 0 0.02 0.04 0.06 0.08 0.1 1985 1990 1995 2000 2005 2010 2015 EVI val u es Years

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which was the earliest image used in terms of the vegetation growing season and may represent an early season greening in this area.

6.5 Conclusion

The results obtained from assessing the vegetation cover change were consistent with previous local studies that have examined the natural vegetation in the Al Jabal Al Akhdar region such as Al Mukhtar (2005), Ben Khaial and Bukhechiem (2005), Ibrahim (2006), the Libyan Agriculture Department (2004), and Oune (2006), and confirms that there has been a decrease in the vegetation cover in the region. The results confirmed that the decrease in the VI may be interpreted as showing a decrease in vegetation cover in the study area over the last 42 years. The spatial and temporal distribution of vegetation cover change across the study area has been examined using time-series spatial resolution satellite imagery and VI. The key results were:

(i) The effectiveness of remote sensing techniques to monitor vegetation change has been demonstrated in the study area, although there were relatively few images covering the study area over the last 42 years.

(ii) The results of the regression analysis of the VI in the Al Jabal Al Akhdar region indicated a change in the VI in some areas over the last 42 years. The negative values of the VI slope showed a decrease in the VI trend and therefore a decrease in

vegetation cover in some areas. In contrast, the areas which have positive values saw no significant change in vegetation cover.

(iii) The correlation coefficient of the VI showed the pixels which revealed a

statistically significant change at a 95% level of confidence in the study area over the 42 years.

(iv) The results of the VI correlation indicated a statistically significant decrease in the VI in some areas.

(v) Although there were some differences between the NDVI, SAVI and EVI in terms of values, all the VI shared areas which had experienced a decrease in the VI

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(vegetation cover) over the period of study. The spatial distribution and causes of significant decrease in vegetation cover are explored in the next two chapters. The aim of this chapter was to assess vegetation cover change in the study area. Vegetation Indices (VI) obtained from Landsat images represent quite a simple and effective technique for quantitative evaluation of vegetation cover for the whole of the study and it can also identify the quantity of vegetation cover in the area. Also, the VI can provide extremely useful insights into vegetation cover change in the study area by examining the time series of VI over a long period.

The statistical analysis of regression and the correlations that were applied in this chapter for the VI index contributed to showing the strength of the linear relationship between the VI and time. It determined which pixels showed a statistically

significant relationship, and then determined which pixels had statistically significant trend in the VI over time.

This chapter has provided statistically significant results for the changes in

vegetation cover and produced maps for the changes in vegetation cover of the Al Jabal Al Akhdar region over 42 years. These were hitherto not available in Libya because most the studies relied on fieldwork and specific locations. The output of this part of the research will be used to detect the factors which caused these changes.