| Oil production is the main economic sector in Nigeria. That West African country is the biggest producer in Africa and one of the largest around the world. The Niger Delta is the main oil production region in the country. However, as a result of the continuous development of the petroleum industry, Nigerian environment facing some serious problems due to frequent oil spilled from different oil facilities. Thousands of oil barrels are spill in the environment every year, especially in the Niger Delta.Since the Niger Delta is the third largest wetland forests in the world, and vegetation is very important for the ecological system; this study has been focused on the negative influence of oil spills on the Niger Delta’s vegetation using GIS and Remote Sensing applications. Rivers State was selected for this study due to its importance in the Nigerian petroleum sector, and due to the vast number of oil spills in this state every year. Oil spills reported by SPDC were used to study vegetation degradation inside oil spills impacted areas between 2011 and 2012 by applying two different techniques; Normalized Digital Vegetation Index (NDVI), and Multi Endmember Spectral Mixture Analysis (MESMA). These two techniques were applied to compare between three different periods of Landsat7 images, and detect vegetation change caused by oil spills inside 163 oil spill impacted areas.Both techniques revealed high ability to detect vegetation in the dry lands environment. However, MESMA represents higher ability to detect vegetation spectra in the wetlands environment due to its sub-pixel analysis method. This result represents the importance of using sub-pixel methods (such as MESMA) to study vegetation (especially vegetation change) in wetlands combining with vegetation health analysis methods, especially in studies based on moderate spatial resolution images such as Landsat images.The net percentage of vegetation degradation analysis results showed that 73% of impacted areas have vegetation degradation in the MESMA analysis. Whereas, only 50% of impacted areas showed vegetation degradation in NDVI analysis. Highest vegetation degradation value inside impacted areas was 66% in MESMA, and 22% in NDVI. Almost 70% of impacted areas in NDVI results didn’t go beyond value of 10% of degradation. Whereas, almost 35% of impacted areas have vegetation degradation more than 30% in MESMA results.Vegetation degradation spatial distribution inside impacted areas has been discussed. Results revealed that oil spill point is the main spatial controller for vegetation degradation inside impacted areas in both NDVI and MESMA (39% and 37% respectively). Only 23.1% of impacted areas have vegetation degradation in the whole impacted area in NDVI results. Whereas, the percentage of impacted areas which have vegetation degradation in the entire impacted area was 33% in MESMA results. Low elevation was the main controller of spatial vegetation degradation inside 19% of the study area impacted areas in both NDVI and MESMA results. Whereas, impacted area boundary is the main spatial controller of vegetation degradation inside 19% of impacted areas in NDVI results, and 11% of impacted areas in the MESMA results. In addition, results revealed that an oil spill could affect vegetation away from the spill point at around 2.5 kilometers in wet environment controlled by open water as a distributer for spilled oil.16 different oil spill determinants were examined to study the effected of each one on vegetation inside oil spill impacted areas. Results of Pearson Chi-Square tests for both NDVI and MESMA vegetation degradation analysis revealed that; there was no significant difference between variables of:cause of incident, time of the incident, soil type, oil type, spilled oil volume on open water, time of response to the spill, facility type, and pipeline size in their influence on vegetation degradation inside impacted areas. Whereas, there were significant difference between variables of: impacted area size, secondary oil spread environment, spilled oil volume, residual oil volume on site, time of recovery, and time of cleanup residual impact in their influence on vegetation degradation inside impacted areas in both techniques results. In addition, there were significant differences between variables of spill point environment, and area burnt by fire in their influence on vegetation degradation inside impacted areas in the NDVI results.Spearman’s rho correlation coefficient test results revealed that; there is a significant positive correlation between vegetation degradation from one side and impacted area size, spilled oil volume, residual oil volume on site, time of recovery, and time of cleanup residual impact in the other side in both NDVI and MESMA techniques results. NDVI results represented a positive relationship between vegetation degradation and size of area burnt by fire. Whereas, MESMA results represented a positive relationship between vegetation degradation inside impacted areas and time of response.Finally, vegetation degradation, and oil spill determinants results were used to create an Oil Spill Risk Assessment Model for Vegetation (OSRAMV) for a selected applied area inside the Rivers State to define different risky areas of vegetation by applying Oil Spill Hazard Model (OSHM) on the applied area vegetation cover. OFHM was tested by shell’s recorded oil spills impacted areas. None of impacted areas has been located in very low, or low risk areas. Whereas,71.6% of impacted areas are located in areas of severe hazard,25.3% in high hazard areas, and only 3.1% were located in medium hazard areas.This study has attempted to gain a comprehensive insight into the severity of vegetation degradation due to oil spills in the Niger Delta and wetlands generally and Rivers state especially. |
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