Multi-pronged assessment of land degradation in West Africa to assess the importance of atmospheric fertilization in masking the processes involved

摘要

Separating human-induced land degradation from that caused by natural processes in the world of global climate and atmospheric change is a challenging task, but important for developing mitigation strategies. Current remote-sensing data and spatio-temporal analyses allow the distinction of climate and human-induced land degradation on a sub-continental scale, but the underlying processes cannot be discerned at this scale. This study is conducted at a river-basin scale to (1) identify land degradation hotspots in a basin or sub-basin, and (2) assess the correspondence and divergence of land degradation assessed by NDV1 shifts with and without accounting for atmospheric fertilization with that based on soil erosion assessment at a sub-basin scale. Long-term remote sensing (NDVI) and rainfall data were used to identify human-induced land degradation hotspot areas in the Volta basin. The results were compared with the critical zone of soil loss in the White Volta sub-basin derived from a spatially distributed soil erosion model, validated by field-measured data. A spatial comparison of the above studies revealed that the biomass productivity (NDVI)-based land degradation assessment grossly underestimated the extent to which soil is being lost, unless a correction was included to account for atmospheric fertilization. Based on inter-annual NDVI signals land degradation was evident in about 8 of the Volta basin's landmass, but when accounting for atmospheric fertilization, as much as 65 of the land is losing some of its vital attributes such as soil quality or vegetation productivity. The study demonstrates the need for using a multi-pronged assessment strategy in land degradation assessment that offers an insight of the processes involved in land degradation.