Inter-annual and seasonal trends of vegetation condition in the Upper Blue Nile (Abbay) basin: dual scale time series analysis.

A long-term decline in ecosystem functioning and productivity, often called land degradation, is a serious environmental and development challenge to Ethiopia that needs to be understood so as to develop sustainable land use strategies. This study examines inter-annual and seasonal trends of vegetation cover in the Upper Blue Nile (UBN) or Abbay basin. Advanced Very High Resolution Radiometer (AVHRR) based Global Inventory, Monitoring, and Modelling Studies (GIMMS) Normalized Difference Vegetation Index (NDVI) was used for course scale long-term vegetation trend analysis. Moderate-resolution Imaging Spectroradiometer (MODIS) NDVI data (MOD13Q1) was used for finer scale vegetation trend analysis. Harmonic analyses and non-parametric trend tests were applied to both GIMMS NDVI (1981-2006) and MODIS NDVI (2001- 2011) data sets. Based on a robust trend estimator (Theil-Sen slope) most part of the UBN (∼77 %) showed a positive trend in monthly GIMMS NDVI with a mean rate of 0.0015NDVI units (3.77%yr^-1), out of which 41.15% of the basin depicted signif icant increases (P < 0.05) with a mean rate of 0.0023NDVI units (5.59%yr^-1) during the period. However, the finer scale (250 m) MODIS-based vegetation trend analysis revealed that about 36% of the UBN shows a significantly decreasing trend (P < 0.05) over the period 2001-2011 at an average rate of 0.0768NDVI yr^-1. This indicates that the greening trend of vegetation condition was followed by browning trend since the mid-2000s in the basin, which requires the attention of land users and decision makers. Seasonal trend analysis was found to be very useful in identifying changes in vegetation condition that could be masked if only inter-annual vegetation trend analysis was performed. The finer scale intra-annual trend analysis revealed trends that were more linked to human activities. This study concludes that integrated analysis of course and fine scale, inter-annual and intra-annual trends enables a more robust identification of changes in vegetation condition. [ABSTRACT FROM AUTHOR]