资源环境科技发展态势分析平台

After the frequent and long drought episodes of the 1980s, a plethora of case studies have shown that West Africa remains a hot spot in the continent where despite its numerous water resources, extreme rainfall variability remains a profound challenge to the availability of freshwater for agriculture and ecosystem services. In this paper, we assess the recent flux in water availability over West Africa by investigating variations in net-precipitation (i.e. the maximum available freshwater flux) using wavelet analysis. Net-precipitation was obtained as a residual of the atmospheric water balance, and its variability compared to precipitation, temperature, evaporation, soil moisture and normalized difference vegetation index using wavelet power transforms and coherence analysis. Results from the study indicate that the variance in water flux over the region has been progressively reducing, suggesting a relative reduction in extreme hydrological conditions. Also, the wavelet coherence analysis revealed that the observed decreasing rate of available freshwater is highly coupled to a low frequency modulating El-Nino activity that induced lower changes in rainfall variance, as well as higher evaporation variance. Spatial trends in the annual-scaled average wavelet power indicated that the south-western parts of the region experienced the most reduction in rainfall flux. The highest deficit in net-precipitation flux was found in the dry sub-humid climatic zone, which is drained by major regional rivers, including the Niger and Volta. Considering the long-term variability in freshwater (i.e. from 1979 to 2010), we found the 1980s to be the driest decade and the 1990s being its recovery period, while the 2000s proved to be a considerably dry decade, suggesting a strong multi-decadal variability.