A conceptual model for assessing rainfall and vegetation trends in sub‐Saharan Africa from satellite data

Abstract

Policymakers, governments and aid agencies require operational environmental monitoring in support of evidence-based policy-making and resource deployment in crisis situations. For Africa, this is only feasible at sub-continental scale with a large network of automated meteorological stations, a large number of highly coordinated field observers or with satellite remote sensing. The challenge with satellite data lies in the derivation of meaningful environmental indicators. This article describes a conceptual framework for understanding satellite-derived indicators of rainfall and vegetation greenness trends over Africa. It attributes observed vegetation changes to climatic (i.e. rainfall linked) and non-climatic drivers. A decade of annual rainfall and vegetation data over sub-Saharan Africa was analysed using satellite-based rainfall estimates [Famine Early Warning System Rainfall Estimation 2.0 (FEWSNET RFE 2.0)] from National Oceanic and Atmospheric Administration's (NOAA's) Climate Prediction Centre and the Normalized Difference Vegetation Index (NDVI) obtained from the Satellite Pour l'Observation de la Terre Vegetation (SPOT-VGT) sensor. Rainfall and vegetation greenness trends were analysed for 759 administrative regions of sub-Saharan Africa to identify those regions that have experienced a negative, positive or stable rainfall/vegetation trend over the period [2001][2002][2003][2004][2005][2006][2007][2008][2009][2010]. The character of the relationship between the annual rainfall and max NDVI trends were examined to identify areas where the changes in greenness could be attributed to climatic (rainfall) and non-climatic (human land use or ecological disturbance) changes. Regions where increasing rainfall was associated with vegetation greening were found in West Africa, Central African Republic, West Cameroon and northeastern part of South Africa, whereas areas with evidence of 'climatic vegetation degradation' were located in Southern Madagascar, Nigeria, Kenya and the Garden Route region of South Africa.

Spatio-temporal vegetation dynamics and relationship with climate over East Africa

“…The current analyses and results supports the evidence that East Africa has been characterized by geographically extensive patterns of varied vegetation dynamics (also found by a number of studies e.g Hoscilo et al, 2015;Landmann and Dubovyk, 2013;Pricope et al, 2013). The results presented here, which to a large extend correspond with the findings of Hoscilo et al (2015), also provide a view of vegetation dynamics that could be used 20 to fully appreciate where significant changes have occurred in the region. In Northern Uganda and southern parts of Tanzania, persistently increasing LAI trends were found which indicates continued increasing trends in the future.…”

“…The predominant sustained and positive LAI trends found in the region give a positive picture on regional sustainability of vegetation cover. The significant and persistent negative trends in north-central and southern Kenya coincide with significant decline in precipitation and can be attributed to climatic effects as also reported by Hoscilo et al (2015). The pattern of 25 consistently declining vegetation extending from Southern Ethiopia, through central Kenya into central Tanzania (an area categorized as a pastoral zone) was also reported by Pricope et al (2013) (Gao et al, 2016;Nemani et al, 2003;Wu et al, 2015), and human activities which alter the seasonal and gradual changes in vegetation (Pfeifer et al, 2013).…”

Vegetation dynamics and responses to climate anomalies in East Africa

“…Widespread decreasing LAI trend found in Tanzania during the OND season 20 coincides with Vrieling et al (2013)'s finding of a decrease in the length of growing season. The significant and persistent negative trends in north-central and southern Kenya coincide with a significant decline in precipitation and can be attributed to climatic effects, as also reported by Hoscilo et al (2015). In addition, the decline in LAI shown in our analysis could also be a combined effect of climate as well as replacement of shrubs by grass and crops with lower LAI values in areas characterised by intensive pastoral activities.…”

“…These ecosystems are often over-utilised for pastoral grazing and mixed cereal cropping systems thus exacerbating their vulnerability to extended 10 drought occurrences leading to severe negative implications on food security and community livelihoods (Hoscilo et al, 2015;Landmann and Dubovyk, 2013;Pricope et al, 2013). The results presented here provide a view of vegetation dynamics that could be used to fully appreciate where significant changes in ecosystem functioning have occurred in the region.…”

Tropospheric NO<sub>2</sub> concentrations over West Africa are influenced by climate zone and soil moisture variability

“…It is also a determinant of the type and condition of vegetation in addition to overall ecosystem health in a region (GCOS, 2015). Over parts of West Africa, decadal-scale trends in rainfall have been analysed and showed 10 flooding in the western Sahel between 2008 and 2010 (Hoscilo et al, 2015). Information on the temporal dynamics of soil moisture is important to identify the start of the wet season and drought events, especially in semi-arid/Sahel regions, where vegetation growth is driven by soil moisture variations (Ibrahim et al, 2015).…”