Variability of African farming systems from phenological analysis of NDVI time series

Abstract

Food security exists when people have access to sufficient, safe and nutritious food at all times to meet their dietary needs. The natural resource base is one of the many factors affecting food security. Its variability and decline creates problems for local food production. In this study we characterize for sub-Saharan Africa vegetation phenology and assess variability and trends of phenological indicators based on NDVI time series from 1982 to 2006. We focus on cumulated NDVI over the season (cumNDVI) which is a proxy for net primary productivity. Results are aggregated at the level of major farming systems, while determining also spatial variability within farming systems. High temporal variability of cumNDVI occurs in semiarid and subhumid regions. The results show a large area of positive cumNDVI trends between Senegal and South Sudan. These correspond to positive CRU rainfall trends found and relate to recovery after the 1980's droughts. We find significant negative cumNDVI trends near the south-coast of West Africa (Guinea coast) and in Tanzania. For each farming system, causes of change and variability are discussed based on available literature (Appendix A). Although food security comprises more than the local natural resource base, our results can perform an input for food security analysis by identifying zones of high variability or downward trends. Farming Climatic Change (2011) 109:455-477 systems are found to be a useful level of analysis. Diversity and trends found within farming system boundaries underline that farming systems are dynamic.

Length of Growing Period over Africa: Variability and Trends from 30 Years of NDVI Time Series

“…To ascertain that for each pixel we consider the same season in the annual LGP retrievals, we added a searching algorithm to White's method. This searching algorithm is described and illustrated in detail by Vrieling et al [30]. In summary, for each pixel we first search for minima and maxima in the long-term average NDVI profile.…”

“…We fully acknowledge that these trends should be further evaluated based on field evidence for individual locations and countries. We refer the reader to Vrieling et al [30] for a detailed discussion on factors that may explain trends in phenology, with focus on SOS and cumulated NDVI over the season, for farming systems of Africa.…”

Early assessment of seasonal forage availability for mitigating the impact of drought on East African pastoralists

“…forage) is not developing during the dry periods and consequently NDVI provides limited information about forage conditions in these periods , good scope exists to shorten the temporal windows without losing valuable information on seasonal forage availability. An effective approach to better define the period of forage development across the season is to estimate season start-and end-dates directly from temporal NDVI profiles by analyzing vegetation green-up and decay using phenological analysis Vrieling et al 2011). …”

Fodder Biomass Monitoring in Sahelian Rangelands Using Phenological Metrics from FAPAR Time Series

“…Many authors have endorsed the applicability of these phenological metrics for crop [9] and grassland [32] phenological monitoring, particularly for analyzing African farming systems [33]. Recently, phenological metrics based on the FAPAR time series were recommended by [34] for providing relevant early information on biomass production in the Sahel.…”