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

Anton Vrieling,Mohammed Said,Jan De Leeuw

doi:10.3390/rs5020982

Abstract

The spatial distribution of crops and farming systems in Africa is determined by the duration of the period during which crop and livestock water requirements are met. The length of growing period (LGP) is normally assessed from weather station data—scarce in large parts of Africa—or coarse-resolution rainfall estimates derived from weather satellites. In this study, we analyzed LGP and its variability based on the 1981–2011 GIMMS NDVI3g dataset. We applied a variable threshold method in combination with a searching algorithm to determine start- and end-of-season. We obtained reliable LGP estimates for arid, semi-arid and sub-humid climates that are consistent in space and time. This approach effectively mapped bimodality for clearly separated wet seasons in the Horn of Africa. Due to cloud contamination, the identified bimodality along the Guinea coast was judged to be less certain. High LGP variability is dominant in arid and semi-arid areas, and is indicative of crop failure risk. Significant negative trends in LGP were found for the northern part of the Sahel, for parts of Tanzania and northern Mozambique, and for the short rains of eastern Kenya. Positive trends occurred across western Africa, in southern Africa, and in eastern Kenya for the long rains. Our LGP analysis provides useful information for the mapping of farming systems, and to study the effects of climate variability and other drivers of change on vegetation and crop suitability.

Highlights

Agricultural crops grow during periods of favorable weather conditions for crop emergence, vegetative growth, and ripening
Climate change can bring about shortening or lengthening of the length of growing period (LGP) [2], which impacts the range of crops that can be cultivated in a region
We do not aim at attributing our identified LGP trends to different processes. Rather we present these trends to identify areas that merit further study given the significant trends found over large areas in our data mining exercise of the new NDVI3g dataset

Summary

Introduction

Agricultural crops grow during periods of favorable weather conditions for crop emergence, vegetative growth, and ripening. For rainfed agriculture in the semi-arid regions of Africa, water availability is the main constraint that limits the time during which crops can grow. We refer to this period of favorable conditions as the length of growing period (LGP). Inter-annual variability of the water availability can, in dry years, result in crop failure, when the LGP does not fulfill the demands of the crop to complete its crop cycle Farmers select their crops carefully to both optimally use the growing period, while reducing the risks of not meeting the crop demands in specific years. The food security of African subsistence farmers and farming systems strongly depends on the crop choice, the year-to-year LGP variability, and longer-term trends in LGP [3]

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Results

Conclusion