Thirty-two Years of Sahelian Zone Growing Season Non-Stationary NDVI3g Patterns and Trends

Assaf Anyamba,Edwin Pak,Compton Tucker,Jennifer Small

doi:10.3390/rs6043101

Assaf Anyamba, Edwin Pak + Show 2 more

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https://doi.org/10.3390/rs6043101

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Journal: Remote sensing	Publication Date: Apr 4, 2014
Citations: 63	License type: CC BY 3.0

Affiliation: Goddard Space Flight Center

Abstract

We update the Global Inventory Modeling and Mapping Studies (GIMMS) analysis of Sahelian vegetation dynamics and trends using the normalized difference vegetation index (NDVI; version 3g) 1981 to 2012 data set. We compare the annual NDIV3g and July to October growing season averages with the three rainfall data sets: the Africa Rainfall Climatology from 1983 to 2012, the Variability Analyses of Surface Climate Observations Version-1.1 from 1951 to 2000, and the Nicholson ground-station precipitation rainfall data from 1981 to 1994. We use the Nicholson ground-station annual precipitation data to determine the reliability of the two continental precipitation data sets for specific locations and specific times, extrapolate these confirmed relationships over the Sahelian Zone from 1983 to 2012 with the Africa Rainfall Climatology, and then place these zonal findings within the 1951 to 2000 record of the Variability Analyses of Surface Climate Observations Version-1.1 precipitation data set. We confirm the extreme nature of the 1984–1985 Sahelian drought, a signature event that marked the minima during the 1980s desiccation period followed within ten years by near-maxima rainfall event in 1994 and positive departures is NDVI, marking beginning of predominantly wetter conditions that have persisted to 2012. We also show the NDVI3g data capture “effective” rainfall, the rainfall that is utilized by plants to grow, as compared to rainfall that evaporates or is runoff. Using our effective rainfall concept, we estimate average effective rainfall for the entire Sahelian Zone for the 1984 extreme drought was 223 mm/yr as compared to 406 mm/yr in during the 1994 wet period. We conclude that NDVI3g data can used as a proxy for analyzing and interpreting decadal-scale land surface variability and trends over semi arid-lands.

Highlights

The Sahel Zone is an arid and semi-arid area on the south side of the Sahara, stretching more than km across Africa, from the Atlantic Ocean to the Red Sea, with an unusual precipitation gradient of 1 mm/yr/km in a north to south direction
We compared the NDVI3g data set against the ARC rainfall data using ordinary least squares linear regression
The results show a positive correlation between normalized difference vegetation index (NDVI) and rainfall for all the epochs with coefficients of determination as follows: r2 = 0.37 for 1983–1992; r2 = 0.36 for 1993–2002, r2 = 0.38 for 2003–2012 and for the entire period 1983–2012, r2 = 0.38

Summary

Introduction

The Sahel Zone is an arid and semi-arid area on the south side of the Sahara, stretching more than km across Africa, from the Atlantic Ocean to the Red Sea, with an unusual precipitation gradient of 1 mm/yr/km in a north to south direction. Many workers [1,2,3,4,5,6,7] describe the Sahel as the long-term to 400 mm/yr mean annual precipitation zone, with a 100 to 200 mm/yr transition zone with the Sahara Desert to the north, and a 400–600 mm/yr transition zone with the Sudanian Zone to the south. We will use the 200 to 600 mm/yr long-term precipitation isolines as our region of interest in this paper. The Sahel and its transition zones occupy an area of ~2.5 million km in a 400 to 500-km wide belt with precipitation isolines more or less parallel to lines of equal latitude. Other aspects of the Sahelian Zone and its transition zones are summarized in [2,8]

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