Assessing Land Degradation/Recovery in the African Sahel from Long-Term Earth Observation Based Primary Productivity and Precipitation Relationships

The ‘rain use efficiency’ (RUE) may be defined as the ratio of above-ground net
primary productivity (ANPP) to annual precipitation, and it is claimed to be a conservative
property of the vegetation cover in drylands, if the vegetation cover is not subject to
non-precipitation related land degradation. Consequently, RUE may be regarded as means
of normalizing ANPP for the impact of annual precipitation, and as an indicator of
non-precipitation related land degradation. Large scale and long term identification and
monitoring of land degradation in drylands, such as the Sahel, can only be achieved by use
of Earth Observation (EO) data. This paper demonstrates that the use of the standard
EO-based proxy for ANPP, summed normalized difference vegetation index (NDVI)
(National Oceanic and Atmospheric Administration (NOAA) Advanced Very High
Resolution Radiometer (AVHRR) Global Inventory Modeling and Mapping Studies 3rd
generation (GIMMS3g)) over the year (ΣNDVI), and the blended EO/rain gauge based
data-set for annual precipitation (Climate Prediction Center Merged Analysis of
Precipitation, CMAP) results in RUE-estimates which are highly correlated with precipitation, rendering RUE useless as a means of normalizing for the impact of annual
precipitation on ANPP. By replacing ΣNDVI by a ‘small NDVI integral’, covering only
the rainy season and counting only the increase of NDVI relative to some reference level,
this problem is solved. Using this approach, RUE is calculated for the period 1982–2010.
The result is that positive RUE-trends dominate in most of the Sahel, indicating that
non-precipitation related land degradation is not a widespread phenomenon. Furthermore,
it is argued that two preconditions need to be fulfilled in order to obtain meaningful results
from the RUE temporal trend analysis: First, there must be a significant positive linear
correlation between annual precipitation and the ANPP proxy applied. Second, there must
be a near-zero correlation between RUE and annual precipitation. Thirty-seven percent of
the pixels in Sahel satisfy these requirements and the paper points to a range of different
reasons why this may be the case.