Rain-Use-Efficiency: What it Tells us about the Conflicting Sahel Greening and Sahelian Paradox

Abstract

Rain Use Efficiency (RUE), defined as Aboveground Net Primary Production (ANPP) divided by rainfall, is increasingly used to diagnose land degradation. Yet, the outcome of RUE monitoring has been much debated since opposite results were found about land degradation in the Sahel region. The debate is fueled by methodological issues, especially when using satellite remote sensing data to estimate ANPP, and by differences in the ecological interpretation. An alternative method which solves part of these issues relies on the residuals of ANPP regressed against rainfall (-ANPP residuals‖). In this paper, we use long-term field observations of herbaceous vegetation mass collected in the Gourma region in Mali together with remote sensing data (GIMMS-3g Normalized Difference Vegetation Index) to estimate ANPP, RUE, and the ANPP residuals, over the period 1984-2010. The residuals as well as RUE do not reveal any trend over time over the Gourma region, implying that vegetation is resilient over that period, when data are aggregated at the Gourma scale. We find no conflict between field-derived and satellite-derived results in terms of trends. The nature (linearity) of the ANPP/rainfall relationship is investigated and is found to have no impact on the RUE and residuals interpretation. However, at odds with a stable RUE, an increased run-off coefficient has OPEN ACCESS Remote Sens. 2014, 6 3447 been observed in the area over the same period, pointing towards land degradation. The divergence of these two indicators of ecosystem resilience (stable RUE) and land degradation (increasing run-off coefficient) is referred to as the -second Sahelian paradox‖. When shallow soils and deep soils are examined separately, high resilience is diagnosed on the deep soil sites. However, some of the shallow soils show signs of degradation, being characterized by decreasing vegetation cover and increasing run-off coefficient. Such results show that contrasted changes may co-exist within a region where a strong overall re-greening pattern is observed, highlighting that both the scale of observations and the scale of the processes have to be considered when performing assessments of vegetation changes and land degradation.

Impact of Climate Change on Vegetation Growth in Arid Northwest of China from 1982 to 2011

“…This difference might be due to the difference in the study period analyzed. The latest AVHRR GIMMS-3g datasets were calibrated with improved algorithms [36][37][38], and this study excluded agricultural lands that were influenced by irrigation. According to Liu and Kuang [48], cultivation represents one of the major land cover changes in recent decades in Northwest China.…”

“…This index reflects vegetation growth because it is closely related to the amount of photosynthetically absorbed active radiation [27,34,35]. The newly improved, third-generation Global Inventory Modeling and Mapping Studies (GIMMS-3g) NDVI dataset was obtained from the Advanced Very High Resolution Radiometer (AVHRR) sensors [36][37][38]. The GIMMS-3g dataset extends the widely used GIMMS dataset, the latter of which is only available for the period of 1981-2006 [35].…”

Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

“…Overall, LULCC are characterized by greening, which is strongly linked to vegetation production. Two maps synthesizing the information were built in the more relevant and regional covering references published in recent years [10][11][12][13][14]33,34]. The maps are the synthesis of six sources documenting changes in NDVI metrics assessing annual vegetation production through the Sahelian strip [10,[12][13][14]33,34].…”

“…This raises questions about (i) the hydrological efficiency of the present vegetation cover and (ii) the contributions of other factors such as rainfall intensification and more local changes (e.g., the extension of catchments due to endorheism bursting). This increase in runoff, which has been occurring in parallel to re-greening, has recently been called the second "Sahelian paradox" [13]. Human and material losses due to floods clearly exceed those due to drought in recent decades, as summarized by [17] from observations in western Niger.…”

“…Almost all remote -based vegetation maps demonstrate a dominant increase in NDVI signal since the early 1980s [10][11][12][13][14][15][16]. These studies did not always determine the respective contribution of the annual and perennial vegetation in this progression.…”