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

Remote Sensing

卷:6

Pierre Hiernaux¹  Cam-Chi Nguyen¹  Laurent Kergoat¹  Manuela Grippa¹  Eric Mougin¹  Cécile Dardel¹  Philippe Ciais² 

[1] Geosciences Environnement Toulouse (GET), Observatoire Midi-Pyrénées, UMR 5563 (CNRS/UPS/IRD/CNES), 14 Avenue Edouard Belin, F-31400 Toulouse, France;

[2] Laboratoire des Sciences du Climat et de l'Environnement (LSCE), UMR 8212 (CNRS/CEA/UVSQ/), F-91190 Gif Sur Yvette, France;

关键词: Sahel;    re-greening;    degradation;    RUE;    RESTREND;    NDVI;    GIMMS-3g;    MODIS;    herbaceous vegetation;    Sahelian paradox;    run-off;   

DOI  :  10.3390/rs6043446

来源: DOAJ

【 摘 要 】

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 overthe Gourma region, implying that vegetation is resilient over that period, when dataare 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 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.

【 授权许可】

Unknown