Remote Sensing

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 OPEN ACCESS Remote Sens. 2013, 5 665 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.

[1]  A. Diouf,et al.  AVHRR monitoring of savanna primary production in Senegal, West Africa: 1987-1988 , 1991 .

[2]  Molly E. Brown,et al.  Famine Early Warning Systems and Remote Sensing Data , 2008 .

[3]  A. Diouf,et al.  Monitoring land-cover changes in semi-arid regions: remote sensing data and field observations in the Ferlo, Senegal , 2001 .

[4]  Jacques Claude,et al.  Un espace sahélien : la mare d'Oursi, Burkina Faso , 1991 .

[5]  Christian Töttrup,et al.  Regional desertification: A global synthesis , 2008 .

[6]  Jennifer Small,et al.  Can human-induced land degradation be distinguished from the effects of rainfall variability? A case study in South Africa , 2007 .

[7]  J. Zak,et al.  Convergence across biomes to a common rain-use efficiency , 2004, Nature.

[8]  Thomas T. Wilheit,et al.  A satellite technique for quantitatively mapping rainfall rates over the oceans , 1977 .

[9]  P. Sen.  Estimates of the Regression Coefficient Based on Kendall's Tau , 1968 .

[10]  Compton J. Tucker,et al.  Satellite remote sensing of total herbaceous biomass production in the Senegalese Sahel - 1980-1984 , 1985 .

[11]  C. Tucker,et al.  Recent trends in vegetation dynamics in the African Sahel and their relationship to climate , 2005 .

[12]  S. Goward,et al.  Objective assessment of the NOAA global vegetation index data product , 1993 .

[13]  F. Veroustraete,et al.  Estimation of carbon mass fluxes over Europe using the C-Fix model and Euroflux data , 2002 .

[14]  Per Jönsson,et al.  TIMESAT - a program for analyzing time-series of satellite sensor data , 2004, Comput. Geosci..

[15]  Kjeld Rasmussen,et al.  Desertification in reverse? Observations from northern Burkina Faso , 2001 .

[16]  P. Xie,et al.  Global Precipitation: A 17-Year Monthly Analysis Based on Gauge Observations, Satellite Estimates, and Numerical Model Outputs , 1997 .

[17]  Stephen D. Prince,et al.  Evidence from rain‐use efficiencies does not indicate extensive Sahelian desertification , 1998 .

[18]  Le Houérou,et al.  Rain use efficiency: a unifying concept in arid-land ecology , 1984 .

[19]  M. Schaepman,et al.  Proxy global assessment of land degradation , 2008 .

[20]  P. Hiernaux,et al.  Sahelian rangeland response to changes in rainfall over two decades in the Gourma region, Mali , 2009 .

[21]  L. Hein,et al.  Desertification in the Sahel: a reinterpretation , 2006 .

[22]  Sharon E. Nicholson,et al.  The nature of rainfall variability over Africa on time scales of decades to millenia , 2000 .

[23]  F. Mosteller,et al.  Understanding robust and exploratory data analysis , 1985 .

[24]  Michael Stuart,et al.  Understanding Robust and Exploratory Data Analysis , 1984 .

[25]  Frédéric Baret,et al.  FAPAR over Europe for the past 29 years: A temporally consistent product derived from AVHRR and VEGETATION Sensors , 2010 .

[26]  Per Jönsson,et al.  Seasonality extraction by function fitting to time-series of satellite sensor data , 2002, IEEE Trans. Geosci. Remote. Sens..

[27]  Edwin W. Pak,et al.  An extended AVHRR 8‐km NDVI dataset compatible with MODIS and SPOT vegetation NDVI data , 2005 .

[28]  P. Ozer,et al.  Analysis of the vegetation trends using low resolution remote sensing data in Burkina Faso (1982–1999) for the monitoring of desertification , 2006 .

[29]  Geoffrey M. Henebry,et al.  Land surface phenology and temperature variation in the International Geosphere–Biosphere Program high‐latitude transects , 2005 .

[30]  C. Tucker,et al.  Satellite remote sensing of total dry matter production in the Senegalese Sahel , 1983 .

[31]  Rasmus Fensholt,et al.  The spatio-temporal relationship between rainfall and vegetation development in Burkina Faso , 1999 .

[32]  H Breman,et al.  Rangeland Productivity and Exploitation in the Sahel , 1983, Science.

[33]  S. Nicholson,et al.  A comparison of the vegetation response to rainfall in the Sahel and East Africa, using normalized difference vegetation index from NOAA AVHRR , 1990 .

[34]  Benjamin Smith,et al.  Precipitation controls Sahel greening trend , 2005 .

[35]  Abdou Ali,et al.  Recent trends in the Central and Western Sahel rainfall regime (1990–2007) , 2009 .

[36]  J. Paruelo,et al.  ANPP ESTIMATES FROM NDVI FOR THE CENTRAL GRASSLAND REGION OF THE UNITED STATES , 1997 .

[37]  G. Rybicki.  Radiative transfer , 2019, Climate Change and Terrestrial Ecosystem Modeling.

[38]  Sharon E. Nicholson,et al.  On the relation between rainfall and the Normalized Difference Vegetation Index for diverse vegetation types in East Africa , 1993 .

[39]  C. Tucker,et al.  Desertification in the Sahel: a reinterpretation of a reinterpretation , 2007 .

[40]  Benjamin W. Heumann,et al.  AVHRR Derived Phenological Change in the Sahel and Soudan, Africa, 1982 - 2005 , 2007 .

[41]  Sharon E. Nicholson,et al.  A STUDY OF RAINFALL AND VEGETATION DYNAMICS IN THE AFRICAN SAHEL USING NORMALIZED DIFFERENCE VEGETATION INDEX , 1990 .

[42]  C. Tucker,et al.  Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981–2003 , 2005 .

[43]  K. S. Shifrin.  Transfer of Microwave Radiation in the Atmosphere , 1970 .

[44]  M. S. Rasmussen.  Developing simple, operational, consistent NDVI-vegetation models by applying environmental and climatic information : Part I. Assessment of net primary production , 1998 .

[45]  Mike Hulme,et al.  Climatic perspectives on Sahelian desiccation: 1973–1998 , 2001 .

[46]  Wang Shaoqiang,et al.  Precipitation‐use efficiency along a 4500‐km grassland transect , 2010 .

[47]  Lars Eklundh,et al.  Vegetation index trends for the African Sahel 1982–1999 , 2003 .

[48]  Emilio Chuvieco,et al.  Debating the greening vs. browning of the North American boreal forest: differences between satellite datasets , 2010 .

[49]  J. Ardö,et al.  A recent greening of the Sahel—trends, patterns and potential causes , 2005 .

[50]  H. Theil.  A Rank-Invariant Method of Linear and Polynomial Regression Analysis , 1992 .

[51]  Lars Hein,et al.  The impacts of grazing and rainfall variability on the dynamics of a Sahelian rangeland , 2006 .

[52]  J. Monteith.  SOLAR RADIATION AND PRODUCTIVITY IN TROPICAL ECOSYSTEMS , 1972 .