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Title:   Author  item
                                            KARNIELI, ARNONitem
                                            AGAM, NURITitem
                                            PINKER, RACHELitem
                                            Anderson, Marthaitem
                                            IMHOFF, MARKitem
                                            GUTMAN, GARIKitem
                                            PANOV, NATALYAitem
                                            GOLDBERG, ALEXANDER
  Submitted to: Journal of Climate
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 8/24/2009
                                    Publication  Date: 2/1/2010
                                    Citation: Karnieli, A., Agam, N., Pinker, R.T., Anderson, M.C., Imhoff, M.L., Gutman, G.G., Panov, N., Goldberg, A. 2010. Use of NDVI and land surface temperature for assessing vegetation health: merits and limitations. Journal of Climate. 23:618-633.Interpretive Summary: A large number of water- and climate-related applications, such as drought monitoring, are based on spaceborne-derived relationships between land surface temperature (LST) and the Normalized Difference Vegetation Index (NDVI).  The majority of these applications rely on the existence of a negative slope between the two variables found from site-and time-specific studies.  The current paper investigates the generality of the LST-NDVI relationship over a wide range of moisture and climatic/radiation regimes encountered over the North American continent (up to 60° N) during the summer growing season (April – September).  Information on LST and NDVI comes from long-term (21-year) datasets obtained by the Advanced Very High Resolution Radiometer (AVHRR).  It was found that when water is the limiting factor for vegetation growth (typical situation for low latitudes of the study area and during the mid-season), the LST-NDVI correlation is negative.  However, when energy is the limiting factor for vegetation growth, (in higher latitudes and elevations, especially at the beginning of the growing season) a positive correlation exists between LST and NDVI.  Forward multiple regression analysis revealed that during the beginning and the end of the growing season, solar radiation is the predominant factor driving the correlation between LST and NDVI, while other biophysical variables play a lesser role.  Air temperature is the primary factor in mid summer.  It is concluded that there is a need to use the LST-NDVI relationship with caution and to restrict its applications as a drought index to areas and periods where negative correlations are observed.
                                Technical Abstract: To date, most drought indices used in drought monitoring are based on precipitation and meteorological data collected on the ground from distributed monitoring networks.  Few satellite-based drought indices are currently in production, although these afford better spatial and temporal coverage and resolution than do indices derived from surface observations.  Therefore, evaluation and improvement of satellite drought indices will improve our ability monitor global drought conditions for forecasting yield and impending food security crises.   Prominent among existing satellite indices are the Vegetation Health Indices (VHI), which use anomalies in land-surface temperature and vegetation cover amount to identify areas of potential vegetative stress.  This paper assesses the primary assumption in the VHI and other related approach – that land-surface temperature and vegetation cover are typically negatively correlated, and that deviations from this relationship are signatures of drought.  The assumption is tested using 21-years of data collected over the contiguous United States using the Advanced Very High Resolution Radiometer (AVHRR) at 1km resolution.  The study concludes that a negative correlation does indeed predominate at lower latitudes during the summer months; however, positive correlations are found at higher latitudes and during the early spring even under healthy growing conditions.  In these cases, vegetation growth is limited by energy (warmth and sunshine) rather than moisture availability, where higher surface temperatures signify positive rather than stressed growth conditions.  Simple empirical indices such as the VHI may falsely interpret these signatures as drought.


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