GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Rescaling NDVI from the VEGETATION instrument into apparent fraction cover for dryland studies E. Bartholomé*, P. Bogaert †, M. Cherlet°, P. Defourny †, P. Mathoux †, P. Vogt* * Joint Research Centre Ispra † Université Catholique de Louvain-la-Neuve °FAO - Rome

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 The problem In arid regions soil is a major component of the NDVI signal Time series keep atmospheric contamination and angular effects A biophysical measurement is more explicit than an index

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Bare soil Yearly NDVI average over bare soils shows a high degree of variability

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Solution: an original per-pixel adjustment procedure Iterative procedure to eliminate vegetated pixels in temporal profile Values above a threshold defined according to a gaussian probability function are eliminated The iterative process stops when difference between previous and current averages <1% If NDVI is never < 0.2, NDVI bare soil is set to 0.1, i. e. close to most frequent average bare soil NDVI value

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Bare soil NDVI offset correction

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Adjustment to the max cover NDVI max = 0.85 ≈ Cover max Defined from the time series over the whole window

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Max cover in % 50% 100%

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Meaning of the new product NDVI – fractional cover relationship is well documented in the literature For VEGETATION data this translates into: For pixels where bare soil value cannot be measured

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 assumptions LAI>3  cover ≈ fPAR Both models and statistical fitting indicate that the relationship is not linear, But linear assumption is usually considered as sufficient for low res. applications

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Identification of vegetated pixels Based on the analysis of NDVI variation on bare soils, a pixel is declared vegetated if NDVI >NDVI bare soil + 2% cover To further reduce the risk of contamination, pixels are retained as vegetated if they were identified as vegetated on the previous product

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Time-series smoothing 5 th degree Polynomial applied on periods of 21 decades Iterative process (7 cycles) to replace low NDVI values by polynomial values 5-decade overlap between periods At the end the highest NDVI value is retained (polynomial or actual value)

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Jebel Haruj es Sawda Known limitations sparse vegetation or bare soil High NDVI variability over some dark (volcanic) surfaces (above the normal threshold) Tibesti

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 Known limitations – dense vegetation Residual atmospheric effects ?

GLC 2000 – “FIRST RESULTS” WORKSHOP JRC – Ispra, March 2002 conclusions The fractional cover makes the data interpretation easy for land cover mapping in arid regions Procedure easily implementable for inter- instrument data comparison Soil effects are well removed, marginal improvement to be implemented Needs further improvement for regions with high aerosol load.