Environmental Engineering Reference
In-Depth Information
the best value of terrestrial NPP. Research into mass and energy l ows in the bio-
sphere expanded during the 1960s and 1970 (Eckardt 1968; Lieth and Whittaker
1975), and thanks to better regression models linking NPP and key climatic vari-
ables, estimates for both terrestrial and marine NPP began to fall within a much
more constrained range during the 1970s: the lowest estimate was 45 Gt C/year
(Lieth 1973), the highest, by Ajtay et al.(1979), was 60 Gt C/year, and perhaps the
most widely cited total, by Whittaker and Likens (1975), put the terrestrial NPP at
55 Gt C/year.
All subsequent global estimates of NPP have benei ted from satellite monitoring
and from better models of primary productivity. Remote sensing of photosynthetic
productivity is based on the phenomenon that chlorophyll, the principal plant
pigment, rel ects less than 20% of the longest wavelengths of visible light but about
60% of near infrared radiation. These differences in rel ectance can be used in rela-
tively detailed ecosystem mapping and can be converted into a normalized difference
vegetation index (NDVI). Multispectral Scanning Systems (MSS) on the i rst Landsat
satellite, launched in July 1972, recorded rel ected radiation in four different spectral
bands between 0.5 and 1.1
m, with a resolution of 80 m. Later Landsat satellites
had the Thematic Mapper, with a resolution of 30 m, and the French SPOT satellite,
launched in 1985, revealed details at 10-20 m.
The high cost of Landsat and SPOT imagery made the Advanced Very High
Resolution Radiometer (AVHRR, installed on the National Oceanic and Atmo-
spheric Administration's polar-orbiting satellites) the tool of choice for monitoring
global NPP. AVHRR's maximum resolution of 1-4 km sufi ced to capture large-
scale patterns of vegetation coverage and to detect seasonal variability (Gutman and
Ignatov 1995). The i rst NDVI calculations used rel ectances in the visible band
(0.58-0.68
μ
μ
m) and the near infrared band (0.73-1.1
μ
m); calculations for rel ec-
tances in the thermal infrared band (11 and 12
m) and compensations for changing
illumination conditions, surface slope, and viewing aspect were added later (Gutman
et al. 1995).
At the same time, the results of a growing number of i eld NPP studies were
collated in a standardized format thanks to the Global Primary Production Data
Initiative based at the Oak Ridge National Laboratory in Tennessee. The database
was begun in 1994 and now contains data on 65 intensively studied sites (mainly
grasslands and tropical and boreal forests, with georeferenced climate and site
characteristics data) compiled between 1930 and 1996 (ORNL 2010). NPP totals
published during the last two decades of the twentieth century had a range of
48-65 Gt C/year, and improvements in satellite monitoring and expanding primary
μ
