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CHAPTER 7. VALUE ADDED PRODUCTS
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Here, pwv is the water vapor partial pressure (millibars = hPa), and Ta is the air temperature (K).
Figure 7.1 shows pwv as a function of air temperature for relative humidities of 20 - 100%. The
partial pressure is computed as :
pwv = RH es / 100
(7.14)
where RH is the relative humidity in per cent, and es is the water vapor partial pressure in saturated
air (Murray 1967) :
es (Ta ) = es0 exp{
a(Ta − 273.16)
}
Ta − b
(7.15)
The constants are a = 17.26939, b = 35.86, and es0 = es (273.16K) = 6.1078 hP a. An alternative
to equation (7.13) is the following approximation (Idso and Jackson 1969) which does not explicitly
include the water vapor and holds for average humidity conditions, compare Figure 7.2.
a = 1 − 0.261 exp{−7.77 × 10−4 (273 − Ta )2 }
(7.16)
Figure 7.1: Water vapor partial pressure as a function of air temperature and humidity. Relative humidities are 20% to 100% with a 10% increment, bottom to top curves, respectively (eq. 7.14).
The calculation of the heat fluxes G, H, and LE on the right hand side of equation 7.7 requires
different models for vegetated and man-made surfaces. For vegetated or partially vegetated surfaces,
we employ a simple parametrization with the SAVI and scaled NDVI indices (Choudury 1994,
Carlson et al. 1995) :
G = 0.4 Rn (SAV Im − SAV I)/SAV Im
(7.17)
where SAV Im = 0.814 represents full vegetation cover. The sensible heat flux is computed as :
H = B (Ts − Ta )n
(7.18)
B = 286 (0.0109 + 0.051 N DV I ∗ )
(7.19)