Environmental Engineering Reference
In-Depth Information
Fig. 3.9 Upward and downward facing 4-component net radiometer deployed over a large
wetland in southern Oregon. Other sensors also are labeled (Photo printed with kind permission
of
David Stannard, U.S. Geological Survey 2014. All Rights Reserved)
©
radiation emitted from the land or water surface proportional to the temperature of
the surface. Several broad-spectrum radiometers with sensors facing both upward
and downward can provide a single net-radiation value. Another option is to deploy
four sensors, two facing upward to separately measure shortwave and longwave
radiation, and two facing downward to separately measure shortwave and longwave
radiation (Fig.
3.9
). Although deploying four sensors provides greater accuracy, it
comes at a substantially larger cost. If only two upward-facing sensors are deployed,
the upward radiation vectors can be calculated with reasonable accuracy (Sturrock
et al.
1992
; Parkhurst et al.
1998
;Winteretal.
2003
). Sensors need to be deployed on
a stable platform and maintained in a level, horizontal orientation to minimize bias.
All evaporation methods require measurement of air temperature and most also
require measurement of humidity at the same location. For gradient-based methods,
sensors usually are deployed that provide both temperature and relative humidity.
With both parameters, humidity output can be converted to vapor pressure, vapor
density, or whatever form of humidity is needed for a specific ET method. Several
ET methods require measurement of the water-surface or land-surface temperature
and make the assumption that vapor pressure is at saturation based on the surface
temperature. Methods that quantify change in heat stored in the water body (
Q
x
)
require a temperature sensor for each horizontal slice of water contained in the
