Environmental Engineering Reference
In-Depth Information
surface around it, is subject to what commonly is termed the oasis effect; the pan
represents a surface that is evaporating at the maximum potential rate. However, if
the surface around the pan has a limited moisture source, and the rate of ET is
substantially smaller, then the cooling effect associated with evaporation is
reduced. The air mass that passes over the pan is both warmer and drier than
what it would be if the surface surrounding the pan was losing water at the same
rate as the pan. The warmer, drier air enhances evaporation from the pan, just as
described for the complementary-relationship method.
Sources of error, in addition to simple measurement error associated with each
sensor, often are the result of prolonged sensor deployment. Some sensors, such as
humidity sensors and radiometers, drift and require regular recalibration. Bearings in
anemometers wear out and require replacement. Thermistors deployed in water
experience algal growth that delay the response time of the sensor. If the wetland
water level changes and the sensor heights are not adjusted, then bias can occur in
temperature and vapor-pressure differences. Loss of data due to power interruptions,
sensor failure, or breaks in the sensor wires also occur. An error that often is
overlooked is extrapolation of measured evaporation rates over the evaporating
surface. As wetland stage changes, along with the corresponding change in shoreline
location and wetland surface area, substantial error can occur by applying measured
evaporation rates to an incorrect wetland surface area (see Sect.
3.2.1
).
3.5.6 Cost Effectiveness
All scientists would like to quantify fluxes and processes as accurately as possible,
but the benefit associated with greater accuracy has to be balanced with the cost of
instrumentation and methodology. The eddy-covariance method may yield ET rates
with the smallest uncertainty, but sensors are expensive, data processing is lengthy,
and installation costs can be large. At the opposite extreme, temperature is one of
the least expensive parameters to measure, making methods such as Thornthwaite
particularly attractive if the scientist is willing to sacrifice accuracy in the interest of
economy. To a large extent, the choice of method depends on the importance of
accurate quantification of ET. If ET is a large component of a wetland water budget,
then a substantial investment in time and money is warranted. If the wetland is
dominated by surface-water flow and ET is a relatively small component, then
perhaps a method based on temperature, or temperature and radiation, is sufficient.
At a small lake in New Hampshire where all components of the water budget were
characterized as accurately as possible, the Priestley-Taylor method was deemed
the best compromise between accuracy and cost. It produced data nearly as good as
the energy-budget method but did not require measurement of surface-water tem-
perature or quantification of advected energy sources and sinks (Rosenberry
et al.
2007
). Another study of a reservoir in northern Florida came to a similar
conclusion regarding use of the Priestley-Taylor method despite the water budget
being dominated by surface-water inputs and losses (Dalton et al.
2004
).
