Abstract:
A stream's temperature is a major factor in its ability to support
fish life and to be utilized for other beneficial purposes. The
approach most generally used for stream temperature prediction is
the Energy-Budget method, which involves the inventory of all the
energy entering and leaving the stream. A temperature prediction
study on the coast fork of the Willamette River was conducted in 1963
using this method. Subsequent analysis of the data revealed a poor
correlation between predicted and measured stream temperatures at
night. Due to the relative magnitudes of the terms in the Energy-Budget, it became evident that the term for evaporative heat loss was
in error. Since the evaporative heat loss can be computed directly
from a value of evaporation rate, one can state that an accurate determination
of evaporation rate is essential to the successful analysis
of any Energy-Budget Equation applied to streams. Several types of mass transfer equations, both theoretical and
experimental, have been developed for predicting evaporation from
lakes and reservoirs. There has not yet been developed, .
however,
a method which relates specific stream parameters, such as surface
configuration and stream turbulence, to evaporation. Therefore, as
an initial step in the search for needed knowledge, this thesis undertook
as its purpose the evaluation of the effect of surface configuration
on the magnitude of evaporation from a modeled stream surface.
There are three types of mass transfer equations which are
commonly used for the prediction of evaporation. The most common
type is a general equation which employs Dalton's relationship between
evaporation and vapor pressure differences together with a
correction for wind velocity. The other two types (the discontinuous
and continuous mixing approaches) are based upon the structure of
the turbulent boundary layer above the evaporation surface. The
equations of Norris and of Thornthwaite and Holzman, based on the
discontinuous mixing approach, and the equations of Sutton, based on
the continuous mixing approach, were evaluated using experimental
data to establish their utility as tools for predicting stream evaporation.
The research involved the measuring of evaporation rates from
porous stream models of five surface configurations placed in a low
velocity wind tunnel and the measurement of physical parameters which are included in various evaporation equations. Several test
conditions were used for each surface configuration.
The analysis of the data had two parts. First, the evaporation
rates computed from several existing equations were compared to
measured evaporation, and second, an effort was made to establish
a relationship between surface configuration and evaporation.
Several statistical tests were used in these analyses, and the following
conclusions were reached:
1. Equations based on the structure of the overlying air may
be used to predict evaporation rates, without any specific knowledge
concerning the stream surface configuration.
2. Of the three theoretical methods tested, Sutton's method gave
the best correlation between computed and measured evaporation
rates. The method of Norris was next, followed by the equation of
Thornthwaite and Holzman.
3. The mass transfer coefficient, B', in an experimentally developed
equation of the form E = B' u(e[subscript s] - e[subscript a]) is related to model
surface configuration. Using wave steepness, H/L, as a characteristic
of model surface configuration, B' increases as H/L increases.
Therefore, when an equation of this form is used, evaporation rate
increases as wave steepness increases.
4. The failure during the 1963 study made on the coast fork of
the Willamette River to compute accurate stream temperatures during nighttime periods can be attributed, at least in part, to errors
in underestimating the evaporation rates.
5. Surface configuration does affect evaporation rates from
streams, but its full quantitative evaluation awaits further research.