Abstract:
This study is part of the Alsea Basin Logging-Aquatic Resources
Study research program. It was initiated to determine the applicability
of energy budget theory to stream temperature prediction on small
forested streams. The study was also designed to evaluate the energy
budget technique as a tool in the management of mountain streams
for the production of high quality water.
Temperature predictions were made on four stretches of three
streams in the Coast and Cascade Ranges in Oregon during the summers
of 1965 and 1966. Three of these stretches were forested.
The fourth was completely exposed to direct radiation.
Stream temperature change occurring within a stretch of
stream was determined by evaluating the radiative, evaporative,
and conductive fluxes incident at the surface of the water as it moved
down stream. Net radiation was measured directly. Evaporation was
computed using a Dalton type equation. Conduction was determined with the Bowen ratio. These fluxes were then added to determine
the net flux. Stream temperature change was computed as: [see pdf for equation]
Stream temperature was predicted with varying degrees of
accuracy on the four stretches. Tests suggested that the predictions
could be improved slightly on long stretches by subdivision of the
stretch and by separating data into daytime or nighttime units. Additional
tests indicated that under a broken canopy. net radiation estimates,
and thus stream temperature predictions, may be improved
by measuring, or utilizing in the predictive equation, only the diffuse
radiation penetrating the canopy. Integration of the net radiation
recorded to include occasional spots of sunlight results in an overestimation
of temperature change during sunny periods.
A stream bottom may act as a heat sink during the day and as
a heat source at night. This phenomenon was measured on one
stretch. This helped explain predictive errors of about 10° F.
The study on the open stretch showed that during the day, conductive and evaporative fluxes were small compared to the
radiative flux. This led to modification of the original formula for
predicting temperature maxima. This formula is:
[see pdf for equation]
This permits field personnel to make estimates of maximum
temperature changes attainable by opening a stretch of stream.
