Habitat - Research
Date Published: October 2001
Number of Pages: 61
Author(s): EDAW, Inc. for the Washington Department of Fish and Wildlife
The purpose of this report is to summarize the proceedings and discussion from two workshops on the subject of heat transfer processes in forested stream environments. The workshops, held in Lacey, WA in February and May of 2001, were organized as part of the Cooperative, Monitoring, Evaluation, and Research (CMER) program, and sponsored by the Riparian Scientific Advisory Group (RSAG).
The goals of the Temperature Workshops were to identify where scientific consensus exists and where it is lacking on heat transfer processes in forested watersheds, to provide overviews of past and current research, and to identify future priorities based on stakeholder review of this information. Specific topics addressed included:
- The effects of direct solar radiation to surface waters and the cumulative effects of heating from upstream sources;
- Currently used temperature models, addressing their inputs, strengths, and weaknesses;
- Heat transfer processes via groundwater; and
- Heat transfer processes via microclimate conditions (both in the riparian zone and over the stream).
Recognized scientific leaders in current research efforts were identified and invited as panelists in the workshops. Invited panelists included Dr. George Ice, NCASI (who addressed solar radiation inputs); Dennis Schult, Western Watershed Analysts (who discussed current temperature modeling efforts); Dr. Patricia Olson, Pacific Watershed Institute (who addressed groundwater inputs); Dr. Sam Chan, PNW Lab/USFS (who addressed microclimate conditions in riparian areas); and Dr. Sherri Johnson, OSU (who addressed microclimate effects on stream systems).
Areas of Consensus Among Panelists.
The panelists noted that the best science to date has confirmed that solar insolation (i.e., direct solar radiation to the water's surface) is the dominant source of heat energy to surface water. Although other heat sources received considerable attention in recent years, validation of these effects is lacking.
Although older reviews on water temperature frequently refer to microclimate, successful measurement of this effect on surface water temperature has been elusive. In the past four years, a number of careful studies have taken advantage of the availability of reliable low cost submersible data loggers to isolate the microclimate effect. These data loggers should be reliable enough to detect differences in water temperature 0.5 centigrade units or less. These studies (Brosofke et al 2000, Johnson and Jones 2000, James pers. comm.) have not been able to measure a microclimate effect on water temperature where there was a buffer 15 meters (50 feet) wide or greater. Where buffers are narrower or absent, it becomes impossible to separate the microclimate effect from the more significant solar insolation effect.
The microclimate hypothesis suggests water temperatures will always move towards equilibrium with the surrounding air. Panelists noted that this was still a fundamental fact. However, elevated air temperature occurs only during the middle of the day. Air has a significantly lower heat capacity than water, thus it takes significant time for air to bring a body of water into equilibrium. Furthermore, microclimate effects from timber harvest are a combination of three effects; higher mid-day air temperatures, lower mid-day humidity, and higher wind speeds. The latter two effects combine to increase evaporation from the water's surface, which has a cooling effect on water temperature.
Several panelists suggested that a better measure of solar insolation would to measure the shade in the path of the summer sun, i.e., solar tracking, rather than measuring the shade from the entire 'view to sky'. The current board manual densiometer method assumes the latter.
More research is needed to determine forest practices induced groundwater effects on surface water temperature. At this time relatively little is known with certainty.
Headwater Temperature Transfers
Panelists agreed that surface water temperature in headwater streams did re-establish temperature equilibrium with air upon re-entering shaded stream reaches The distance and time that it takes to re-establish equilibrium is a function of many variables.
Areas of Non-Consensus
There were no major areas of non-consensus among the panelists.
Future Research Priorities
No future research is needed to validate the fundamental effects of solar insolation.
Solar Insolation Measurement
Research is needed on the most effective measure of solar insolation. Current rules require a densiometer, which is time consuming to use and readings are subjective. In recent years, there have been a number of additional tools available that appear to be more precise and eliminate user subjectivity. Research into the utility of these tools for research measurements and rule implementation would be desirable.
Research on this subject as it applies to forest channels is sparse. If solar tracking proves to be a better predictor of water temperature response, this would create flexibility to manage for other riparian functions on the north bank of stream channels. Evaluation of tools for measuring shade along summer solar pathway is needed. This is a moderate priority for research.
Headwater Temperature Transfers
Additional research is needed to validate the distance and/or time needed to achieve equilibrium with surrounding physical conditions. This is a moderate priority for research.
In light of recent findings and current riparian buffer requirements, additional research on the effects of microclimate on water temperature is a low priority. It may be worthwhile reviewing the scientific literature in several years. The data logger technology will likely facilitate additional scientific publications.
Research on groundwater is in a very early phase of development. Both the theory and field methodology need development. A workshop discussion group identified that need for a conceptual model of heat transfer to groundwater, and then from groundwater to surface water. The model will be used to identify priority areas for initial research. This is a high priority.
Eastern Washington Nomograph
There was a broad consensus that the eastern Washington nomograph that is currently in the Board Manual should be revised using current datasets. If possible, a model that considers more that elevation should be developed. This is a high priority
Western Washington Nomograph
With the current 50 foot core zone and an additional inner zone, the western Washington nomograph is not likely to see much use, and thus, it is a low priority for research.
Initial research by Johnson and Jones 2000 suggests that hyporheic heat exchange in alluvial streambeds and valley floodplains could have a significant effect on surface water temperature. It appeared to be considerably more significant than the microclimate effect. Other studies also suggest this effect may be under-rated. If significant, the restoration of bedrock channels that were historically alluvial channels, and the restoration of incised channels may be legitimate methods for water temperature restoration. Although this in not a Schedule L-1 question, further research on this subject may be worth considering.
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