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CADDIS Volume 2: Sources, Stressors & Responses


Ways to Measure Temperature

When considering temperature as a candidate cause of impairment, measurements of stream temperature should incorporate spatial and temporal variability of the thermal regime to the extent feasible. These considerations are particularly important in terms of seasonal and inter-annual variation in stream temperatures. Various automated temperature monitoring devices can be used to incorporate temporal variability in water temperature measurements (e.g., min/max thermometers and automated temperature loggers). One can measure spatial differences in temperature over an area of water surface using airborne and satellite remote sensing techniques (Torgersen et al. 2001). One-time measurements of stream temperature (called spot or point measurements), can be unreliable indicators of thermal modification and are best reserved for situations where the source is highly localized (e.g., point source discharges of heated cooling water). If spot measurements are the only available technique for measuring stream temperatures, care should be taken to control for diurnal variation: for example, measurements should be taken at the same time of day at reference and impaired sites. Measurements also should be made in a variety of habitats within the stream to capture within-channel spatial variability. Some common metrics for stream thermal regimes, and their relationship to biological effects, are provided in Table 1.

Table 1. Common metrics for stream thermal regimes, and related biological effects.
Temperature metric Relationship to biological effects
Daily maximum, minimum, and average Often used for evaluating the occurrence of acutely lethal conditions via comparison with laboratory toxicity data.
Daily range Increases may indicate changes associated with reduced riparian cover or base flow, and may increase physiological stress.
7-day average of daily maximum Used by U.S. EPA Region 10 for setting protective thermal criteria for salmonids during summer (U.S. EPA 2001).
Maximum weekly average Traditionally used by EPA to evaluate chronic effects of temperature on organism growth (U.S. EPA 1985c).
Weekly average, monthly average Can be used to characterize and evaluate seasonal changes in temperature regime, and related cues to reproduction and development.
Cumulative degree days A summation of average daily temperatures that represents an integrative measure of thermal exposure history; may be linked to the onset of developmental or reproductive events.

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