Using sound pressure to estimate reaeration in streams

The reaeration coefficient (k) is an essential and sensitive component of the equations used to calculate whole-stream metabolism (WSM) for a stream reach by the open-channel method. However, the empirical methods used to estimate k (e.g., by propane or sulfur hexafluoride evasion rates) are time consuming and costly. We reasoned that the reaeration rate (the product of k and the dissolved O sub(2) deficit or surplus) and sound level (noise) are related to turbulence in a stream and, thus, sound level should be related to reaeration. We used a simple and inexpensive sound level meter to measure sound levels at a fixed height (30 cm) above key geomorphic features (e.g., pools, riffles, cascades) in several streams in Alaska and Vermont. We calculated a feature-weighted average sound pressure for selected stream reaches based on the proportion of these geomorphic features within each reach. We calculated k in the Alaskan stream reaches based on propane evasion rates and found a strong linear relationship to the feature-weighted average sound pressure (n = 11, r super(2) = 0.94, p < 0.001). We concluded that the sound pressure method, which requires 0.5 h to complete and relies on inexpensive instrumentation, provides estimates of k that are comparable to estimates from the more resource-intensive volatile gas-evasion method. In the Vermont stream reaches, we were able to create a sound pressure-stage rating curve, similar to a hydrologic rating curve. Combining the k-sound pressure relationship with the sound pressure-stage rating curve holds promise as a way to estimate reaeration continuously, at modest cost and effort, and on a time scale similar to that on which dissolved O sub(2), temperature, and light values are recorded to calculate WSM.