Surface Reaeration in Tropical Headwater Streams: the Dissolution Rate of a Soluble Floating Probe as a New Variable for Reaeration Coefficient Prediction
2020
de Souza Ferreira, Murilo | de Campos Jordão, Carlos Eduardo Kolb Maynardes Araújo | de Souza Inácio Gonçalves, Julio Cesar | Dodds, Walter Kennedy | Cunha, Davi Gasparini Fernandes
The reaeration coefficient (Kₐ₂₀) is one of the main indicators of dissolved O₂ movement to and from aquatic systems via the atmosphere. Direct gas tracer measurements, physical models, and models of O₂ dynamics have been used for Kₐ₂₀ estimation, especially in temperate aquatic ecosystems, with fewer examples in their tropical counterparts. Here we investigate a less commonly employed soluble floating probe (SFP) method, based on the dissolution rate of a soluble solid (VS₂₀) as an auxiliary variable for Kₐ₂₀ estimation. Our objectives were to test the SFP method for estimating Kₐ₂₀, validate such estimates through the traditional gas tracer method, and develop empirical models for Kₐ₂₀ prediction using VS₂₀ and additional physical variables. Five reaches (with lengths from 20 to 250 m) of four tropical headwater streams were selected, and their main physical, hydrological, and hydraulic variables were measured in eight sampling periods. The gas tracer (using NaCl and SF₆ tracers) and the SFP (spherical format made with oxalic acid dihydrate) methods were carried out in each reach. The Kₐ₂₀ values ranged from 16.94 to 373.79 days⁻ ¹ and the VS₂₀ ranged from 0.079 to 0.778 mm min⁻ ¹. We observed a significant linear relationship between Kₐ₂₀ and VS₂₀. The best model (R² = 0.78) combined the variables depth, Froude number, and VS₂₀. Our study highlighted that models developed for temperate aquatic systems can underestimate Kₐ₂₀ in tropical streams, bringing uncertainties for modeling metabolic rates, self-depuration capacity, or any other processes that depend on reaeration.
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