Surface Reaeration in Tropical Headwater Streams: the Dissolution Rate of a Soluble Floating Probe as a New Variable for Reaeration Coefficient Prediction

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.