Assessment of the Performance of Evaporation Suppressant Films: Analysis and Limitations of Simple Trialling Methods

Abstract

The potential utility of monomolecular layers (“monolayers”) and other surface film materials for the reduction of open water evaporation has long been argued. However, outside the laboratory, trials to quantify the effectiveness of artificial surface films have produced highly variable results after application to water surfaces, whether natural water bodies or managed farm storages. This paper briefly reviews the physical mechanisms involved in evaporation suppression and the biophysical literature on aquatic surface microlayers. The wide-ranging results from 16 months of outdoor trough-scale and (simultaneous) replicated bucket-scale evaporation reduction trials are interpreted using biophysical measurements made on microlayer and immediate subsurface water samples taken from the experimental troughs. When the prevailing environmental conditions and other ancillary measurements are taken into account, plausible hypotheses arise to account for at least some of the observed trial-to-trial differences in evaporation reduction and surface film performance. Results for the commercial monolayer product are inconclusive, as the concentration of the active ingredient in its formulation lacked the uniformity required for the accuracy at which these trials were conducted. Results for the temperature differential between open (unprotected) water and film-covered water, and the influence of windspeed on evaporative loss, indicate that the mechanism of evaporation suppression for mono-molecular (monolayer) organic films differs from that for thicker silicone oil films. These results have implications for both small-scale trialling of evaporation suppressants and the deployment and management of artificial surface film materials on agricultural water storages. In addition, it concluded that despite the attractiveness of simple side-by-side comparisons of performance, meaningful interpretation of results must consider prevailing meteorological conditions, and a timescale of hours rather than days.