Evaporation suppression from water reservoirs: Efficiency considerations of partial covers

Abstract

[1] Reservoirs enhance availability and temporal stability of water resources, however, water losses due to evaporation may significantly reduce their operational efficiency. Studies have evaluated the efficacy of various types of chemical or physical evaporation barriers often on a qualitative basis. Certain design and maintenance advantages are offered by self-assembling modular floating covers especially for large reservoir surfaces. Evidence suggests that evaporation suppression efficiency of these floating covers is not proportional to the covered surface fraction. Capitalizing on recent experimental results carried out on small vessels, we show that evaporation suppression efficiency of floating covers in much larger reservoirs (3 to 5 order of magnitude in volume) is proportional to the square root of open area. Thermal effects of evaporation from open and partially covered surfaces on surface water temperature are proportional to evaporation rate and thus provide a potential mean for real-time monitoring of cover performance. Floating covers seem to suppress evaporation more effectively than other suspended evaporation barriers. For a fixed covered surface fraction, larger ''holes'' suppress evaporation more efficiently than many smaller and distributed openings. The concepts and relationships developed in this study provide quantitative means for engineering design and economical evaluation of evaporative barriers for water reservoirs in terms of cover geometry and water saving efficiency.

Evaporation suppression and energy balance of water reservoirs covered with self-assembling floating elements

“…These nonlinearities are attributed to vapor diffusion 10 from water gaps across air viscous boundary layer (Schlünder, 1988;Shahraeeni et al, 2012; and potential feedback on the gap temperature (Aminzadeh and Or, 2013). The combined effects of gap size, spacing and thickness of the air boundary layer (Shahraeeni et al, 2012) support laboratory experimental results of Assouline et al (2011) that have shown higher evaporation rates from small water gaps (per unit gap area) relative to evaporation rates from larger gaps (with similar 15 uncovered surface fraction). These nonlinear relationships and additional energetic constraints must be considered in design and deployment of evaporation suppression floating covers.…”

“…where w f and c f are the areal fractions of free and covered surface, respectively ( 1 comparison with the thickness of viscous sublayer (Assouline et al, 2011). Hence, the reduction of vapor diffusion resistance from individual gaps due to the formation of three-dimensional vapor shells (governed by the combined effect of gap size g a , boundary layer thickness and lateral spacing) would enhance vapor diffusion and result in values of ϕ 1 ≥ (Schlünder, 1988;Shahraeeni et al, 2012):…”

“…Interest in methods for suppressing evaporation have led to upsurge in the use of self-assembling floating covers over water reservoirs (e.g., Los Angeles reservoir in Sylmar, California), yet the selection, performance and 15 implementation of such measures remain largely empirical. Recent studies (Assouline et al, 2011;Ruskowitz et al, 2014) have shown that evaporation suppression is a highly nonlinear process that depends on the properties of the covers (size, radiative properties and shape).…”

“…Laboratory studies of evaporation from partially covered water surfaces (Assouline et al, 2010;Assouline et al, 2011) suggest nonlinear relationship between the covered area fraction and evaporative losses (see Figure 1 in Assouline et al (2011)). These nonlinearities are attributed to vapor diffusion 10 from water gaps across air viscous boundary layer (Schlünder, 1988;Shahraeeni et al, 2012; and potential feedback on the gap temperature (Aminzadeh and Or, 2013).…”

“…The available strategies include deepening the water reservoirs (to reduce evaporative surface per stored volume), covering the surface, underground storage, or introducing wind breakers to reduce exchange with wind. Among these different measures for evaporation suppression, the use of self-assembling floating elements appears most promising for smallscale reservoirs due to its simplicity, cost-effectiveness and scalability (Craig, 2005;Assouline et al, 2011;Gallego-Elvira et al, 2012;Chaudhari and Chaudhari, 2015). Floating covers spontaneously 5 rearrange in response to changes in water level or external conditions, e.g., wind (in contrast with chemical films that may break up due to the wave action, UV radiation, or biological activity).…”