Evaporation Suppression From Water Bodies Using Floating Covers: Laboratory Studies of Cover Type, Wind, and Radiation Effects

Abstract

Water reservoirs have been used to mitigate seasonal water shortages since the dawn of civilization. Present day use of water reservoirs continues to expand to meet the increasing demand for irrigation water and to mitigate effects of climate change and droughts. Losses to evaporation are an important challenge to water storage efficiency in arid regions. We study the use and efficiency of self-assembling floating covers as a simple and scalable solution to evaporation suppression. We report laboratory-scale studies of evaporation suppression using different cover types under various external conditions that jointly affect surface flux partitioning. Experiments using floating spheres and disks of different colors, sizes, and materials were conducted for a range of external conditions (radiation, wind, and wind + radiation) over a 1.44 m 2 water basin placed in a wind tunnel. Considering maximum cover density using spherical and disk-shaped elements (91% surface cover), the evaporation was suppressed by 80% (for disks) and 70% (for spheres) relative to uncovered water surfaces in agreement with physically based model estimates. Surprisingly, evaporation suppression using black and white floating covers was similar despite significantly different surface energy partitioning pathways. The observed similarity was attributed to thermal decoupling between the top of cover elements and water surfaces. The study shows a high and nearly constant evaporation suppression efficiency of floating covers for a range of external conditions supporting the potential of such simple and scalable solution of evaporation suppression for regions with limited infrastructure or alternative water solutions.

Evaporation Suppression From Small Reservoirs Using Floating Covers—Field Study and Modeling

“…As reported in other studies (Lehmann et al, 2019), despite the very high surface temperatures of the black covers compared to the white covers (up to 30°C difference, see Figure 3), the overall evaporation suppression performance of the different color covers was similar. Nevertheless, the high black cover temperatures affected the water temperatures in the reservoirs top layer as evidenced by considerably higher temperatures under black relative to white disks (see Figure S3).…”

“…Other studies have shown that suppression efficiency of floating covers varies with cover properties (cover fraction, geometries, and material properties) and atmospheric forcing of evaporation process (Assouline et al, 2011). Based on lab experiments in a shallow water basin using floating spheres (0.04 m in diameter), Lehmann et al (2019) have found that evaporation suppression was about 70% irrespective of boundary forcing (wind and radiation). Despite the higher temperature of black covers, no difference in evaporation suppression was measured by Lehmann et al (2019) compared to white spheres and disks.…”

Distribution of small seasonal reservoirs in semi-arid regions and associated evaporative losses

“…Cheap floating cover elements could be a simple and scalable solution for suppressing evaporative losses from the many small reservoirs that serve largely rural populations in semi-arid regions. Results from recent studies have shown that across a wide range of climatic conditions, such covers suppress between 70 and 85% of the evaporation relative to uncovered water surfaces [33,46]. Floating covers reduce the exposed water surface, increase the effective boundary layer over the surface, and shift radiation and heat exchange from the water surface to the top surfaces of the covers [33,46].…”

“…Results from recent studies have shown that across a wide range of climatic conditions, such covers suppress between 70 and 85% of the evaporation relative to uncovered water surfaces [33,46]. Floating covers reduce the exposed water surface, increase the effective boundary layer over the surface, and shift radiation and heat exchange from the water surface to the top surfaces of the covers [33,46]. The packing of floating spheres or discs permit light penetration and gas exchange through the 9% uncovered area (gaps formed between the elements), thus permitting ecological functions (we report preliminary results from an ongoing field experiment in SI-section 3 and figure S8).…”

“…The cover effectiveness is influenced by atmospheric conditions [46], but for simplicity we have used a constant evaporation suppression efficiency of 80% over the dry season as specified by Aminzadeh et al [33] for Styrofoam disks. In figure 4, we add regional context by presenting savings percentage for each country of SAWS and highlighting social information about the population and agricultural land areas in each continent.…”