The definition of potential evaporation remains widely debated despite its centrality for hydrologic and climatic models. We employed an analytical pore-scale representation of evaporation from terrestrial surfaces to define potential evaporation using a hypothetical steady state reference temperature that is common to both air and evaporating surface. The feedback between drying land surfaces and overlaying air properties, central in the Bouchet (1963) complementary relationship, is implicitly incorporated in the hypothetical steady state where the sensible heat flux vanishes and the available energy is consumed by evaporation. Evaporation rates predicted based on the steady state reference temperature hypothesis were in good agreement with class A pan evaporation measurements suggesting that evaporation from pans occurs with negligible sensible heat flux. The model facilitates a new generalization of the asymmetric complementary relationship with the asymmetry parameter b analytically predicted for a wide range of meteorological conditions with initial tests yielding good agreement between measured and predicted actual evaporation.

Abstract. The growing pressure on natural freshwater resources and the projected climate variability are expected to increase the need for water storage during rainy periods. Evaporative losses present a challenge for the efficiency of water storage in reservoirs, especially in arid regions with chronic water shortages. Among the available methods for suppressing evaporative losses, self-assembling floating elements offer a simple and scalable solution, especially for small reservoirs. The use of floating elements has often been empirically based; we thus seek a framework for systematic consideration of floating element properties, local climate and reservoir conditions to better predict evaporative loss, energy balance and heat fluxes from covered water reservoirs. We linked the energy balance of the water column with energy considerations of the floating elements. Results suggest significant suppression of evaporative losses from covered reservoirs in which incoming radiative energy is partitioned to sensible and long wave fluxes that reduce latent heat flux and thus increase the Bowen ratio over covered water reservoirs. Model findings were consistent with laboratory-scale observations using an uncovered and covered small basin. The study offers a physically based framework for testing design scenarios in terms of evaporation suppression efficiency for various climatic conditions; it hence strengthens the science in the basis of this important water resource conservation strategy.

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.

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