Abstract
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.
Highlights
The competition over dwindling freshwater resources is expected to intensify with the projected increase in human population and expansion of irrigated land (Assouline et al, 2015), and with changes in precipitation and drought patterns (Dai, 2011)
The low thermal diffusivity of Styrofoam discs resulted in negligible heat conduction to the water body, whereas the intercepted radiative flux on the cover surfaces results in a considerable increase in cover temperature (Fig. 9a) with higher sensible heat and long wave radiate exchange
To meet the design and prediction challenges, we developed and tested a simple energy balance model for quantifying surface fluxes and vertical temperature profiles in a water reservoir
Summary
The competition over dwindling freshwater resources is expected to intensify with the projected increase in human population and expansion of irrigated land (Assouline et al, 2015), and with changes in precipitation and drought patterns (Dai, 2011). The reliance on water storage in reservoirs (Fig. 1) is likely to increase to mitigate seasonal shortages due to projected precipitation variability, and to meet water needs for increased population and food production. By some estimates up to half of stored water in small reservoirs is lost to evaporation (Craig, 2005; Rost et al, 2008), thereby exacerbating the water scarcity problem. Interest in methods for suppressing evaporation has led to an 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 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, shape, radiative and thermal properties)
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