In this paper we evaluate the enhancement of nonequilibrium concentration fluctuations induced by the Soret effect when a binary fluid layer is subjected to a stationary temperature gradient. Starting from the fluctuating Boussinesq equations for a binary fluid in the large-Lewis-number approximation, we show how one can obtain an exact expression for the nonequilibrium structure factor in the long-wavelength limit for a fluid layer with realistic impermeable and no-slip boundary conditions. A numerical calculation of the wave-number dependence of the nonequilibrium enhancement and of the corresponding decay rate of the concentration fluctuations is also presented. Some physical consequences of our results are briefly discussed.
In the last half-century in most semi-arid or arid developed and developing countries, many aquifers have become intensively used. This means that the hydrogeological conditions may have changed, with groundwater storage being modified significantly. This intensive use has been performed mainly by millions of modest farmers with scarce public or governmental planning. This silent revolution has been market driven. The cost of groundwater abstraction is usually a small fraction of the value of the irrigated crop obtained. The benefits have been clear and important in most cases. In poor countries groundwater development has provided reliable drinking water supply and increased food security through irrigation, and consequently groundwater as become a relevant means of eradicating poverty. The impact of this groundwater development in developed countries is also significant but not yet well recognised. But sometimes, there are other associated negative effects. These sometimes have been utilised by certain lobbies to expand the hydro-myth of the unreliability (or fragility) of groundwater development, in order to continue to promote the construction of large hydraulic infrastructures.
In Spain, groundwater irrigation accounts for more than 50% of the total economic value of irrigated agricultural production, while it only consumes about 20% of the total volume of water. In spite of its benefits, groundwater has not been a significant part of Spanish water policy.
In semi-arid regions, aquifers provide a series of practical advantages that make them preferential sources of water supply. In Spain, groundwater meets about one-fifth of the total water demand and is used to irrigate over one-third of the total irrigated land. This article examines groundwater use in Spain from the perspective of the EU Water Framework Directive. Analysis of different sector uses suggests that core problems (and solutions) related to groundwater lie in agricultural uses and that the Directive's environmental requirements remain distant from reality on the ground, where economic, political and social reasons prevail on legal obligations set by national and supranational authorities.
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