Nuclear power plants sited in the vicinity of a large source of water, either a sea, lake or river, are generally cooled by running a large amount of water into the condenser and discharging it back to the original source a few degrees warmer, with little or no evaporation. This is known as once-through cooling. The water withdrawal requirements for a nuclear power plant of 1 GW employing this cooling method are about 50 m3/s, which corresponds to 1.3 billion tonnes of water per year. If the water availability is limited, heat removal is generally accomplished via evaporation in cooling towers with a re-circulating system: water passes through the condenser, is pumped to the top of the tower and is sprayed downwards to a collection basin, while being cooled by an up-drought of air. In a wet recirculating system, about 4-5% of the water evaporates and must be replaced.