<P> In some areas of the world, irrigation is necessary to grow any crop at all, in other areas it permits more profitable crops to be grown or enhances crop yield . Various irrigation methods involve different trade - offs between crop yield, water consumption and capital cost of equipment and structures . Irrigation methods such as furrow and overhead sprinkler irrigation are usually less expensive but are also typically less efficient, because much of the water evaporates, runs off or drains below the root zone . Other irrigation methods considered to be more efficient include drip or trickle irrigation, surge irrigation, and some types of sprinkler systems where the sprinklers are operated near ground level . These types of systems, while more expensive, usually offer greater potential to minimize runoff, drainage and evaporation . Any system that is improperly managed can be wasteful, all methods have the potential for high efficiencies under suitable conditions, appropriate irrigation timing and management . Some issues that are often insufficiently considered are salinization of groundwater and contaminant accumulation leading to water quality declines . </P> <P> As global populations grow, and as demand for food increases in a world with a fixed water supply, there are efforts under way to learn how to produce more food with less water, through improvements in irrigation methods and technologies, agricultural water management, crop types, and water monitoring . Aquaculture is a small but growing agricultural use of water . Freshwater commercial fisheries may also be considered as agricultural uses of water, but have generally been assigned a lower priority than irrigation (see Aral Sea and Pyramid Lake). </P> <P> It is estimated that 22% of worldwide water is used in industry . Major industrial users include hydroelectric dams, thermoelectric power plants, which use water for cooling, ore and oil refineries, which use water in chemical processes, and manufacturing plants, which use water as a solvent . Water withdrawal can be very high for certain industries, but consumption is generally much lower than that of agriculture . </P> <P> Water is used in renewable power generation . Hydroelectric power derives energy from the force of water flowing downhill, driving a turbine connected to a generator . This hydroelectricity is a low - cost, non-polluting, renewable energy source . Significantly, hydroelectric power can also be used for load following unlike most renewable energy sources which are intermittent . Ultimately, the energy in a hydroelectric powerplant is supplied by the sun . Heat from the sun evaporates water, which condenses as rain in higher altitudes and flows downhill . Pumped - storage hydroelectric plants also exist, which use grid electricity to pump water uphill when demand is low, and use the stored water to produce electricity when demand is high . </P>

What is earth's biggest source of fresh water