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Not all water that falls as precipitation runs off into lakes and rivers. Some of it seeps down into the ground through spaces between soil and rock particles. Over time, this infiltrating water can accumulate into vast underground reservoirs. 

A common misconception about groundwater is that it builds up in subterranean caverns or rivers. Although they do exist in some locations, such caves are relatively rare. Nearly all groundwater exists in geologic formations called aquifers that consist of sand, gravel or rock, and are saturated with water just like a sponge. 

These water-bearing rocks that readily transmit water to wells and springs have different porosity and permeability characteristics. This means that water moves differently through them. As such, aquifers have a different rate of "recharge," or percolating ability to replenish the water within the aquifer.

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Think of a time when you you dug a hole at the beach and watched it fill with water, The upper surface of this water-filled area, or "zone of saturation," is called the water table. The saturated area beneath the water table is called an aquifer. The hole was much like a "well" that exposed the water table, with an aquifer beneath. 

Most aquifers take thousands of years to refill, relying on snowmelt and rain to fill depleted reservoirs. The global hydrological water cycle is a closed system, which means that no water is ever added or removed from the cycle. Liquid water evaporates into water vapor, condenses into clouds, precipitates back to the earth in the form of rain and snow, and then in different phases, moves through the atmosphere and ground systems. The system is driven by solar energy and gravitational potential energy. 

Also, a relationship does not necessarily exist between the water-bearing capacity of rocks and the depth at which they are found. A very dense granite that yields little or no water to a well may be exposed at the land surface. Or, a porous sandstone may lie hundreds or thousands of feet below the land surface and yield hundreds of gallons per minute of water.

Rocks that yield freshwater have been found at depths of more than 6,000 feet, and salty water has come from oil wells at depths of more than 30,000 feet. But, on the average, the porosity and permeability of rocks decrease as their depth below land surface increases because the pores and cracks in rocks at deeper depths are closed or greatly reduced in size because of the weight of overlying rocks. 

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In dry, desert climates, the places where groundwater surfaces are rare and easily threatened sources. Perennial springs and seeps are often the only source of water. The small patches of vegetation that pop up in the middle of vast desert areas do so because they grow in an area made fertile by a freshwater source beneath the surface called an oasis. Rivers that flow through fertile desert regions are called elongated oases (plural for oasis.) 

     READ | Oasis 

Climate issues impact the global hydrological water cycle in a number of ways from the increase in extreme weather patterns to the decrease in water vapor and precipitation. When earth surface temperatures rise, it affects the rate of evaporation, which means less water vapor and precipitation cycle. This causes the soil to dry out faster, creating harder and less permeable soil, which then influences the ability of aquifers to recharge naturally.

     READ |  How Climate Change Affects Water Access

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