Around 71 percent of the earth's surface is covered in water. This massive quantity of water is hard to visualize: the total water resources of the earth equal roughly 326 million cubic miles, with each cubic mile equal to around 1 trillion gallons of water. To imagine just one trillion gallons of water, try to picture 40 million swimming pools, or 24 billion baths. Now, multiply those numbers by 326 million!
Of all of this water, only about 2.5 percent is freshwater: the other 97.5 percent is saltwater. Almost 69 percent of freshwater resources are tied up in glaciers and ice caps, about 30 percent is groundwater, and a mere 0.27 percent is surface water. While all kinds of water resources are important for the survival of the planet, accessible freshwater is especially important for humans.
TL;DR (Too Long; Didn't Read)
Water resources come in many forms, but the three main categories are saltwater, groundwater and surface water.
As mentioned, saltwater is abundant in the surface of the planet. However, saltwater is currently not particularly useful when it comes to potable water supplies. Desalination plants, while they do exist, are scarce because the energy required for desalination makes the process extremely expensive.
That said, there are saltwater resources from which humans benefit, aside from beautiful ocean views. Saltwater fish are a staple in much of the world's diet (although overfishing and pollution has put much of the marine life population at risk). Furthermore, tidal waters are being used as a source of hydroelectric energy. So, while saltwater is not helpful in dealing with scarce water supplies, it does provide resources that humans rely on.
Groundwater is the most plentiful of all freshwater resources. As water percolates into the ground through layers of soil, clay, and rock, some of it adheres to the topmost layers to provide water to plants. This water is in what is called the unsaturated, or vadose, zone. Most of the pores in the vadose zone are filled with air, rather than water.
Gravity continues to move the water down through the ground. Eventually, the water reaches the saturated zone, where all the pores are filled with water. The separation between the saturated and unsaturated zone is called the water table.
Aquifers are areas of permeable rock that hold water. Typically, aquifers are made of bedrock that has many fractures and connected pores, such as limestone, sandstone and gravel. Shale and clay layers are impermeable, and therefore make poor aquifers. An aquifer is "recharged" through precipitation from above percolating through the layers of soil and rock. Therefore, there is significant interaction between surface water and ground water.
In turn, groundwater feeds surface water through springs, and surface water can also recharge groundwater supply.
Most often, groundwater is accessed by humans via wells. To build a well, one must drill down past the water table. In most cases, a pump is placed in the bottom of the well, and it is pumped into homes, businesses and water treatment plants, where it is then dispersed. As water is pumped from the ground, a cone of depression forms around the well. The groundwater from the surrounding area moves towards the well. Wells can run dry during times of drought, or if surrounding wells are pumping too much water, causing the cone of depression to be large.
Water pumped from wells is generally very clean. The layers of soil, clay and rock acts as a natural filter. However, contaminants from nearby contaminated soils, leaky underground tanks, and septic systems can pollute a well, rendering it unusable. Furthermore, salt water intrusion can occur when the rate of pumping near a shoreline exceeds the rate of recharge. Saltwater gets pulled from the ocean into the cone of depression, and enters the well.
Subsidence, the gradual settling of the land due to continuous pumping and development, has also become an issue as groundwater is mined. This occurs when groundwater is pumped out faster than it can be replenished, and the sediment beneath becomes compacted. Subsidence is a permanent phenomenon. It can cause structural problems to foundations, an increased incidence of sinkholes and flooding problems. To top it off, subsidence is extremely costly. In some areas, such as the San Joaquin Valley in California, the land has subsided over 30 feet due to groundwater withdraw.
Surface Water Resources
Surface water is the water that exists in streams and lakes. This water is primarily used for potable water supply, recreation, irrigation, industry, livestock, transportation and hydroelectric energy. Over 63 percent of the public water supply is withdrawn from surface water. Irrigation gets 58 percent of its water supply from surface water. Industry gets almost 98 percent of its water from surface water systems. Therefore, surface water conservation and quality is of the utmost importance.
Watershed organizations continuously measure the stream flow and quality of surface water. Stream flow is monitored to warn of flooding and drought conditions. Water quality is very important, as the majority of the water used in the United States comes from surface water. It is the measure of how suitable the water is from a biological, chemical and physical perspective. Water quality can be impacted negatively by both natural and human causes: electrical conductivity, pH, temperature, phosphorus levels, dissolved oxygen levels, nitrogen levels and bacteria are tested as a measure of water quality.
Water that runs off into the stream can naturally carry sediment, debris and pathogens. Turbidity, the measure of suspended sediment in a stream, is also a measure of water quality. The more turbid the water, the lower the water quality.
Manmade contaminants such as gasoline, solvents, pesticides, and nitrogen from livestock can wash over the land and can leach in to waterways, degrading the quality of nearby waters. The Clean Water Act in the United States protects the quality of the stream and issues fines to those contributing to the degradation in water quality. By protecting and conserving the water supply, there is a greater guarantee of future water resources for human use.