Pumping water from aquifers for use in irrigation can lower the water table.
The groundwater table is the underground division line between the groundwater, or saturated or phreatic, zone below and the unsaturated, or vadose or aeration, zone above. In the groundwater zone, every pore in the sediment or rock of which it is composed is saturated with water. In the vadose zone, the sediment or rock is only partially water filled; water has room to move downward, pulled by the force of gravity toward the water table.
Natural Water Table Variations
The distance from the surface of the ground to the water table is variable from place to place. It tends to be deeper under hills and less deep under valleys. Water table depth is not static; change in its level can depend on a variety of factors. Some water tables are subject to frequent changes by a number of feet, while others tend to fluctuate occasionally and by only inches.
When rainfall is frequent and heavy, water percolates down, formerly unsaturated ground becomes saturated and the water table rises. During seasonally dry periods or drought, water is drawn towards the surface and the water table falls. In locations where the water table is hundreds of feet below the surface however, it may take a long time for water to travel through the vadose zone to the saturated zone; seasonal variation in the water table may be minimal to nonexistent.
The geologic makeup of the saturated zone in different geographic areas varies considerably, and different types of sediment and rock have different levels of porosity, or ability to hold water. This water-holding ability is expressed as a ratio: pore space to solid material per unit volume. For example, sand that is saturated may have 30 percent pore space to 70 percent solids. Fractured rock, by comparison, may have as little as 1 percent pore space to 99 percent solids. Sand is much more porous than rock and can hold more water. Thus, the water table above a sand groundwater zone will rise or fall more slowly than the water table above a rock groundwater zone.
A groundwater zone sufficiently porous to allow water to move through easily is called an aquifer. People drill wells into aquifers to withdraw water for use. Rainfall naturally recharges, or refills, aquifers, but at different replenishment rates. If water is pumped out faster than the natural recharge rate from surface infiltration or streams, it may draw down the level of the water table so far that wells tapping into the aquifer go dry.
Groundwater recharges less quickly and the water table therefore fluctuates less in areas that are covered with thick vegetation than in areas of the same type of soil that are bare of vegetation. Vegetation retains moisture and returns it to the atmosphere through evapotranspiration, thereby allowing less water to infiltrate the soil and percolate down to groundwater level.
Wells that tap aquifers to withdraw water for irrigation purposes are one of the biggest challenges to maintaining a stable water table. Irrigation typically requires high withdrawal rates for the growing season. Municipal and industrial wells also withdraw large volumes of water but do so more intermittently or uniformly throughout the year. When a water table has been lowered more than is considered healthy, surface water is purposefully injected into the groundwater, which is known as artificial recharge.
Other Factors that Affect the Water Table
Stormwater retention or recharge ponds and wastewater treatment systems, such as cesspools and septic tank drain fields, hold water that would otherwise flow down to the groundwater and raise the water table. An increase in the construction of impervious surfaces, such as concrete, also hinders the penetration of water into the ground. In a poor neighborhood of Cairo, a leaking sewage system actually caused the water table to rise, which was not a desirable outcome, and threatened to decompose the limestone ruins of ancient Heliopolis, the 6,000-year-old first great priestly city of Egypt. In places like Florida, which is underlain by porous limestone, the water table is sensitive to ground movement caused by earthquakes. The South Florida Water Management District reported that there was a rise of up to 3 inches in the water table about 30 minutes after the Japanese earthquake of March 11, 2011, even though it occurred over 7,000 miles away.