Precipitation, which includes rain and snow, is the primary source of water for the planet. Groundwater is the water that is stored underground in soil and rock fractures. A decrease in precipitation directly leads to a reduction in the amount of available groundwater. Understanding this relationship requires examining how surface water becomes groundwater and the factors that influence this exchange.
The Process of Groundwater Recharge
Groundwater recharge is the process by which water from the surface moves downward to replenish underground water sources. When rain falls or snow melts, the water seeps into the ground in a process called infiltration. This water first moistens the upper layers of the soil, in a region known as the vadose zone, before gravity pulls it further down.
As the water continues its descent, it percolates through layers of soil, sand, and porous rock. This movement eventually brings it to the water table, which is the upper level of the saturated zone. The water collects in underground layers of rock or sediment known as aquifers.
The rate of this recharge depends on several factors, including the type of soil and rock, the land cover, and the intensity of the precipitation. For example, sandy soils allow for much faster infiltration than dense clay soils. The entire journey from the surface to an aquifer can take anywhere from days to centuries, depending on the depth of the aquifer and the geological makeup of the land above it.
Impact of Reduced Precipitation on Aquifers
A reduction in precipitation curtails the amount of water available for recharge. With less water infiltrating the ground, the natural replenishment of aquifers slows or even halts. As a result, the water table, which marks the upper boundary of an aquifer, begins to drop.
During periods of low precipitation, the ground surface and upper soil layers become drier. Any moisture that does fall is absorbed by this soil, where it is more likely to be taken up by plant roots or evaporate. This means an even smaller fraction of the reduced precipitation makes its way deep enough to recharge the aquifer.
Over time, this imbalance causes the total volume of water stored in an aquifer to decline. This decline is a consequence of recharge rates falling below the natural discharge rates, where groundwater exits into springs, rivers, or lakes.
The Role of Human Water Consumption
Human activities place additional pressure on groundwater supplies, especially when precipitation is low. Aquifers are a major source of water for agriculture, which uses it for irrigating crops. Cities and towns rely on groundwater for their municipal supplies, providing drinking water to millions. Industries also draw on this underground resource for various manufacturing and cooling processes.
During droughts or extended periods of low rainfall, the demand for groundwater often increases. Farmers need to irrigate more frequently to compensate for the lack of rain, and shrinking surface water sources force municipalities to rely more heavily on their wells. This creates a combination of reduced supply and increased demand.
This situation accelerates groundwater depletion. While natural recharge has slowed due to the lack of precipitation, human extraction continues or intensifies. When the rate of water being pumped out exceeds the rate of water flowing in, it causes a drop in groundwater levels beyond what would occur naturally.
Consequences of Groundwater Depletion
Groundwater depletion has several consequences:
- As the water table drops, existing wells may no longer be deep enough to reach water, causing them to dry up. This can leave homes, farms, and communities without their primary water source, requiring the expensive process of drilling deeper wells.
- Aquifers are composed of rock and sediment, and the water within them helps support the ground above. As water is pumped out, the pore spaces can collapse, causing the land to sink. This process, known as land subsidence, can damage buildings and roads and is generally irreversible.
- Many rivers, lakes, and wetlands are fed by groundwater seeping into them. When this source is depleted, the flow can slow or reverse, causing surface waters to shrink and harming the ecosystems that depend on them.
- In coastal areas, excessive pumping can cause saltwater from the ocean to move into freshwater aquifers, contaminating the water supply.