Droughts & Water Scarcity
Groundwater is the hidden water stored beneath our feet. During drought, aquifers can become the last backup source for farms, towns, springs and rivers — but when pumping exceeds recharge, wells dry up, land sinks and water systems can take decades or centuries to recover.
What is groundwater?
Groundwater is water stored below the ground in pores, cracks and spaces between soil, sand, gravel and rock. It fills part of the subsurface called the saturated zone, where empty spaces are completely filled with water.
This hidden water feeds wells, springs, streams, wetlands and some rivers. In dry regions, groundwater may be the most important long-term water reserve.
Key idea
Groundwater is often invisible, but it is one of the most important parts of the freshwater system. When it declines, the surface may not show the damage immediately.
What is an aquifer?
An aquifer is a layer of permeable rock, sand, gravel or sediment that can store and transmit groundwater. Aquifers are natural underground reservoirs, but they do not behave like empty caves full of water. Most groundwater is held in tiny spaces between grains or fractures in rock.
Main aquifer types
- Unconfined aquifers: shallow aquifers directly connected to the land surface and rainfall recharge.
- Confined aquifers: deeper aquifers trapped between less permeable layers, often under pressure.
- Alluvial aquifers: aquifers made of river-deposited sand, gravel and sediment.
- Fractured-rock aquifers: groundwater stored in cracks and fractures in solid rock.
What is groundwater depletion?
Groundwater depletion happens when water is removed from an aquifer faster than it is replaced by recharge. This can occur slowly over decades or rapidly during severe drought when pumping increases.
Depletion lowers the water table, reduces well yields, weakens springs and may reduce the baseflow that keeps rivers alive during dry periods.
Common causes
- Overpumping: wells withdraw more water than the aquifer receives.
- Drought: rainfall and snowmelt recharge decline.
- Irrigation demand: farming uses large volumes of groundwater in dry regions.
- Urban growth: more homes, lawns and industries increase demand.
- Reduced recharge areas: pavement, drainage and land-use change limit infiltration.
- Climate stress: heat increases evaporation and water demand.
Overpumping and dry wells
Overpumping occurs when wells remove water faster than the aquifer can recover. At first, deeper pumps may still reach water. Over time, the water table drops and shallow wells begin to fail.
What happens when groundwater drops?
- Wells produce less water
- Shallow wells go dry
- Pumping costs rise because water is deeper
- Springs and wetlands weaken
- Streams lose groundwater support
- Water quality may worsen as deeper or saltier water moves in
Dry wells are one of the clearest human-visible signs of aquifer stress, but the aquifer may have been declining long before wells fail.
Land subsidence: when the ground sinks
Land subsidence is the sinking of the ground surface. It can happen when groundwater is removed from fine-grained sediments such as clay and silt. As water pressure declines, the sediment layers compact.
Subsidence is one of the most serious consequences of groundwater depletion because it can be permanent. Once compacted, some aquifer layers lose storage capacity forever.
Why subsidence matters
- Can damage canals, roads, bridges and buildings
- Changes drainage patterns and flood risk
- Reduces aquifer storage capacity
- Can create cracks, fissures and uneven ground
- Makes drought recovery harder
Recharge and groundwater recovery
Recharge is the process by which water enters an aquifer. It happens when rain, snowmelt, river water or irrigation water infiltrates through soil and sediment into the groundwater system.
Natural recharge sources
- Rainfall soaking into the ground
- Snowmelt infiltrating mountain basins
- Water seeping from rivers and streams
- Floodplain infiltration
- Wetlands and seasonal ponds
Managed aquifer recharge
In some regions, water managers intentionally recharge aquifers by spreading floodwater over basins, using recharge ponds, restoring floodplains or injecting treated water underground.
Recharge is slow. Some shallow aquifers respond within months or years. Deep aquifers may take decades, centuries or longer to recover.
Ogallala Aquifer
The Ogallala Aquifer, part of the High Plains Aquifer system, is one of the most important groundwater systems in North America. It supports large areas of irrigated agriculture across the Great Plains.
The challenge is that some parts of the aquifer recharge slowly, while irrigation withdrawals can be intense. In heavily pumped areas, water levels have dropped significantly, creating long-term concerns about dry wells, farming limits and groundwater sustainability.
Why it matters
- Major agricultural water source
- Slow recharge in many areas
- Large regional variation in depletion
- Strong example of fossil groundwater use
California Central Valley
California’s Central Valley is another major groundwater depletion case study. During drought, reduced surface water supplies often lead to heavier groundwater pumping for agriculture and towns.
Parts of the Central Valley have experienced severe groundwater decline and land subsidence. This makes the region one of the clearest examples of how drought, irrigation demand, aquifer compaction and water management interact.
Key issues
- Heavy irrigation demand
- Drought-driven pumping spikes
- Land subsidence in some basins
- Groundwater quality concerns
- Need for recharge during wet years
Warning signs of groundwater depletion
- Wells need to be drilled deeper
- Pumps run longer or fail more often
- Springs weaken or stop flowing
- Streams dry earlier in the year
- Wetlands shrink or disappear
- Ground cracks or fissures appear
- Land surface slowly sinks
- Water quality becomes saltier or more mineralized
- Recovery becomes slow even after wet years
FAQ: Groundwater and aquifer depletion
What is an aquifer?
An aquifer is a layer of permeable rock, sand, gravel or sediment that stores and transmits groundwater.
What causes groundwater depletion?
Groundwater depletion happens when wells pump water from an aquifer faster than rain, snowmelt, rivers or managed recharge can replace it.
Why do wells go dry during drought?
During drought, recharge declines and pumping often increases. If the water table drops below the depth of a well, the well can produce less water or go dry.
Can aquifers recover after depletion?
Some shallow aquifers can recover after wet years, but deep or heavily depleted aquifers may take decades, centuries or longer to recharge.
What is land subsidence?
Land subsidence is the sinking of the ground surface. It can occur when groundwater removal causes underground sediments to compact.
Why is overpumping dangerous?
Overpumping lowers groundwater levels, dries wells, weakens springs and streams, increases pumping costs, worsens water quality and can cause permanent land subsidence.
