Groundwater is water stored beneath the Earth’s surface within the pores and fractures of rock and soil formations, often in reservoirs called aquifers. A drought is a prolonged period of abnormally low precipitation, creating a deficit in the natural water supply. While surface water sources like rivers and lakes respond quickly to a lack of rain, groundwater systems react much more slowly because of the time it takes for water to seep deep underground. This lag means that initial drought impacts on groundwater are not immediately obvious.
The Immediate Loss of Soil Moisture
The first area to feel the effects of a lack of precipitation is the unsaturated zone, the layer of soil and rock above the water table. This zone holds soil moisture and functions as the initial buffer against dry conditions. As soon as a drought begins, this soil moisture is quickly depleted through evaporation from the surface and transpiration by plants, a combined process known as evapotranspiration.
The drying soil begins to hold onto any minimal precipitation that does fall, preventing it from draining deeper. When soil becomes extremely dry, it can even become water-repellent, a condition known as hydrophobicity, which encourages water to run off the surface rather than soaking in. This means the first step in the groundwater renewal process—infiltration—is significantly impaired almost immediately after a meteorological drought starts. This depletion of the unsaturated zone is the prerequisite for the deeper hydrological impacts that follow.
Interrupting the Recharge Process
Groundwater renewal, or recharge, is the downward movement of water from the surface into the saturated zone. For deep percolation to occur, the soil moisture deficit in the unsaturated zone must first be fully satisfied. During a drought, this pre-condition is not met because nearly all the available precipitation is lost back to the atmosphere or held by the dry soil.
Evapotranspiration interrupts recharge, as plants pull water from the soil and release it as vapor, intercepting the water that would otherwise filter down. High temperatures and low humidity common during drought conditions accelerate this loss, increasing the amount of water needed to satisfy the soil layer. Consequently, the water that might normally push past the root zone and into the aquifer is instead consumed locally, halting the flow of new water to the deeper underground reserves. This interruption in the recharge process means that the water table is not replenished, leading to a net loss of stored groundwater over time.
Early Decline in Shallow Water Tables and Baseflow
The direct consequence of interrupted recharge is the decline in water levels, particularly in shallow, unconfined aquifers. An unconfined aquifer is in direct contact with the surface and is highly responsive to changes in precipitation and recharge. These shallow systems are the first groundwater sources to reflect the stress of a drought, often within weeks or months of the initial lack of rain.
As the water table in these unconfined aquifers drops, the continuous contribution of groundwater to surface water bodies, known as baseflow, begins to diminish. Baseflow keeps streams and rivers flowing during dry periods when there is no runoff from recent rainfall. When baseflow is reduced, streams and wetlands that rely on this groundwater discharge quickly experience stress, leading to reduced flow or drying up entirely. This visible decline in surface water flow is one of the earliest hydrological signs of groundwater system stress.
Observable Signs of Initial Stress
The initial impacts of a drought on the shallow groundwater system result in tangible problems for users. The most immediate signs of stress are the failure of domestic wells and springs. These sources draw water from the shallow, unconfined aquifers that respond most rapidly to a lack of recharge.
As the water table drops, a shallow well’s pump intake may sit above the water level, causing the well to run dry or produce a reduced yield. Springs, which are natural outlets for groundwater, will also slow or cease flowing as the pressure head in the shallow aquifer declines. These observations are indicators of the initial, localized groundwater drought.