The movement of water from the atmosphere into the earth’s crust is a fundamental part of the global water cycle. When precipitation falls, a portion is absorbed by the ground, beginning a slow, downward progression. This initial entry into the soil surface is called infiltration, and its subsequent downward movement through soil and rock is known as percolation. These actions replenish the vast underground reservoirs that supply drinking water and sustain ecosystems worldwide.
Infiltration: Water’s Entry Point
Infiltration is the first step in the subsurface water journey, defining the rate at which water passes through the ground surface. This process is governed by the soil’s infiltration capacity—the maximum rate at which it can absorb water at a given time. If rainfall exceeds this capacity, the excess water becomes surface runoff, often flowing into streams and rivers.
The infiltration rate is highest when precipitation first begins because the soil is dry and has a greater capacity to absorb moisture. As the soil becomes wetter, pore spaces fill, causing the infiltration rate to decrease. Vegetation helps maintain a higher infiltration capacity by creating pathways and organic material, reducing runoff and promoting groundwater recharge.
Movement Through the Unsaturated Zone
After infiltration, water enters the unsaturated zone, also known as the vadose zone, which is the layer of soil and rock above the water table. This zone is not fully saturated; the pores and spaces between soil particles contain both air and water. The movement of water through this area is percolation, driven primarily by the force of gravity pulling the water downward.
Capillary action also plays a role, especially in finer-textured soils. Capillary forces cause water molecules to adhere to soil particle surfaces and draw into smaller pore spaces, often resisting the downward pull of gravity. The water held within the unsaturated zone is called soil moisture, a supply necessary for plant roots to draw upon for survival and growth.
Reaching the Water Table and Groundwater
As percolating water continues its descent, it eventually reaches the water table, the upper boundary of the saturated zone. The water table is not a fixed line but a dynamic surface separating the unsaturated soil above from the saturated ground below. Below this surface lies the saturated zone, where all available cracks, crevices, and pore spaces in the soil and rock are filled with water.
The water stored within this saturated zone is defined as groundwater, a massive reserve found almost everywhere beneath the earth’s surface. Geologic formations that can store and transmit usable quantities of groundwater are known as aquifers. The depth of the water table is constantly changing, rising during heavy rain or snowmelt, which replenishes the aquifer, and falling during dry seasons or when groundwater is heavily pumped for human use.
How Soil Characteristics Influence Water Flow
The speed and volume of water’s subterranean journey are controlled by the physical characteristics of the soil and rock it passes through. Porosity refers to the total amount of open space or void volume within a material, determining how much water the ground can potentially hold. While high porosity means high storage capacity, it does not guarantee easy water flow.
The ability of water to move through these spaces is defined by permeability, which measures how well the pores are interconnected. For water to flow freely, the void spaces must be large and connected enough to allow movement. For example, sand has high porosity and high permeability, allowing water to pass through quickly, making it a good aquifer material. Conversely, clay can have high porosity due to many tiny spaces but very low permeability because these microscopic pores are not well connected, causing water movement to be extremely slow.