In most cases, groundwater originates as precipitation that infiltrates at the ground surface and percolates down to the water table, which is a conceptual surface existing everywhere at some depth below the land surface (Figure 2). The depth of the water table varies from location to location. Its depth can be measured by drilling (or if the water is close to the surface, digging) a borehole to below the level where water begins to flow into the hole, and measuring depth from the ground surface to water surface. The water surface in a lake or river becomes the water table where the land surface meets the edge of the lake or river (Figure 2). That is, the water surface along a shoreline extends beneath the land surface. Along the shoreline, water is generally flowing (discharging) from the land into the lake or river but, in some instances, the direction of flow is from the water body into the earth.
Water rises a small distance into soil pores above the water table due to capillary forces that result from the adhesion of water molecules to subsurface solids and the cohesion of water molecules to one another. These capillary forces pull soil water upward, opposing the force of gravity that pulls the water downward. This is similar to a dry sponge that pulls water upward from a kitchen countertop to fill the pore spaces within the sponge. This zone is called the capillary fringe. Above the capillary zone and extending up to the surface is the “unsaturated” or “vadose” zone in which some of the spaces are filled with air and some with water. Below the water table, water occupies all the space between particles of sediment (pores), within cracks (fractures) and in channels (caves) of rocks as shown in Figure 3. All sediments and rock within the upper few thousand meters of the land surface have connected open spaces between the particles, fractures, or caverns and those space are called porosity. Porosity is the fraction of the geologic material that is open space and can hold fluid. Interconnected pore spaces render the geologic material permeable, that is, allow groundwater to flow through the material. Permeability is a measure of the ease by which water passes through the cracks and pores of geologic material.
Aquifers are defined as geologic layers that store and transmit useful volumes of water. Aquifers are more porous and permeable than the geologic layers surrounding them. A clean bed of coarse sand below the water table is a good example of an aquifer. Confining units are impediments to groundwater flow to and from aquifers. A layer of low permeability clay between layers of coarse sand or a layer of unfractured basalt rock between layers of fractured basalt are examples of confining units.
Hand-dug wells are common in many regions around the globe where the water table is shallow enough to be reached using shovels and picks or small excavators. We can see the water table by looking down into a well that was dug long ago by nomads in the Gobi Desert (Figure 4). In most cases, water wells are deeper and are drilled with heavy-duty, truck-mounted drill rigs rather than being dug by hand. Drilled wells typically have a small diameter and it can be difficult or impossible to see the water surface in the well, so a hand dug well is useful for the image of Figure 4.
Groundwater is an integral part of the hydrologic cycle. Figure 5 provides a link to a video that illustrates the hydrologic cycle in action.
Water enters the atmospheric portion of the hydrologic cycle by evaporation and leaves by precipitation. Movement of water in the atmosphere is powered by the sun and controlled by the continuous exchange of water molecules between liquid surface-water bodies (e.g., lakes, streams, oceans) and water vapor in the air.
The sun’s energy heats surface water bodies causing individual water molecules to break their bonds with neighboring molecules and escape into the atmosphere. At the same time, individual gaseous water molecules make contact with the liquid water and are absorbed. Depending on the temperature of the air and surface water, as well as the amount of water vapor in the air, this exchange results in either evaporation (a net movement of water from the surface into the air), or condensation (a net movement of water from the air to the surface). Air has limited ability to hold water vapor, so if the amount of water vapor in the air exceeds the maximum amount it can hold at the prevailing temperature (saturation), excess water vapor condenses as liquid water; forming dew or fog.
Evaporation creates humid air. As air near Earth’s surface is heated by the sun, it expands and rises. The Earth’s atmosphere is warmer near the surface and cooler aloft, thus rising humid air reaches a height where it has cooled to the point that the water-vapor content exceeds saturation. The excess water vapor then condenses as small droplets that are clouds (or fog, if near the ground). The small droplets grow by collision with one another and by condensation of water vapor on their surfaces. If they become large enough, they are too heavy to be held by the buoyancy of the air and they fall from the clouds as precipitation in the form of rain drops, snow, sleet or hail.
When precipitation falls on the ground surface it may:
- evaporate back into the atmosphere;
- flow over the ground (overland runoff) or through temporarily saturated shallow layers of soil above the water table (interflow) to surface water bodies such as streams and become storm flow in the stream; or,
- infiltrate into the soil.
Infiltrated water enters the vadose zone and some of it is used by vegetation. When there is surplus water in the vadose zone it percolates down to the water table becoming recharge to the groundwater system. When recharge occurs, the water table rises, water is stored in the groundwater system and flows slowly toward rivers, lakes, marshes and oceans where it discharges to the surface. This slow-moving water is stored in the subsurface as it moves toward discharge areas, and acts as a regulator (or buffer) that provides water to surface water bodies even during droughts. Stream flow that persists in dry seasons is called baseflow. In most watersheds, groundwater discharge is the principal, and often the only, source of baseflow.
Water moves between the surface, subsurface, and the atmosphere as it makes its way across the continents to the oceans. Most groundwater reaches the ocean as the baseflow component of stream flow, but some groundwater discharges directly to the ocean as seepage near coastlines and some rejoins the atmospheric portion of the hydrologic cycle by evaporation. Much of groundwater that enters the atmosphere without reaching the ocean occurs in closed depressions. The Aral Sea (between Kazakhstan and Uzbekistan), Caspian Sea (between Europe and Asia), and Dead Sea (in the Jordan Rift Valley), as well as the saline ponds and marshes of Death Valley (in the southwestern United States) are examples of such closed drainage systems.
Evaporation occurs from both the land and the ocean, but the ocean covers 71% of our planet and, unlike the land surface, the ocean surface does not dry out so there is an endless supply of water available for evaporation from the ocean, thus we often simplify the hydrologic cycle by saying it starts from the ocean. However, evapotranspiration from the land surface, which includes transpiration from vegetation, can move a large quantity of water from the shallow subsurface into the atmosphere. Plant roots draw water from the soil and the water table and transport it up through the plant (Figure 6). It is not uncommon for a single tree to move 500 liters of water a day from the soil into the atmosphere. Globally between 70 and 75% of water falling on the land is transpired or evaporated back to the atmosphere (Dai and Trenberth, 2002). That is, only 25 to 30% of rainfall flows through the ground and rivers back to the oceans. On a yearly average, approximately half of the river water reaching the ocean is groundwater (Reitz, et al., 2017) and about half flow over the surface or through the shallow subsurface as “quick flow”.
The combination of evaporation from small water bodies such as lakes, ponds, streams, as well as from the ground surface, and transpiration of water through the pores of plants, are collectively called evapotranspiration which comprises the movement of water from the Earth to its atmosphere.
Climate aridity is defined based on the ratio of the annual precipitation (P) and the potential evapotranspiration (PET) of a locale. PET is the amount of water that could be evapotranspired if there was an unlimited supply of water in the soil. PET is higher in areas with higher temperatures, lower humidity, and higher wind speeds. The level of aridity is defined by the Aridity Index (AI) which is the amount of precipitation divided by the potential evapotranspiration.
Humid areas are defined as areas with an AI greater than 0.65 and typically receive more than 500 millimeters per year (mm/yr) of precipitation. Although it varies widely from place to place, in humid areas:
- groundwater recharge typically occurs several times a year.
Dry sub humid areas have an AI between 0.5 and 0.65.
Semi-arid areas are defined as areas with an AI between 0.2 and 0.5. They typically receive on the order of 250 to 500 mm/yr. Arid areas have an AI between 0.05 and 0.2. Arid and semi-arid regions:
- lose a large fraction of precipitation to evapotranspiration with only a small amount reaching the water table (from near 0up to about 4% of precipitation); and,
- groundwater recharge occurs in frequently.
Hyper-arid areas have an AI of less than 0.05, and:
- groundwater recharge only occurs in depressions that collect sufficient runoff following a storm event to infiltrate enough water through the soil such that excess water reaches the water table; and,
- groundwater is often the only source of water because surface waters tend to be saline given that evaporation removes water molecules and leaves the dissolved salts behind in the surface water body.
The distribution of semi-arid and arid regions is shown in Figure 7. Arid regions are home to approximately 2.5 billion people, grow roughly 44% of the world’s food and raise about 50% of the world’s livestock.
The distribution of evapotranspiration on Earth is illustrated by the video of Figure 8. The video was created using a United States National Center for Atmospheric Research (USNCAR) Community Climate System Model simulation of water vapor and precipitation. In the video, clouds (white wisps) rise from both the oceans and continents, especially from tropical rainforests, such as the Amazon rainforest. Bursts of orange in the video indicate initiation of precipitation when the clouds become heavy with water and precipitation begins. These precipitation events are “fed” by evaporation from both the ocean and continents. As shown in the video, the rising water vapor from a location on land can join the moving air mass above, contributing to its vapor content, and then be carried downwind, and “rained out” at another location on the continent. This process is called precipitation recycling because this precipitation began as rain that fell on land then evaporated and became precipitation again.
Recycled precipitation can be a large portion of the total precipitation received in regions far from the ocean (Figure 9a). For example, more than half of the precipitation over the southern part of South America originates from water that is transpired in the Amazon forest (van der Ent et al, 2010). The video link of Figure 8 shows that water vapor rises from the Amazon Basin and moves to the south, along the eastern flank of, and bent by, the Andes mountain range, and eventually precipitates on the La Plata River Basin far to the south. A basin is the area drained by a river system as defined by a line connecting the topographic divide between river systems where a drop of water on one side flow to one river and on the other side flows to the other river. Groundwater basins may coincide with river basins, but can differ and are delineated by the line connecting the highest water levels between adjacent groundwater basins. The arrows of Figure 9b correspond with the direction of vapor transport in the atmosphere simulated in the video of Figure 8. The farther away a region is from the location where moist marine air reaches the continent, the more its precipitation depends on the evapotranspiration from the land upwind of its location. Because the La Plata Basin in the south depends on transpiration from the Amazon, the Amazon has been called a “green ocean”.