Once the infiltrated precipitation reaches the water table, the water table rises. Gravity causes groundwater to flow laterally from locations where the water table elevation is higher to locations where the water table is lower as shown on the cross-sectional face of Figure 12. The brown frame on the vertical face of Figure 12 places the earlier view of one-dimensional flow through the vadose zone (Figure 10) into a larger, two-dimensional spatial flow pattern. Groundwater flow occurs in three-dimensional patterns, but first we consider flow a two-dimensional cross section to simplify visualization and discussion.
The water table is higher beneath upland areas because the precipitation entered the land at a higher elevation, and the groundwater flows downward into the groundwater system and toward the lowlands. The rate of water flow from the uplands to the discharge area depends on the rate of recharge (water infiltrating to the water table), the elevation difference between recharge and discharge areas, and the permeability of the soils and rocks through which the groundwater is flowing. If there is a drought, the water table in the uplands declines because water is flowing toward the stream and not being recharged by infiltration at the surface, thus water that is stored in the pores or fractures of the geologic units is released and the water table is lowered. When the upland water table declines, the rate of groundwater flow toward the streams slows, but water continues to discharge to the stream until the water table drops below the streambed surface.
A piezometer is a small-diameter well that is open to the inflow of groundwater over a short section of its length. The elevation of the water surface in a piezometer relative to sea level is known as hydraulic head (often simply called “head”), which is a measure of the potential energy of the water.
Groundwater flows from points with high hydraulic head (high potential energy) to points with low hydraulic head (low potential energy). As water that recharged the water table beneath an upland moves downward and toward lowlands, it loses energy due to friction between the moving water and the aquifer framework so the hydraulic head declines along the flow path as the potential energy is converted to heat, though the heat is too small to be measured.
The flow system on the front face of Figure 12 represents an idealized cross section with flow moving along a section perpendicular to ridges on the left and right to a stream valley in the center. Water infiltrates at the ground surface and percolates down to the water table, then flows in paths with combinations of downward, lateral, and upward components of flow to discharge at the stream. This is an “idealized” view because all the flow appears to occur in the plane of the drawing. That is, flow is two-dimensional with water entering and exiting only at the top boundary and no water moving into, or out of, the plane of the page. In natural settings, groundwater flow is three-dimensional, as illustrated in the schematic of Figure 13. Blue dashed lines in Figure 13 connect locations of equal hydraulic head and solid blue arrows represent groundwater flow moving down the hydraulic head gradient. Conceptually, Figure 12 follows the thick white line cutting perpendicular to the stream in Figure 13. Although Figure 12 indicates that flow is perpendicular to the ridge and stream, in a natural system flow is not perpendicular to them as shown in Figure 13. That is, components of flow enter from the upgradient areas in the foot hills and exit to down gradient areas of the plains.
This lateral flow from hills to valleys is important. The water table in a valley is closer to the surface than in the uplands and does not rise and fall as much as water tables under uplands, because it is regulated by the recharge over the entire hillslope above it. If it does not rain for many days, the hills may be parched, but the valleys are still receiving groundwater from the uplands, because groundwater flow is relatively slow. In dry times, the groundwater recharge on the hills from many days, or even many years, ago is still “on its way” toward the valleys. Thus, the delayed delivery of groundwater from the hills to the valleys ensures that the valleys will receive water in dry times.
As such, the groundwater system is similar to a bank account, water is stored under the hills as “funds in an account” that steadily sends “cash” to the valleys, through a slow “postal service”, arriving in the valley when it is not receiving “cash from local customers”. Thus, the valley has a steady income, so it is prepared for lean times. This is part of the reason why trees in the valleys are larger and healthier than the trees on the hills in places that have long dry seasons.