4.1 The Shallow View
When precipitation moves below the ground surface it is called infiltration (dark blue downward dashed arrows in Figure 10). Figure 10 shows a fractured medium environment on the left and a porous medium environment on the right. In fractured rock, water moves through cracks, while in porous media water moves through spaces between granular particles. Flow processes are the same in both systems, but the character of the geologic material differs.
Water from a short rain event only infiltrates to a shallow depth, but long, gentle rains infiltrate deeper, sometimes reaching the water table (solid horizontal line marked with a filled triangle in Figure 10) where it becomes groundwater recharge. Although there are localized areas of lateral water movement, flow is primarily one-dimensional in the zone above the water table.
Below the water table, the fractures and pores are filled with water and the water pressure is greater than the atmospheric pressure (positive gauge pressure). In contrast above the water table, the pores and fractures are filled with a mixture of water and air and the water pressure is less than the atmospheric pressure (negative gauge pressure).
The zone above the capillary fringe is called the unsaturated zone because soil pores are only partially filled with water; it is also called the vadose zone (Figure 10). Even though the subsurface material may be rock or sand (that is, may be different from surficial soil) the water in the vadose zone is often called soil water in order to differentiate it from groundwater in the saturated zone below the water table. Soil water is indicated as blue shapes throughout the vadose zone in Figure 10. The negative gauge pressure of water in the vadose zone are due to capillary forces.
Near the water table, capillary forces create a capillary fringe (Figure 10) in which the pores and fractures are completely full of water but the water pressure is negative. The capillary fringe extends a few tens of millimeters above the water table in large pores such as in sand deposits, and several meters above the water table in small pores such as in clay deposits or small hair-line fractures in rocks. Although the capillary fringe is saturated with water like the groundwater zone, it is considered part of the unsaturated zone because the water has negative pressure. This is why the term vadose zone includes the capillary fringe.
When the soil water reservoir is filled to the point that no more water can be held by capillary forces, gravity pulls the extra water downward to reach the water table. This process is referred to as groundwater recharge because infiltrated precipitation is replenishing the groundwater reservoir. The groundwater is recharged only where and when there is a soil water surplus. The recharged water is stored in the groundwater system as it slowly flows toward discharge locations.
Capillary forces have less influence when the subsurface is wetter, so water is more likely to migrate down to the water table when the shallow subsurface contains more moisture. Consequently, in the wet and cool seasons, or during periods of steady rain or snowmelt, infiltrating water can reach the water table and add water to the groundwater reservoir. Clearly, not all rain events recharge the groundwater reservoir; most events only wet the vadose zone, replenishing the soil water reservoir and being consumed by transpiration of the vegetation. The timing and magnitude of recharge reaching the water table depends on the soil properties and depth of the water table as well as the duration and intensity of precipitation as illustrated by the rise of the water table following precipitation (Figure 11).
Vegetation on land (terrestrial vegetation) uses the vadose zone reservoir for water supply. Plant roots occupy this zone because it contains air, and oxygen is essential to root respiration (much like our breathing). Plants draw soil water into their roots by lowering the effective water pressure in their roots to enhance the movement of soil water into the plant.
In arid areas, the shallow groundwater may contain too much salt for most types of vegetation to grow, so the vegetation will consist of only salt tolerant plant species. Also, in arid areas, some plants, called phreatophytes, grow their roots deep enough below the surface to tap groundwater. Examples of phreatophytes include cottonwood, willow, eucalyptus and Russian olive trees; brush such as salt cedar; and crops such as alfalfa. Phreatophyte roots commonly extend to depths up to 15 meters, however, some salt cedar roots have been found to extend to 30 meters depth along the Suez Canal.
There is a significant awareness that the introduction of some animal species from one continent to another (e.g., rabbits brought from England to Australia) can result in an extreme ecological imbalance. However, there is far less awareness of the effect that the introduction of non-native, groundwater-consuming vegetation (e.g., eucalyptus trees from Australia; salt cedar from Eurasia and Africa) has on dry regions of other continents. Such plants can become unwanted groundwater consumers, affecting native plants and agriculture.