Unsaturated Flow Models

The previous two chapters dealt with the modeling of groundwater flow in aquifers, i.e., in the saturated zone. However, as emphasized on several occasions already in Chaps. 1 and 2, certain flow processes, which take place in the unsaturated zone, or zone of aeration (Fig. 2.2.1), are highly important also from the regional point of view and should be incorporated in our modeling considerations. The first example is the infiltration process. A phreatic aquifer is replenished from above by water from various sources: precipitation, irrigation, artificial recharge by surface spreading techniques, etc. In all these cases, water moves downward, from ground surface to the water table, through the unsaturated zone. The understanding of, and, consequently, the ability to calculate and predict the movement of water in the unsaturated zone is, therefore, essential when we wish to determine the replenishment of a phreatic aquifer. A second example is related to the contamination of groundwater from sources at ground surface. Contaminants from such sources dissolve in water applied to ground surface. The infiltrating water will then carry the dissolved contaminants as it moves downward towards the water table. As contaminants travel downward with the infiltrating water, various phenomena, e.g., dispersion and adsorption, take place. These affect the concentration of pollutants in the water, which, eventually, reaches the water table. The ability to forecast the movement and accumulation of contaminants in the unsaturated zone is required if we wish to clean the subsurface from these contaminants, or to determine the rate at which they will reach the water table. However, one cannot study the movement of contaminants carried by the water without information on the movement of the water itself. In this chapter we shall discuss the modeling of flow in unsaturated flow.