ABSTRACT
Water exists on Earth in all its three states, liquid, solid, and gas, in various degrees of motion. Evaporation of water from water bodies such as oceans and lakes, formation and movement of clouds, rain and snowfall, streamflow, and groundwater movement are some examples of the dynamic aspects of water. The various aspects of water related to the Earth can be explained in terms of a cycle known as the hydrologic cycle. A brief system description of hydrologic cycle was presented in Chapter 2 (see Figure 4.1). The hydrologic cycle generally includes the water interaction between the atmosphere and the Earth, namely, precipitation, infiltration, evaporation, and evapotranspiration, as well as water movement and storage on and under the surface of the Earth, namely, surface runoff, subsurface flow, groundwater flow, and reservoirs and lakes.
