Science of Water : Concepts and Applications

All about Water 29
FIGURE 2.3 Watershed.
Rain storm
When you trace on a map the course of a major river from its meager beginnings on its seaward
path, that its fl ow becomes larger and larger is apparent. While every tributary brings a sudden
increase, between tributaries, the river grows gradually from the overland fl ow entering it directly
(see Figure 2.3).
Not only does the river grow its whole watershed or drainage basin, the land it drains into grows
too in the sense that it embraces an ever-larger area.
The area of the watershed is commonly measured in square miles, sections, or acres. When taking
water from a surface-water source, knowing the size of the watershed is desirable.
RAW WATER STORAGE
Creek
Surface runoff
Melting snow
Reservoir
Raw water (i.e., water that has not been treated) is stored for single or multiple uses, such as navigation,
fl ood control, hydroelectric power, agriculture, water supply, pollution abatement, recreation,
and fl ow augmentation. The primary reason for storing water is to meet peak demands, or to store
water to meet demands when the fl ow of the source is below the demand. Raw water is stored in
River
Mouth of watershed
Spring
Watershed
divide
Groundwater
seepage