Botkin Environmental Science Earth as Living Planet 8th txtbk

18.2 Water Supply: A U.S. Example 373
Urban land
Agricultural land
Water well
Vacation homes with septic
tanks near reservoir
Sedimentary rock
(sandstone)
Groundwater
movement
Influent stream
(see Figure 21.3)
4
1
2
Sunlight
City
Reservoir
6
Sewage
treatment
3
plant
5
Groundwater table
FIGURE 18.4 Idealized diagram illustrating some interactions between surface water and groundwater for a
city in a semiarid environment with adjacent agricultural land and reservoir. (1) Water pumped from wells lowers
the groundwater level. (2) Urbanization increases runoff to streams. (3) Sewage treatment discharges nutrientrich
waters into stream, groundwater, and reservoir. (4) Agriculture uses irrigation waters from wells, and runoff
to stream from fields contains nutrients from fertilizers. (5) Water from the reservoir is seeping down to the
groundwater. (6) Water from septic systems for homes is seeping down through the soil to the groundwater.
the groundwater level may change a perennial stream into
an intermittent influent stream. Similarly, withdrawing surface
water by diverting it from streams and rivers can deplete
groundwater or change its quality. Diverting surface waters
that recharge groundwaters may increase concentrations of
dissolved chemicals in the groundwater because dissolved
chemicals in the groundwater will no longer be diluted by
infiltrated surface water. Finally, pollution of groundwater
may result in polluted surface water, and vice versa. 5
Selected interactions between surface water and groundwater
in a semiarid urban and agricultural environment are
shown in Figure 18.4. Urban and agricultural runoff increases
the volume of water in the reservoir. Pumping groundwater
for agricultural and urban uses lowers the groundwater level.
The quality of surface water and groundwater is reduced by
urban and agricultural runoff, which adds nutrients from
fertilizers, oil from roads, and nutrients from treated wastewaters
to streams and groundwater.
18.2 Water Supply:
A U.S. Example
The water supply at any particular point on the land
surface depends on several factors in the hydrologic
cycle, including the rates of precipitation, evaporation,
transpiration (water in vapor form that directly enters
the atmosphere from plants through pores in leaves and
stems), stream flow, and subsurface flow. A concept useful
in understanding water supply is the water budget, a
model that balances the inputs, outputs, and storage
of water in a system. Simple annual water budgets
(precipitation – evaporation = runoff) for North America
and other continents are shown in Table 18.2. The total
average annual water yield (runoff) from Earth’s rivers
is approximately 47,000 km 3 (1.2 10 16 gal), but its
distribution is far from uniform (see Table 18.2). Some