Page 112 - Biosystems Engineering

Soil and W ater Conservation 93

The Water Cycle
Condensation
Water storage
in ice and snow Water storage in
Precipitation the atmosphere
Transpiration

Snowmelt runoff Evaporation
to streams Surface runoff

Infiltration Discharge freshwater
ground- groundwater
water storage
Water storage
Storage in oceans
ground- Infiltration
water groundwater Discharge
groundwater

FIGURE 3.1 Schematic representation of the hydrologic cycle. [Adapted from the
United States Geological Survey (USGS).]

water molecules in different forms (vapor, ice, liquid) on, above, and
below the earth’s surface (Fig. 3.1). The energy from the sun (i.e., solar
radiation) is the main driver of the hydrologic cycle. Approximately
50 percent of the sun’s energy reaching the earth’s surface is used to
vaporize water. The average solar radiation reaching the United States
2
is about 400 cal/cm /day. Thus, on average, roughly 200 cal/cm of
2
energy is consumed per day by the earth’s surface and by plants to
vaporize water either by direct evaporation or by transpiration.
The hydrologic cycle includes many components and processes:
evaporation, transpiration, condensation, precipitation in all forms
(rain, snow, hail, sleet, fog, etc.), interception, infiltration, exfiltration,
percolation, runoff, groundwater flow, groundwater recharge, and
snow melt. These components and processes are tightly connected.
However, it is not necessary to have all these processes for a complete
water cycle. Often, some processes are bypassed or the hydrologic
cycle is short circuited. Brief summaries of these components and
processes are given as follows.

Evaporation This is the process of the formation of water vapor mole-
cules from a liquid or gaseous (sublimation) form. This process requires
solar energy. Factors affecting evaporation are temperature, wind speed,
relative humidity, and solar radiation. There are two main limiting fac-
tors for evaporation to occur: water and energy. When there is plenty of
water available, the temperature is the limiting factor for evaporation.
The evaporation rate in this case is mainly driven by temperature.