Page 113 - Biosystems Engineering

94 Cha pte r T h ree

Conversely, when there is not enough water, water is the limiting factor.
Two important terms are actual and potential evaporation. Potential evap-
oration is the rate of evaporation when there is plenty of water, which
corresponds to the temperature-limited case. When there is not enough
moisture in the soil (i.e., water-limited case), the actual evaporation rate is
smaller than the potential evaporation. Actual evaporation is always
smaller than or equal to potential evaporation.
Transpiration This is the process of water molecules escaping from
tree leaves and other vegetation into the atmosphere. This process is
often combined with evaporation and is called evapotranspiration.
Similar to evaporation, actual evapotranspiration is mostly less than
potential evapotranspiration. In addition to the factors affecting evap-
oration rates, physiologic characteristics of the plants, their leaf shapes,
and their leaf densities also affect transpiration rates.
Condensation This is the process of water vapors changing into liq-
uid form as a result of cooling. Formation of clouds and fog is a result
of condensation of water vapors.
Precipitation This is the process of the falling of condensed water
onto the earth’s surface (land or water bodies). Precipitation occurs
mainly in the form of rain. However, snow, sleet, hail, dew, frost, and
icy rain are all forms of precipitation.
Interception This is the process of the interception of a portion of the
precipitation by vegetation or by trees. Intercepted water eventually
evaporates into the atmosphere. In other words, it never reaches the
soil and, therefore, does not contribute to surface runoff or to infiltra-
tion processes.
Infiltration This is the process of the movement of water from the
soil surface into the soil as a result of gravity and capillary forces.
Infiltration takes place at the soil–air interface, or in case water is pon-
ded at the soil–water interface. In addition to soil texture and soil
characteristics, available soil moisture content also plays a major role
in determining the infiltration rate. The term infiltration capacity is
used to define the rate of infiltration when there is plenty of water or
water is ponded on the soil surface. This happens when the rainfall
intensity is larger than the infiltration rate. When the rainfall inten-
sity is low and water cannot pond on the soil surface, then infiltration
rate is equal to rainfall intensity.
Exfiltration This is the opposite of infiltration (i.e., movement of water
from the soil layer to the soil surface). Exfiltration could be due to pres-
sure forcing water to move up or water moving within the soil sud-
denly meeting the atmosphere due to changes in topography (springs).

Percolation This process is similar to infiltration. The difference is
that water movement in percolation is within the soil matrix instead