computer programs can be used to determine the pan coefficient, which relates the
evaporation in the pan to the actual evaporation in the body of water.
In addition to evaporation from a water surface, there is evaporation of moisture
from soil. This is usually a function of the evaporation of recent precipitation, but
where the ground water level is close to the ground surface, capillary action may
maintain moisture at the surface of the ground, where the moisture may evaporate.
As a general rule, ground cover reduces the amount of evaporation from the soil
surface. Compared to the evaporation from bare ground, areas covered by vegetation
may result in only 0.7–0.8 as much evaporation, and a dense forest may result in only
0.2–0.4 as much. In general, the same factors that affect evaporation from an open
water surface also have similar effects upon evaporation of the soil moisture.
5 Transpiration
Transpiration is the vaporization of water from the breathing pores of leaves and
other plant surfaces. This amount can be significant in forested areas and can be as
much as 2 million pounds of water per acre per year (300,000 kg/ha-yr). The amount
of transpiration increases significantly with an increase in temperature, and sunlight
is necessary for the process to occur. It is relatively difficult to measure transpiration
alone, but it can be determined experimentally in special plots, greenhouses, and so
on. However, it is difficult to compare these experimental studies with actual field
conditions. In general under field conditions, it is almost impossible to separate
transpiration from evaporation from the soil surface.
6 Evapotranspiration
Since it is difficult to determine transpiration alone, and since the total amount of
vaporization of water is really the important parameter, evaporation and transpiration
are usually combined in a term called evapotranspiration.
Evapotranspiration may be estimated by the inflow-outflow method, which is
essentially the same as the method for measuring evaporation, where the two are
combined and called evapotranspiration.
Considerable attention has been paid to the total of evapotranspiration by soil
scientists. Meyer [5] has designed a series of charts that show the estimated amount
of evaporation as a function of the rainfall during a month and the potential
transpiration over the same period of time. These charts are available for different
latitudes, but are basically designed for use in the middle latitudes. From the charts
for evaporation and transpiration, the total evapotranspiration for a given area may
be estimated.
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
249
evaporation in the pan to the actual evaporation in the body of water.
In addition to evaporation from a water surface, there is evaporation of moisture
from soil. This is usually a function of the evaporation of recent precipitation, but
where the ground water level is close to the ground surface, capillary action may
maintain moisture at the surface of the ground, where the moisture may evaporate.
As a general rule, ground cover reduces the amount of evaporation from the soil
surface. Compared to the evaporation from bare ground, areas covered by vegetation
may result in only 0.7–0.8 as much evaporation, and a dense forest may result in only
0.2–0.4 as much. In general, the same factors that affect evaporation from an open
water surface also have similar effects upon evaporation of the soil moisture.
5 Transpiration
Transpiration is the vaporization of water from the breathing pores of leaves and
other plant surfaces. This amount can be significant in forested areas and can be as
much as 2 million pounds of water per acre per year (300,000 kg/ha-yr). The amount
of transpiration increases significantly with an increase in temperature, and sunlight
is necessary for the process to occur. It is relatively difficult to measure transpiration
alone, but it can be determined experimentally in special plots, greenhouses, and so
on. However, it is difficult to compare these experimental studies with actual field
conditions. In general under field conditions, it is almost impossible to separate
transpiration from evaporation from the soil surface.
6 Evapotranspiration
Since it is difficult to determine transpiration alone, and since the total amount of
vaporization of water is really the important parameter, evaporation and transpiration
are usually combined in a term called evapotranspiration.
Evapotranspiration may be estimated by the inflow-outflow method, which is
essentially the same as the method for measuring evaporation, where the two are
combined and called evapotranspiration.
Considerable attention has been paid to the total of evapotranspiration by soil
scientists. Meyer [5] has designed a series of charts that show the estimated amount
of evaporation as a function of the rainfall during a month and the potential
transpiration over the same period of time. These charts are available for different
latitudes, but are basically designed for use in the middle latitudes. From the charts
for evaporation and transpiration, the total evapotranspiration for a given area may
be estimated.
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
249