Environmental Engineering Reference
In-Depth Information
the soil. This is an inherent weakness in the conventional
manner in which weather station rainfall data are collected.
The primary driver for net ground surface moisture flux
is precipitation and snowfall. This information is obtained
from weather stations. Figure 6.17 shows a typical plot of
daily rainfall records and the cumulative precipitation over
the period of one year.
Runoff
Runoff
Maximum
infiltration
rate
Maximum
infiltration
rate
6.3.6 Calculation of Relative Humidity
The primary driving mechanism of evaporation of water
from the ground surface is the water vapor pressure gra-
dient. Air consists of a mixture of gases and water vapor.
The partial pressure of water vapor in air forms the basis for
water vapor movement. The water vapor pressure gradient
between two points causes moisture to move through the air
phase of an unsaturated soil. Water vapor pressure gradients
at the ground surface result in the evaporation of moisture
from the surface of the soil. In other words, evaporation is
fundamentally driven by a water vapor pressure gradient.
Most of the mathematical formulations describing water
vapor movement in unsaturated soil mechanics are written
in terms of changes in relative humidity RH or h r . A number
of other variables are also used as a measure of the amount
of water vapor in air.
Relative humidity h r is defined as the ratio of the par-
tial pressure of water vapor in an air mixture, u ai v ,tothe
saturated vapor pressure of water vapor, u air
Time (days)
Figure 6.15 Partitioning of rainfall based on ability of soil to
accept water.
another scenario that shows two distinct rainy seasons. Even
though the total precipitation during a year might be the
same for the two cases, the response of a cover system (or
the near-ground-surface soils) to each situation would be
considerably different.
An unsaturated soil can only accept water at a particular
rate and the rate is mainly dependent upon the hydraulic
conductivity and water storage capabilities near the ground
surface. It is possible for the intensity of rainfall during
a storm to exceed the ability of the soil to accept water.
Figure 6.15 shows a hypothetical plot of the intensity of
rainfall versus time along with the capacity of the soil to
accept water. Moisture begins to run off or pond on the
ground surface when the intensity of precipitation exceeds
the infiltration capacity.
The conventional collection of precipitation data may not
allow for the character of a particular storm to be quantified.
Rainfall is usually recorded on a daily basis. The rainfall
may be spread over an entire day with no consideration as
to whether the storm was 10min long or 10 h long, as shown
in Fig. 6.16. If the rainfall for each day is spread over the
entire day, most of the precipitation will appear to infiltrate
v 0 ,atthesame
temperature. Relative humidity h r can be written either in
decimal form or as a percentage:
u air
v
u air
v 0
h r =
(6.10)
Relative humidity is a fundamental variable associated
with water vapor flow, namely, the gradient of water vapor
partial pressure. Relative humidity is one of the variables
generally collected at weather stations.
5.0
4.5
4.0
Actual storm flux
3.5
Actual storm flux
3.0
2.5
k sat
2.0
Storm average
1.5
1.0
One week average
0.5
0
0
1
2
3
4
5
6
7
Time, days
Figure 6.16 Distribution of typical rainfall intensity diagrams.
 
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