Agriculture Reference
In-Depth Information
Box 6.5
Measurements of Rainfall and Rainfall Intensity
Consider a volume of rain
V
(m
3
) falling on a soil area
A
(m
2
) in time
t
(hr)
(fig. B6.5.1). If none of this rain infiltrates the soil but collects on the surface, we
can calculate the depth
d
of this “ponded” rain as
Volume
V
(m
3
)
1000
d
(mm)
(B6.5.1)
Area
A
(m
2
)
Thus, the amount of rain is usually measured as a
depth
of water in mm.
Nonmetric units, such as inches (in.), are used in the United States (see appendix 15).
Rainfall is measured in a rain gauge (fig. B6.5.2). Because of wind turbulence,
the amount of rain collected decreases with the height of the rain gauge rim
aboveground. For this reason, a rain gauge should be installed at a standard height
(the rim 30 cm above the ground in Australia). Vegetation (preferably grass)
around the gauge should be kept trimmed. No obstacles, like trees or buildings,
should overshadow the gauge, and its position should not be too exposed to wind.
A
manual
gauge is read daily at a standard time (usually 9
A
.
M
.). The rain
collects in a transparent cylinder that is calibrated to give the volume of rain per
unit area. The rainfall intensity is measured with a recording rain gauge. This is a
gauge with two small “tipping buckets” and an electrical device for recording the
time at which each bucket tip occurs, connected to a data logger. A bucket may
tip for every 0.1 or 0.2 mm of rain that falls, which gives a very accurate measure
of the rainfall intensity (mm/hr).
Rainfall
Figure B6.5.1
Water fluxes into and out of a
defined volume of soil.
z
y
Surface area,
A
x
Surface runoff
Infiltr
a
tion
A horizon
Subsurf
a
ce
lateral fl
o
w
B h
o
rizon
Deep drainage
(continued)
