Page 116 - Biosystems Engineering

Soil and W ater Conservation 97

2. Altitude: Air temperature decreases with elevation. Due to this
cooling effect, precipitation increases with elevation. This increase
in precipitation with altitude is called the orographic effect.
3. Distance from moisture sources: Inland areas, therefore, receive
less precipitation than coastal areas.
4. Position within the continental mass.
5. Wind direction.
6. Relation to mountain ranges: Windward sites are typically more
cloudy and rainy and leeward sites are drier and sunnier.
7. Relative temperatures of land and bordering oceans.
The most common form of precipitation is rainfall. Rainfall has a
direct impact on soil erosion. As we will see later, raindrops on soil
surfaces are responsible for the initial detachment of soil particles,
called rainsplash erosion. The intensity of the rain and size of the
raindrop is, therefore, very important for the erosion process. Rain-
drops could be as large as 7 mm in diameter, but not necessarily
spherical in shape. Raindrops larger than 5 mm are not very stable
and usually split in the air. The velocity of raindrops depends on their
size, with larger particles having higher velocity and, therefore, exert-
ing larger forces and energy to the soil. The terminal velocity of rain-
drops typically varies from 5 to 9 m/s for 1- to 5-mm raindrop size,
respectively.

3.3.1 Rainfall Measurement
Rainfall is traditionally measured with rain gauges. To obtain spatial
variation of rainfall, measurements from many rain gauges need to
be combined because measurements from these rain gauges only
reflect point measurements. In areas with high spatial variation, a
very dense network of rain gauges needs to be used. With the recent
advancements in radar technology spatially varied rainfall distribu-
tion of an area can easily be obtained.
There are two types of rain gauges: recording and nonrecording.
Nonrecording rain gauges are simple wedge- or funnel-shaped contain-
ers that collect rainfall over a specific period of time. Only informa-
tion on rainfall depth during that time span can be obtained from
such rain gauges. Nonrecording rain gauges do not record and pro-
vide rainfall intensity and the timing of the rain event. They are rela-
tively cheap and easy to maintain. The recording rain gauges supply
more information on the rainfall event. There are two common types:
weighing and tipping bucket. The weighing type usually collects rain-
fall through a funnel and record the weight as a function of time. A
tipping bucket consists of a container with a funnel at the top leading
to a pair of small buckets. When one bucket fills, it tips, emptying the