Agriculture Reference
In-Depth Information
DESCRIBING RAINFALL PATTERNS
a way as to create predictable wet and dry peri-
ods; the Cuyama Valley, where precipitation is
largely confined to the period from October to
May, is a good example. Within this overall
climatic distribution pattern, however, rainfall
is often distributed differently each year: if the
data for the Cuyama Valley were graphed, for
example, the peaks and valleys for each year
would not correspond, and some years, such as
2004 to 2005, would show much more evenly
distributed rainfall than others.
Each region of the earth has its characteristic patterns of
precipitation. The total amount of precipitation received
in a typical year, its distribution throughout the year, the
intensity and duration of precipitation events, and the
regularity and predictability of the precipitation patterns
are all important determinants of the opportunities for,
and constraints upon, agriculture in a particular region.
Below, these facets of rainfall patterns are described
using rainfall data collected by the author from the Cuyama
Valley, California. These data are shown in Table 6.1.
•
Intensity and duration.
The absolute amount
of rainfall in a long time period such as a
month or even a day does not fully describe
the ecological relevance of the rainfall. How
intense the rainfall is, and for what length of
time that rainfall occurs, are important aspects.
Two inches of rainfall in less than an hour can
have very different ecological impacts than a
2-in. rain spread over 24 h. For example, of
the 12.66 in. of rainfall recorded during Feb-
ruary 1998 in the Cuyama Valley, over 8 in.
fell in one 3-h rainfall event, with associated
excessive runoff and flooding.
•
Average total annual rainfall.
The total
amount of precipitation that falls in an area
during an average year is a good indicator of
the moistness of that area's climate. From an
ecological perspective, however, it is also
important to know how much variability there
can be in this rainfall amount from one year to
the next. Extremes at either end of the average
can have significant negative impact on an agri-
cultural system, even if that extreme only
occurs rarely. Table 6.1 shows that in the
Cuyama Valley the annual total is highly vari-
able: during the data collection period there
were three drought years, 3 years of near-normal
precipitation, one wet year, and one excessively
wet year (associated with El Niño patterns in
the Pacific Ocean).
•
Availability.
It is also important to know how
much of the rainfall becomes available as soil
moisture. Does it penetrate into the root zone?
What were the weather conditions immediately
following the rainfall event? What was the tem-
perature and what were the wind conditions?
•
Distribution and periodicity.
This refers to
how rainfall is spread out through the year, both
on average and during a specific year. In many
parts of the world, rainfall is distributed in such
•
Predictability.
Every region has a characteris-
tic degree of variability in its rainfall patterns.
The higher the variability, the less predictable
is the rainfall for any particular time period.
TA B L E 6 . 1
Monthly and Seasonal Rainfall Totals in Inches at Cottonwood Canyon, Cuyama Valley, Santa Barbara
County, California
Season
Sept
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Total
1996-1997
0.0
2.3
2.12
4.31
5.6
0.37
0.0
0.0
0.0
14.7
1997-1998
0.2
0.1
3.65
4.93
6.75
12.66
3.76
1.78
1.82
35.65
1998-1999
1.43
0.18
0.87
0.93
0.23
3.4
2.29
0.85
0.0
10.18
1999-2000
0.0
0.0
0.9
0.04
1.91
2.99
4.85
2.6
0.18
13.46
2000-2001
0.0
1.06
0.02
0.17
5.32
5.05
5.6
2.35
0.0
19.52
2001-2002
0.5
a
0.58
2.4
2.54
0.08
0.8
0.87
0.03
0.2
8.2
2002-2003
0.0
0.0
3.73
2.06
2.28
1.64
2.3
0.95
1.2
14.16
2003-2004
0.8
b
0.45
0.44
1.88
0.42
1.98
2.90
0.1
0.0
9.05
2004-2005
0.0
4.25
0.06
4.32
7.06
2.25
2.30
0.66
0.75
21.65
Averages
0.33
0.99
1.58
2.35
3.29
3.46
2.76
1.04
0.46
16.29
Rainfall from June to August is usually negligible.
a
Includes 0.5 in. from July.
b
Includes 0.88 from late July/early August.
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