Humidity and Rainfall - Agroecology: The Ecology of Sustainable Food Systems - page 76

ACID PRECIPITATION

Rainfall is the lifeblood of both natural ecosystems and most agroecosystems. Yet in many areas of the world, it
is poisoning the very systems it supports. The rain (and snow) that falls in these areas is acidic enough to damage
crops and forests, kill fish and other aquatic organisms, and acidify the soil.
Acid precipitation is just one of the many consequences of humans' pollution of the atmosphere. The burning
of fossil fuels in automobiles and in power plants dumps large amounts of nitrogen oxides and sulfur oxides into
the atmosphere near urban and industrial areas. These compounds are called acid precursors; they easily combine
with atmospheric water to form nitric acid and sulfuric acid. These acids are then dissolved into the naturally
occurring water droplets of the atmosphere. After drifting for some distance, the droplets eventually fall as acid
precipitation. (Oxides of nitrogen and sulfur can also form nitrates and sulfates in the atmosphere and “rain” down
as solid particles; when they combine with water, these particles are converted to acids and have the same effect
as acid rain.)
Rainfall in an unpolluted environment is naturally slightly acidic — whereas pure water has a neutral pH of
7.0, the pH of natural rainfall is about 5.7. This normal acidity is the result of atmospheric carbon dioxide dissolving
in water droplets in clouds and forming weak carbonic acid. Thus acid precipitation is a problem only where human-
produced acid precursors lower the pH of the precipitation to less than 5.7. Where this will occur depends on rainfall
and wind patterns and on the location of significant anthropogenic sources of acid precursors. Prevailing wind
patterns, for example, tend to carry acid precursors from urban areas and power plants in the northeastern U.S. to
the Adirondack Mountains of New York state. There, the rainfall has an average pH of about 4.1, and has been
measured as low as 2.3. Other areas where acid precipitation is a problem include northern Europe, much of the
eastern U.S., southeastern Canada, and parts of southern California. The distribution of acid rainfall, however, is
extremely variable, and almost any area can receive it.
Acid precipitation has been shown to have many deleterious effects. Aquatic ecosystems are particularly
vulnerable; years of acid precipitation has acidified many of the lakes in mountainous areas of the eastern U.S. and
Canada and left them virtually lifeless. Acid rain also damages forests: it harms needles and leaves, impairs seed
germination, and erodes protective waxes from leaves.
In the U.S., some progress has been made in reducing sulfur and nitrogen oxide emissions from power plants
since 1980, leading to small decreases in acid deposition and a reduction in the acidity of many lakes and streams.
Acid precipitation continues to occur, however, and further reductions in acid-precursor emissions will be necessary
for already-damaged ecosystems to recover (NAPAP, 2005). Acid precipitation continues to be a serious problem
in many parts of the developing world, particularly China and other parts of Asia.
The extent to which acid precipitation damages agroecosystems is difficult to assess. Some studies have shown
decreased crop productivity and inhibition of the dark reactions of photosynthesis. Other studies have documented
damage to leaves and buds and the leaching of calcium from leaves. Where the soil lacks the ability to neutralize
acids, acid precipitation has caused acidification of the soil and changes in nutrient availability. Although the
distribution of acid precipitation varies, and different crops and soils have different levels of sensitivity, acid
precipitation is a global problem with the potential for significant direct and indirect effects on agriculture.

The rainfall data in Table 6.1 show that the
Cuyama Valley has fairly high variability, for
example. Based on these data, a farmer could
not count on there being at least 1 in. of rain in
April, even though the 6-years average for that
month is 1.04 in.

this time, the rains damaged the crop (in most years,
significant rainfall does not occur until early November,
after the grapes have been harvested).

RAINFED AGROECOSYSTEMS

Agriculture in most of the world is carried out using natural
precipitation to meet the water needs of crops. These
rainfed agroecosystems must adjust to the distribution,
intensity, and variability of the rainfall that is characteristic
of the local climate. The challenge is either to maintain a
balance between precipitation (P) and potential evapo-
transpiration (PET) by manipulating evapotranspiration,
or to somehow work around a water deficit (P-PET < 0)
or a water surplus (P-PET > 0).

Additional aspects of rainfall may be relevant from an
agroecological perspective as well. For example, it may
be important to know how much moisture was in the soil
when rainfall occurred, as well as the stage of crop deve-
lopment. In the Paso Robles and Santa Maria regions of
California, for example, two storms with total rainfall of
about 1.5 in. occurred during the first 2 weeks of Septem-
ber in 1998. Since most grapes were still on the vine at