Impacts of Climate Change on Agriculture - Climate Resilient Agriculture for Ensuring Food Security - page 62

water scarce conditions. For example, the effects
of drought can be avoided by early planting of
cultivars with rapid rates of development.
Fallowing and weed control can also help to con-
serve moisture in the soil.
Heat stress and drought stress often occur
simultaneously, the one contributing to the other.
These conditions are often accompanied by high
solar irradiance and high winds. When crops are
subjected to drought stress, their stomata close.
Such closure reduces transpiration and, conse-
quently, raises plant temperatures.
Excessively wet years, on the other hand, may
cause yield declines due to water logging and
increased pest infestations. High soil moisture in
humid areas can also hinder fi eld operations.
Intense bursts of rainfall may damage younger
plants and promote water logging of standing
crops with ripening grain as well as soil erosion.
The extent of crop damage depends on the dura-
tion of precipitation and fl ooding, crop develop-
mental stage, and air and soil temperatures. The
costs of drying corn are higher under wetter cli-
mate regimes.
In most of the tropical and equatorial regions
of the world, and across large areas outside the
tropics, the yield of agricultural crops is limited
more by the amount of water received by and
stored in the soil than by air temperature. Even in
the high mid-latitudes such as in southern
Scandinavia, too little rain can restrict growth of
cereal crops during the summer when evapo-
transpiration exceeds rainfall. In all these areas,
the amount of dry matter a crop produces is
roughly proportional to the amount of water it
transpires (Monteith 1981 ). This, in turn, is
affected by the quantity of rainfall but not in a
straightforward manner: it also depends on how
much of the rainfall is retained in the soil, how
much is lost through evaporation from the soil
surface, and how much remains in the soil that
the crop cannot extract.
The amount of water transpired by the crop is
also determined by air humidity, with generally
less dry matter produced in a drier atmosphere
(Monteith 1981 ). Thus, changes in both rainfall
and air humidity would be likely to have signifi -
cant effects on crop yields.

Reliability of rainfall, particularly at critical
phases of crop development, can explain much of
the variation in agricultural potential in tropical
regions. Thus, many schemes used to map zones
of agricultural potential around the world have
adopted some form of ratio of rainfall to potential
evaporation, r / E o, to delimit moisture-availability
zones, which are then overlaid on temperature
and soil maps to indicate agroecological zones
(Sombroeck et al. 1982 ). The regions are distin-
guished largely on the basis of the length of
growing season determined by the r / E o ratio. In
Kenya, for example, average plant biomass is
estimated to vary by more than an order of mag-
nitude between agroclimatic zones that lie within
100 km of each other (Akong'a et al. 1988 ).
These are characterizations of the effect of differ-
ences in average rainfall on agricultural potential,
but it is important to note that a high degree of
interannual variability of rainfall, particularly in
the drier zones, can lead to very marked variation
in crop yield between wet and dry years, so that
changes in rainfall over time as well as over space
are also likely to have a similar effect on crop
yields.
A strongly positive relationship between rain-
fall and crop yield is generally found in the major
mid-latitude cereal-exporting regions of the
world, such as the US Great Plains and Soviet
Ukraine. For example, in the dry steppe zone of
the Volga Basin (USSR), a 0.5 or 1 °C warming,
with no change in rainfall, is estimated to have
little effect on spring wheat yields, while a 20 %
decrease in rainfall (at current temperatures)
could reduce yields by more than a tenth.
Relatively few studies have been made of the
combined effects of possible changes in tempera-
ture and rainfall on crop yields and those that
have are based on a variety of different methods.
However, a recent review of results from about
ten studies in North America and Europe noted
that warming is generally detrimental to yields of
wheat and maize in these mid-latitude core crop-
ping regions. With no change in precipitation (or
radiation), slight warming (+1 °C) might decrease
average yields by about 5 + 4 %, and a 2 °C warm-
ing might reduce average yields by about 10 + 7 %
(Edwards and Walker 1983 ). In addition, reduced