Agriculture Reference
In-Depth Information
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which is sponsored by the U.S. Agency for International Development (US-
AID), monitors drought conditions in Africa. Because agricultural droughts
occur due to low crop yields, monitoring them requires monitoring the
factors that affect crop yields.
Fa ctors Affecting Crop Yield
According to Diepen and van der Wall (1996), factors influencing yield can
be categorized as (1) abiotic factors, such as soil water, soil fertility, soil
texture, soil taxonomy class, and weather; (2) farm management factors,
such as soil tillage, soil depth, planting density, sowing date, weeding inten-
sity, manuring rate, crop protection against pests and diseases, harvesting
techniques, postharvest loss, and degree of mechanization; (3) land devel-
opment factors, such as field size, terracing, drainage, and irrigation; (4) so-
cioeconomic factors, such as the distance to markets, population pressure,
investments, costs of inputs, prices of output, education levels, skills, and
infrastructure; and (5) catastrophic factors that include warfare, flooding,
earthquakes, hailstorms, and frost. Measuring or estimating some of these
factors is often not feasible, and the influence of some other factors may be
considered insignificant or constant in an economically stable region. It is
therefore weather conditions alone that affect crop yield most significantly.
Various weather parameters such as temperature, precipitation, humidity,
solar radiation, cloudiness, and wind velocity affect crop yield, but tem-
perature and precipitation are most significant.
A change in temperature causes a shift in planting dates that, in turn,
shifts the commencement and termination of the phenological phases. The
entire crop-growth period (period from planting to harvest) may shift,
shrink, or expand due to a change in temperature pattern. McKay (1983)
observed for wheat production in Canada that a drop in the mean annual
temperature of 1°C accompanied by a 9- to 15-day reduction in the grow-
ing season could be critical. However, interannual variation in temperature
is much less than that in precipitation. It is for this reason that precipitation
becomes more important than temperature for monitoring crop yields. An
intricate relationship exists between precipitation and crop yield because
it is the soil moisture, and not precipitation, that ultimately contributes to
crop growth and crop yield.
Soil moisture data are more important than precipitation data for mon-
itoring agricultural droughts, but soil moisture data are not as readily
available as precipitation data are. Unlike precipitation data that are rou-
tinely available via a network of weather stations, soil moisture data are
collected only on an experimental basis or estimated using agrometeorolog-
ical models. Monitoring agricultural droughts requires soil moisture data
for large areas on spatial and temporal scales. With advances in microwave
remote sensing, it is becoming possible to estimate soil moisture for large
areas, as described in detail in chapter 7.
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