M.Sc. Module DX 4017

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M.Sc. Module DX 4017 Crop Production in
Developing Countries
Tropical climates and their impact on crop
production
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General patterns Progressing away from the
equator, a very simplified picture is as
follows-- 1. at the equator, a double rainfall
maximum but with no pronounced dry season 2. a
double rainfall maximum (bimodal) separated by a
more pronounced dry season during the low sun
period and a less pronounced dry season in the
high sun period (to /- 10o N/S gt78 dry for gt
90 days) 3. one rainy season and one dry season
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4. semi-arid region on the equatorial margin of
the sub-tropical high pressure cells short rainy
season in the high sun period and a long dry
period some places may have a very small rain in
the low sun period 5. an arid area associated
with the STHs (sub-tropical high pressure zones)
Sahara, Kalahari etc 6. a semi-arid region on
the poleward margin of the STHs with a short wet
season in the low sun (winter) period under the
influence of extra-tropical climatic disturbances
(Mediterranean climate).
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ANIMATION OF AFRICAN CLIMATE
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Climate classification A. Systems based on ratio
of rainfall to estimated potential (not actual)
evapotranspiration or a proxy such as temperature
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1. Köppen See FAO web site link on
Blackboard Tried to find out how climate
corresponded to major vegetation zones.
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Symbol Köppen's name Equivalent de Candolle zone Climatic limits zone
A Topical rainy Megathermal Coldest month temperature above 18C
C Warm temperate Mesothermal Coldest month between -3C and 18C
D Boreal Microthermal Coldest month below -3C (27F) warm est above 10C(50F.)
E Snow Hekistothermal Warmest month below 10C. (50F)
B Dry Xerophilous Annual rain-fall less than R.
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Defined dry according to some critical
rainfall. Köppen used many different formulae to
determine this critical rainfall (R). In 1928
he adopted (as a final form) the value R
0.44 (T-k) where T mean annual
temperature, and k a constant whose
value is determined by the seasonal concentration
of rainfall.
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• Thornthwaite type systems
• Built on Köppens system. Defined 3 stages
• a period of full storage, when precipitation
  exceeds moisture requirement and a moisture
  surplus (s) accumulates
• a drying season when stored soil moisture and
  precipitation are used in evapotranspiration,
  storage is steadily diminished, the actual
  evapotranspiration falls below the potential and
  a moisture deficiency, d, occurs
• a moistening season when precipitation again
  exceeds water-need and soil moisture is recharged.

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Defined Moisture Index Im
(100s-60d)/PE where s accumulated moisture
surplus the total surplus from all months
having a water surplus, d moisture deficit -
the total of all deficits in those months when
there is a deficit monthly surplus or deficit is
defined by the difference between rainfall and
potential evapotranspiration - all in the same
units
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Im 0 neutral climate Im gt 0 humid
climate lt 0 dry climate Problem 1 the '100'
and '60' are empirical, from US data. Problem 2
ET calculated from temperature Problem 3
assumed 100 mm maximum storage but in reality
depends on soil type Problem 4 ignores
drainage and runoff
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Moisture Province Moisture Index (Im) A
Perhumid 100 and above
B4 Humid 80 to
99.9 B3 Humid
60 to 79 9 B2 Humid
40 to 59.9 B1 Humid
20 to 39.9 C2 Moist
subhumid 0 to 19.9 C1 Dry
subhumid - 19.9 to 0 D
Semiarid - 39.9 to - 20
E Arid - 60 to
- 40
Despite the problems it was a great improvement
and is the basis of many more modern systems
see the crop suitability database using this
system on BB
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Bailey Developed classification system based on
the Köppen System for regions where evaporation
exceeds precipitation. Monthly moisture index SI
is given by
where p is the mean monthly rainfall and T is the
mean monthly temperature Annual moisture index
is the sum of these
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• Systems based on length of growing period
• mean number of days when Tmeangt 5C and moisture
  supply from precipitation exceeds half the PE
• accumulated moisture sufficient for germination
  taken as 50 mm
• more precise def high prob. of gt5 consecutive
  days with wet soil (gt 1bar or some other trigger)
  in any 10 day period - Fourier analysis
• year divided into periods where
• humid (RgtE0)
• moist (0.5E0ltRltE0)
• dry (Rlt 0.5E0)

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Simple classification based on LGP lt 90
arid 90 to 180 semi-arid 180 to 270
subhumid gt 270 humid method more appropriate
than rainfall totals for variety selection as
varieties have differences in length of growth
stages selection of plants for differences in
length of growth stages
ANIMATION OF LENGTH OF GROWING PERIOD
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Agro-ecologoical zones In various ways related
to vegetation and climate now often using LGP
with allowance for soil differences and crop
moisture demands. See the document in Blackboard
for various classification systems. The
Agro-ecologoical zones based on the FAO system
were used by GTZ for detailed maps of Kenya - I
have them and the manuals if anyone is
interested. Also make sure you visit the FAO
Crop Suitability site provided in Blackboard. It
may help with your assignment. You can right
click on the maps to download them to your PC.
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Impact of climatic characteristics on crop
production
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• High intensities
• higher proportion of rain as high intensity
  storms in tropics than in temperate zones
• larger drop sizes - 40 of rain erosive (cf. 5
  in temperate regions)
• in E. Africa ranges from 130 mm/hr over 15 mins
  to 3.7 mm/hr over 24 hrs
• rapid runoff rates especially if soil
  infiltration capacity is low (either due to
  clayey texture or cap on surface) ?
• flash floods
• attention to soil conservation techniques often
  needed
• loss of potential soil moisture -gt water
  conservation
• leaching of soil nutrients if infiltration is
  high (e.g. sandy soils)

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• Dry ground at beginning of rains
• soils may be very dry at beginning of rains
• form a cap very easily
• adversely affects seedling emergence
• showers will not improve soil moisture
• intense storms may cause runoff soil erosion
  even on dry soils
• slaking of unstable soils may lead to soil
  aeration problems
• hard ground at beginning of rains may delay
  planting,
• usually wait for rain to cultivate,
• could use ploughs (usually ox drawn) but money
  fodder required ( so more land)

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• animals lose condition in dry season
• tractors can extend season as well as increase
  productivity but
• shortage of for capital maintenance
• foreign exchange availability
• environmental and sustainability issues

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Other impacts of dry season

• bush fires common if gt3 months dry (interferes
  with forest establishment)
• wind erosion common in dry season
• v. high seasonal labour demand low labour
  demand in dry season ? low returns on labour
  low income

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• Timing of rainfall seasons
• In bimodal areas, plant second crop / ensure 2nd
  peak coincides with extra demand (e.g. fruit
  development, - not ripening or harvest) -
  breeding for duration
• match variety to duration of rains but
• droughts can occur during rainy season drought
  avoiding v. drought tolerant
• make opportunistic use of occasional rain falling
  outside the normal growing period

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• Analysis of rainfall agroclimatology
• analysis of distribution of weekly or decade
  (10-day) rainfall
• used to determine key periods such as best
  planting dates and periods of exceptionally high
  rainfall probability
• CV usually highest at the beginning and end of
  the season - rainfall of first month more
  uncertain than for the total season - early rains
  may be insufficient for crop establishment -
  replant

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• Variation from year to year
• Poor farmers are more interested in yield
  stability than
• yield amount - do not want large swings in
  harvest
• coefficients of variation (or percentage
  variation) of annual
• rainfall usually increases as rainfall
  decreases
• see discussion in Jackson
• in lower rainfall areas, droughts and years
  with exceptionally
• high rainfall may be common
• Cairo annual average 28 mm but only gt 0 in
  13/30 years
• and can have 43 mm in a day
• Tigre, Ethiopia - annual totals vary from 56 to
  183 of mean
• in semi-arid areas, distribution often skewed
  so that drought
• more common than excess - beware of annual
  averages -
• they may be affected by uncommon large
  rainfalls

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• mean sd of limited value - need 1/4 1/10 conf
  limits - Gdistn.
• seasonal distribution may not be constant, e.g.
  Tigre wettest month can be June to August -
  farmers often plant late to avoid early drought
  destroying crop
• failure of rains in year that new technology is
  adopted may lead to a reversion to traditional
  methods
• series of good years often bring about an
  increase in animals - which exacerbates the
  effects of any ensuing drought
• famines usually after several consecutive drought
  years rather than a single year - coping
  mechanisms

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• Spatial variations
• can be due to random variation or the mean may
  vary -
• affected by topography etc.
• can produce considerable differences in yield
  over
• small distances
• experimental results are often suspect if not
  well designed
• to allow for rainfall variation over the area
  the trials were done
• ownership of several farms (and wives!) a
  common way of
• coping with spatial rainfall variations in
  semi-arid areas

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Effect of rainfall on land-use
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Temperatures
Affect of altitude
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Range of temperatures (maximum to minimum)
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• T more equitable throughout the year than
  temperate regions (but usually cooler during the
  rains)
• hot arid regions tend to have cooler winters so
  range begins to increase again for very hot areas
  - see following graph

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• Other effects of temperature
• biological activity increases with temperature ?
  many pests diseases if adequate moisture
• higher temps. allow growing of C4 photosynthetic
  systems - more efficient than C3 systems (e.g.
  wheat, rice, root crops)
• C4 plants no germination lt 12C - cold
  sensitive C4 crops include maize, sorghum,
  pearl millet
• C3 (Calvin) plants - no germination lt 4C cold
  resistant C3 crops include wheat, rice
• C3 crops have a temperature optimum (CO2
  exchange) for vegetative growth between 25
  35C

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• C4 crops have optimum between 30 45C ... but
  high growth rates only common for 2-3 weeks
  towards end of veg. growth (between canopy
  closure 1st flowering)
• increase of temperature above optimum ? leaf
  root detachment
• partitioning of OM and N among plant organs
• (e.g. grain - stem) strongly dependent on
  environmental factors particularly temperature -
  high T (or water deficits) leads to low grain
  size short grain fill period
• flower formation fertilisation depend on T
• crops (e.g. apples) which require period of
  dormancy (cold spell) cannot be grown in tropics
  (without use of hormones)

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Evapotranspiration ET function of net
radiation, wind speed, humidity, temperature,
lower T, greater cloud cover, lower water vapour
def. -gt lower E 8 to 12 mm/day typical (up to
15 mm/d) for semi-arid areas 4 to 8 mm/day more
common in wetter areas dry matter
f(transpiration, water use efficiency)
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• Actual v. Potential ET
• Actual ET depends on the amount (rather the
  energy potential) of soil moisture.
• Rainfall (and / or irrigation) and potential
  evapotranspiration together are the main factors
  determining actual evapotranspiration - also of,
  soil type, drainage crop characteristics.
• Growth of the plant depends on actual
  evapotranspiration (Ea).
• even in moist soils, instantaneous demand may not
  be met by crops - transient leaf water deficits
  common -gt Ea lt Ep or wilt die
• adding fertilisers may produce so much vegetation
  that the plant cannot keep up with the extra
  demand

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Drought Types of drought Physiological drought
Excess of evaporative demand over
ability of plant to supply moisture.
Internal resistance to flow. Root damage or
excess of salts in soil or adding
fertilisers may bring on physiological
drought without climatic drought Climatic
drought Unseasonable shortfall of rain.
Lack of rain in an annual dry season is not
drought.
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Agricultural drought Water supply for
crop use is scarce. Exists when a consistent
Soil Moisture Deficit (SMD) over growing
season. Affected by temperature and wind
and runoff not just rain. Hydrological drought
Low levels of water in reservoir and
ground water
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• Drought strategies
• (a) Drought evading
• small size - e.g. dwarf pigeon peas
• low growth rate - adjust growth rate to
  available moisture
• wide spacing,
• low water requirements
• short growing season to catch the rain and
• avoid dry periods (e.g. Katumani maize
• - not drought resistant but grown in semi-arid
  areas

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• (b) Drought enduring
• low growth temporary wilt
• go dormant until water available

• (c) Drought resisting
• most trop. crops have developed mechanisms to
  minimise
• effects of high evaporative demand
• ( / or low soil moisture)-
• - stomata close
• - pineapple habitually only opens stomata at
  night
• sunflowers - stomata remain open but leaves
  lose turgidity
• ? reduction in radiation interception

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Drought effects
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• Wind
• dust storms, wind erosion
• increases evaporation (another benefit of shelter
  belts is to reduce ET)
• lodging of crops (in Kordofan, sorghum "laid" and
  allowed to ripen)
• breeding, e.g., dwarf rice to resist lodging due
  to heavy rain and wind (and shorter stems needed
  to carry heavier heads)
• transport of insects and diseases

• Humidity
• influences Ep Ea
• diseases
• temperature often drops as humidity increases

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• Day length
• Many plants sensitive to length of day -
  photoperiodism - particularly _at_ 2 to 10 weeks
• Three classes
• long day plants - temperate regions - dark lt 10
  hrs
• short day plants - tropical regions - dark gt 14
  hrs
• day neutral.
• Even within tropics, there are varietal
  differences e.g. farmers in W. Africa have
  informally (i.e. without the help of scientists)
  selected sorghum vars. for day length ? also fit
  seasonal soil moisture pattern.
• In Asia, rice vars. which are essentially
  day-length insensitive have been developed.

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• Hail Frost
• May occur in some parts of tropics gt 6000 ft.
• Much damage in highland areas of Kenya for
  example
• Altitude
• Higher altitude colder temperatures lower
  pressures
• But the principal factor is temperature not air
  pressure.
• Beware of people talking about plants ability to
  grow at certain altitudes

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Niches Because of climatic response of crops
T, E, R and partly because of C3 / C4 mechanisms
even in the tropics, crops have particular
ecological niches. Purseglove among others gives
some ideas of the niches occupied. The following
diagrams give some ideas but are based on fairly
old data please feel free to contact me to
correct the information
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Rainfall - mm
Maize
Altitude - m
T range C
Number of months when E gt R
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Rainfall - mm
Finger Millet
Altitude - m
T range C
Number of months when E gt R
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Rainfall - mm
Altitude - m
T range C
Sorghum
Number of months when E gt R
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Rainfall - mm
Altitude - m
T range C
Pearl Millet
Number of months when E gt R
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Rainfall - mm
Altitude - m
T range C
Pigeonpea
Number of months when E gt R
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Rainfall - mm
Altitude - m
T range C
Cowpea
Number of months when E gt R
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Rainfall - mm
Altitude - m
T range C
Groundnut
Number of months when E gt R
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Rainfall - mm
Altitude - m
T range C
Soybean
Number of months when E gt R
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Rainfall - mm
Altitude - m
T range C
Mung bean
Number of months when E gt R