M.Sc. Module DX 4017Crop Production in Developing Countries

Tropical climates and their impact on crop production

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; >78% dry for > 90 days);

3. one rainy season and one dry season;

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).

ANIMATION OF AFRICAN CLIMATE

Climate classification

A. Systems based on ratio of rainfall to estimated potential (not actual) evapotranspiration or a proxy such as temperature

1. Köppen

See FAO web site link on Blackboard

Tried to find out how climate corresponded to major vegetation zones.

Symbol

Köppen's name

Equivalent de Candolle

zone

Climatic limits zone

A Topical rainy

Megathermal Coldest month temperature above 18°C

C Warm temperate

Mesothermal Coldest month between -3°C and 18°C

D Boreal Microthermal Coldest month below -3°C (27°F) warm est above 10°C(50°F.)

E Snow Hekistothermal

Warmest month below 10°C. (50°F)

B Dry Xerophilous Annual rain-fall less than R.

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.

Thornthwaite type systems

Built on Köppen’s system. Defined 3 stages :

1. a period of full storage, when precipitation exceeds moisture requirement and a moisture surplus (s) accumulates

2. 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

3. a moistening season when precipitation again exceeds water-need and soil moisture is recharged.

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

Im =0 = neutral climateIm >0 = humid climate

<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

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

BaileyDeveloped classification system based on the Köppen System for regions where evaporation exceeds precipitation.

Monthly moisture index SI is given by

S i

pT 0 1 8

1 0 4 5

.

.

where p is the mean monthly rainfall and T is the mean monthly temperature

Annual moisture index is the sum of these

Systems based on length of growing period

• mean number of days when Tmean>= 5°C and moisture supply from precipitation exceeds half the PE

• accumulated moisture sufficient for germination – taken as 50 mm

• more precise def: high prob. of >5 consecutive days with wet soil (> 1bar or some other trigger) in any 10 day period - Fourier analysis

• year divided into periods where:• humid (R>E0)

• moist (0.5E0<R<E0)

• dry (R< 0.5E0)

Simple classification based on LGP:< 90 = arid90 to 180 = semi-arid180 to 270 = subhumid> 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

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.

Impact of climatic characteristics on crop production

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 -> water conservation leaching of soil nutrients if infiltration is high (e.g.

sandy soils)

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)

• 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

Other impacts of dry season

• bush fires common if >3 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

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

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

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 > 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;

• mean & sd of limited value - need 1/4 & 1/10 conf limits - Γdistn.

• 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

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

Effect of rainfall on land-use

0

5

10

15

20

25

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100

Rainfall (mm)

Nomadic grazing

Semi-nomadism;

shortfallows

Seminomadic grazing / arable; medium to long

fallows

Arable; medium to long fallows. Forest plantations

Long fallows; permanenttree crops; natural & planted forests

Land use in relation to rainfall in West & West Equatorial Africa

Temperatures

Affect of altitude

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

Mean T (degrees C)

Ran

ge

of

T (

deg

rees

C)

Range of temperatures (maximum to minimum)

• 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

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 < 12°C - cold sensitive; C4 crops include maize, sorghum, & pearl millet

• C3 (Calvin) plants - no germination < 4°C = cold resistant; C3 crops include wheat, rice

• C3 crops have a temperature optimum (CO2 exchange) for vegetative growth between 25 & 35°C

• C4 crops have optimum between 30 & 45°C ... 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)

Evapotranspiration

ET – function of net radiation, wind speed, humidity, temperature,

lower T, greater cloud cover, lower water vapour def. -> 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)

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 -> Ea < Ep or wilt & die;

• adding fertilisers may produce so much vegetation that the plant cannot keep up with the extra demand

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.

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

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;

(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

Drought effects

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

Day lengthMany plants sensitive to length of day - photoperiodism

- particularly @ 2 to 10 weeks

Three classes: long day plants - temperate regions - dark < 10 hrs short day plants - tropical regions - dark > 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.

Hail & FrostMay occur in some parts of tropics > 6000 ft.Much damage in highland areas of Kenya for example

AltitudeHigher altitude – colder temperatures – lower pressuresBut the principal factor is temperature not air pressure.Beware of people talking about plants ability to grow at certain altitudes

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

2000 >1500 800 <400

1500 15-27

16-29

1000

18-32

500 19-33

20-35

0

3 6 9 12

Rainfall - mm

T r

an

ge °

C

Number of months when E > R

Alt

itu

de -

m

Maize

2000 >1500 800 <400

1500 15-27

16-29

1000

18-32

500 19-33

20-35

0

3 6 9 12

Rainfall - mm

T r

an

ge °

C

Number of months when E > R

Alt

itu

de -

m

Finger Millet

2000 >1500 800 <400

1500 15-27

16-29

1000

18-32

500 19-33

20-35

0

3 6 9 12

Rainfall - mm

T r

an

ge °

C

Number of months when E > R

Alt

itu

de -

m

Sorghum

2000 >1500 800 <400

1500 15-27

16-29

1000

18-32

500 19-33

20-35

0

3 6 9 12

Rainfall - mm

T r

an

ge °

C

Number of months when E > R

Alt

itu

de -

m

Pearl Millet

2000 >1500 800 <400

1500 15-27

16-29

1000

18-32

500 19-33

20-35

0

3 6 9 12

Rainfall - mm

T r

an

ge °

C

Number of months when E > R

Alt

itu

de -

m

Pigeonpea

2000 >1500 800 <400

1500 15-27

16-29

1000

18-32

500 19-33

20-35

0

3 6 9 12

Rainfall - mm

T r

an

ge °

C

Number of months when E > R

Alt

itu

de -

m

Cowpea

2000 >1500 800 <400

1500 15-27

16-29

1000

18-32

500 19-33

20-35

0

3 6 9 12

Rainfall - mm

T r

an

ge °

C

Number of months when E > R

Alt

itu

de -

m

Groundnut

2000 >1500 800 <400

1500 15-27

16-29

1000

18-32

500 19-33

20-35

0

3 6 9 12

Rainfall - mm

T r

an

ge °

C

Number of months when E > R

Alt

itu

de -

m

Soybean

2000 >1500 800 <400

1500 15-27

16-29

1000

18-32

500 19-33

20-35

0

3 6 9 12

Rainfall - mm

T r

an

ge °

C

Number of months when E > R

Alt

itu

de -

m

Mung bean