S.S. Jagtap, A.K. Chan Agricultural and Forest Meteorology 103 2000 59–72 65
less in the northern and southern guinea savanna. The 900, 1100, 1300 and 1500 mm rainfall isohyets were
shifted south throughout the country by 167, 138, 108 and 50 km during the same 20 years. The great-
est changes occurred in the early season rainfall. The growing season shortened by nearly 1 month during
the period 1981–1990 compared with the 1961–1970 decade. The temperatures during the growing season
are typically above 23–24
◦
C. Further decline in rain- fall combined with green house warming could ad-
versely affect crop yields. Furthermore, the 1981–1990 decade recorded fewer wet days and therefore much
higher rainfall intensities Jagtap, 1995a. A high pro- portions of rains in Africa are associated with wave
disturbances giving rise to strong convective activities which results into short intense storms with peak inten-
sities sometimes exceeding 100 mmh. These intense rains frequently result in considerable compaction, soil
erosion and large runoff particularly in the absence of surface cover. Using Fournier 1962 index Jagtap
1995b estimated that one-thirds of the sub-humid zone lies in moderate erosion risk category while re-
maining two-thirds in the high-risk class. Although detailed information is still lacking on both the ero-
sivity of the rains and the erodibility of the soils in many parts of Africa, it is clear that the risks of accel-
erated erosion are very high in these areas. Improved water harvesting techniques and soil erosion preven-
tion methods are rarely used. The zone is widely con- sidered to be the zone at greatest risk of declining
agricultural production at present, and parts of it have been severely affected by erosion, soil degradation and
drought in recent years. At the same time, the parts that receive adequate rain have an enormous potential
for the expansion of rainfed agricultural production.
3. Asia
3.1. Monsoon Areas in Asia, which have humid and sub-humid
climate, generally come under the influence of mon- soon. In India, for example, if monsoon is weak or ar-
rives late, crop productions may be severely affected. A strong monsoon creates havoc through wide spread
floods damaging crops and property. Most floods occur along lowland areas that are near the rivers,
resulting from high rainfall in upstream catchment areas.
3.2. El NinoLa-Nina Beside the monsoon, these areas are also affected by
El-NinoLa-Nina events that could either enhance or reduce precipitation significantly. The occurrence of
dry spells amidst the generally wet weather is not un- common. For example, subsidence farmers in Indone-
sia and Malaysia who practice shifting cultivation, lost most of their crops during the dry weather associated
with the recent 1997–1998 El-Nino episode. Irriga- tion is either not practised at all or is in primitive form.
3.3. Wet and dry weather Water resource is a major constraint in agriculture
production in humid and sub-humid Asia. Excessive rain raises water table, which restrict the development
of crops. For shallow rooted crops, if the land is in- undated following heavy rainfall, severe damage and
even death can occur. In areas with a well-defined dry season, rubber trees which are grown in many areas
of the region will ‘winter’ drop their leaves during that period. The trees subsequently grow new leaves
to form a new canopy, without undue damage by dis- ease. However, in areas without clear dry season, the
trees winter during periods of intermittent rain and high humidity. This sustained wet conditions favor
the development of Odium, Phytophthora and Col- letotrichum diseases Peries, 1979. Sometimes it is
not the amount of rain, which affect agricultural pro- duction but the frequency and the timing of the rainfall
events that govern production. Rubber is a good ex- ample. Because latex of rubber trees is tapped in the
morning, the yield of rubber is affected by the diur- nal pattern of rainfall. Heavy rainfall in the morning
interferes with the tapping process. More generally, the trickling of rainwater down the trunk from the wet
canopy and branches interfere with flow of latex into the collection cup. When rains fall before tapping, the
trees are wet with water trickling down the trunk, thus forcing the tapping process to be postponed or can-
celled. When rains fall during the tapping, there may be time to collect the latex before interference. If col-
lection is not possible, then the heavy rains may wash
66 S.S. Jagtap, A.K. Chan Agricultural and Forest Meteorology 103 2000 59–72
the latex out of the cups, resulting in what is known as ‘wash-out’. When rains fall after tapping, the trick-
ling of water down the trunk may interfere with the ‘late-drip’ Wycherley, 1967.
3.4. Strong wind In the Asian humid and sub-humid regions, wind
speeds are generally in the region of 1–3 ms with stronger winds in coastal areas, reaching about 4 ms.
However, strong winds in the form of downdrafts from thunderstorms occur frequently. Damages from such
strong winds happen in various forms. Annual crops may be lodged and trees may be uprooted on soils with
impeded drainage or where rooting has been limited to the surface layers. Young plants may suffer stem
bending which requires corrective pruning or roping. Wind can also desiccate the plants during dry spells,
thus weakening and predisposing them to diseases. In Indonesia it was found that susceptibility of rubber tree
to wind damage was greatest at the time of maximum girthing and canopy development which was related to
the branching structure Dijkman, 1951. Trees with narrow crotches are particularly prone to breakage.
3.5. Forest fires Forest fires which usually occur during the dry
spells, result in not only loss of forest materials but also in widespread haze problem. During the 1997–1998
ENSO warm episode, thousands of hectares of forest in Indonesia were burnt, causing a thick blanket of
haze to envelop Indonesia, Malaysia, Singapore and Brunei. Haze not only causes health problems but
also reduces incoming solar radiation substantially, thereby lowering crop productions. It has been ob-
served that a significant decrease in oil extraction ratio in oil palm bunches in Malaysia appears to be
related to the occurrence of widespread haze caused by forest fires in one of her neighbouring country.
3.6. Pests and diseases Pests and diseases are common in the hu-
mid and sub-humid Asia. The perceptual warm and moist conditions favor the development of
many bacteriavirusvectors. It is not uncommon for widespread and often excessive use of pesti-
cideinsecticidefungicide to check the spread of these pests and diseases. However, it leads to pollution
of the environment. There is a growing opposition among the people to the use of chemicals to con-
trol pest and diseases. Pests and diseases particularly affect cocoa, which is grown in many parts of the hu-
mid and sub-humid areas of Asia. Estimates of losses in cocoa production due to diseases indicate values
of between 20–30 which are relatively high Pad- wick, 1956. Air humidity, temperature and rainfall
are the major meteorological factors influencing the development of cocoa diseases.
4. Challenges and future prospects
