2. Reasons of seasonal variations in energy metabolism
Climatic-environmental conditions have an enormous influence on
the vegetative cycle of the food and cash crops on which the whole economy of
rural households in Less Developed Countries (LDC) depends. The same factors
that regulate the agricultural cycle can therefore be expected to impinge on the
energy metabolism of the individuals. Essential factors are: temperature, water
availability, nature and structure of the soil, solar radiation, and variation
in the photoperiod.
In equatorial or tropical areas, where temperature, solar
radiation and photoperiod do not undergo substantial seasonal fluctuations, the
only factor that limits vegetal development is water availability. For example,
even sorghum, a drought-resistant vegetal species, needs about 250,000 L of
water per ton of total dry matter produced. Since artificial irrigation is
rarely possible in LDC, rain and other factors which limit or increase the loss
of water through percolation, evaporation and/or transpiration (i.e., the nature
of the ground) are critical. Therefore, the best indicators of agricultural
production potential in equatorial and tropical areas are absolute and relative
seasonality, derived from pluviometric data.
Absolute seasonality is the proportion of dry months among
all months of a year. Dryness is not an absolute value depending only on
pluviometric data; it is corrected for the evapotranspiration in relation to the
culture, temperature, exposure to winds, relative humidity, nature and
granulometrical structure of the soil, direct and indirect radiation, etc. The
range of absolute seasonality extends from 0 (sufficient rains all year long) to
1 (total lack of a period favouring the optimal growth of plants).
Relative seasonality is the ratio between the amount of
rain in different months and annual pluviosity. This index ranges from 0 (no
seasonality) to 1.8 (extreme seasonality).
By combining data on annual rainfall with values of absolute and
relative seasonality, we have constructed a rough, global index; it describes
the likelihood and intensity of seasonal fluctuations on simultaneous demand and
availability of food energy (Figure 1). Desert areas, characterized by
permanent shortage, represent a peculiar and extreme situation and have not been
included in this analysis. The remaining areas are subdivided into three
In the very low seasonality category are areas with annual
rainfall of more than 1000 mm, reasonably distributed throughout the year, and
with an index of absolute seasonality of less than 0.40. No seasonal
interference with the energy balance of the population is expected.
In the moderate seasonality category are areas with an annual
rainfall between 500 and 1000 mm, an absolute seasonality between 0.40 and 0.75,
and a relative seasonality between 0.8 and 1. The high absolute pluviosity or a
bimodal distribution of rains ensures that the vegetative cycle is prolonged. In
these areas, the likelihood of seasonal fluctuations of energy metabolism of
individuals is very limited. Since agricultural work is spread throughout the
year and food crops are harvested on a semicontinuous basis, energy is available
at all times, without occurrence of a "hungry season", except in particular
situations such as rain failure in unusually dry years.
In the severe seasonality category are areas with an annual
pluviosity of less than 500 mm, an absolute seasonality index higher than 0.75,
and values of relative seasonality higher than 1. In these areas, the vegetative
period is very brief, and farmers are forced to concentrate all their physical
efforts on a time usually coinciding with depleted food stores. Furthermore, the
shortage of water imposes wide spacing between furrows, and the brevity of the
vegetative period creates the need to utilize seed cultivars which have a short
cycle and are less productive than those with a longer cycle. All this leads to
a reduced potential for food production in these areas. BAYLISS-SMITH (1981)
calculated that, in one year, a hectare of soil in an area with an absolute
seasonality index below 0.5 and a relative seasonality index below 0.6 will
theoretically produce 40 to 44 tons of total dry matter. An area where the two
indexes are above 0.8 can, in the same period and for the same unit of surface,
produce at most a theoretical quantity of 10 tons of total dry matter.
Figure 1. Grading of climatic
seasonality of some areas of the world (see text for explanation).
The conclusions are obvious:
1. In LDC areas with an essentially agricultural
economy, climatic seasonality is undoubtedly the overriding cause of seasonal
energy imbalance of the population, either through its impact on energy
expenditure or on food energy availability, or on both.
2. The statement that seasonality is "... the rule among adults in
rural areas of developing countries" (TEOKUL, PAYNE and DUGDALE, 1986)
represents an unwarranted generalization.
3. In areas with non-seasonal or bimodal climatic conditions,
major fluctuations of human energy balance are not to be expected.
4. As climatic seasonality increases, physical labour demand - and
thereby energy expenditure - tends to concentrate in a brief annual bout. Food
availability is limited in a similar seasonal manner. The two fluctuations are
often asynchronous, compound each other's effect and cause marked seasonal
imbalances of energy