|Chronic Energy Deficiency : Consequences and Related Issues (International Dietary Energy Consultative Group - IDECG, 1987, 201 pages)|
|Seasonality 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 categories.
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.
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 metabolism.