Definition and Measures of Seasonality

Seasonality is defined as the cyclical change of food availability and agricultural labor induced by climatic changes in rural areas of least developed countries (LDC). In seasonal climates a bad, lean, or slack season, also known as the saudure (French, meaning junction between two agricultural cycles), is present for 2-3 months a year, often coinciding with rains, leading to cyclical stress on the health and nutrition of rural populations. Agroclimatic seasonality is relevant in populations practising subsistence agriculture and among hunter-gatherers, or in other agricultural systems in which background food security is poor; such as in areas where cash crops are mainly planted. Human interventions can alter this pattern, by changing the water and sun exposure conditions, with irrigation and greenhouses, but these techniques are not accessible to the majority of peasants in LDC.

Changes in rainfall, temperature, exposure to winds, and relative humidity, with respect to the water retention capacity of the soil, are responsible for a cyclical change of water balance that may restrict the period for plant growth to some parts of the year. The proportion of dry months in a year, named absolute seasonality, can vary between 0 (sufficient rains all year long) to 1 (lack of a period suitable for plant growth). If the vegetative cycle of corn (maize) is considered (120 days), then the areas of the world can be classified as follows: low seasonality when there are more than 200 days' vegetative season per year and two harvests are possible; moderate seasonality with 120200 days' vegetative season and one to two corn harvests possible; and severe seasonality with less than 120 days' vegetative season and barely one corn harvest possible. With even shorter vegetative periods, agricultural production is impossible in the absence of irrigation. The different areas of agrocli-matic seasonality in the world are shown in Figure 1.

Factors aggravating the climatic seasonal effects may be the occurrence of pests, for example, the arrival of locusts arriving with the rainy season in Sahel. Furthermore, seasonal patterns of food production are often superimposed upon longer term cycles, which leads to the periodic appearance of drought and famines in sub-Saharan Africa and in Central Asia.

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