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close this bookDrought and Famine (DHA/UNDRO - UNDP, 1994, 53 p.)
close this folderPART 1 Drought
View the document(introduction...)
View the documentDefinitions
View the documentCauses of reduced rainfall
View the documentTypes of drought
View the documentFactors affecting the severity of drought
View the documentThe impact of droughts
View the documentReducing the impact of drought
View the documentSummary

Factors affecting the severity of drought

While it is usually true that decreased rainfall results in decreased crop yields, the following factors influence the strength of the relationship.

The proportion of production which is irrigated

The correlation between rainfall and yields is weaker in irrigated rather than rainfed areas. The extent to which this is the case however, will be determined by the importance of local rainfall in the irrigated water supply and whether all or only part of the crops’ moisture requirements are normally met through irrigation.

The moisture retention capacity of the soil

Different soil types have different capacities to “hold” or retain moisture. For instance the water retention capacity of sandy soils is generally significantly lower than that of clay soils. In areas where soil moisture retention capacities are high, crop growth may not be affected by prolonged dry periods (as much as 20 days) and some moisture may actually be held over from one wet season to another.

In contrast, in areas where retention capacity is low, dry periods of only a week may result in reduced yields and moisture present in the soil at the end of one season will not last to the next. In many arid and semi-arid areas of the tropics the predominant soil types are sandy. To attain optimum crop growth such areas need frequent and evenly spaced rainfall throughout the growing season.

National soil surveys are available for most countries, either from the Ministry of Agriculture or the local FAO office. Geography departments at local universities frequently have information on soil types and characteristics.


Timeliness of the rainfall

Deficiencies in moisture supply at critical stages during the growth cycle (e.g. germination and flowering) can significantly reduce yields. Consequently the distribution or timeliness of the rainfall during the growing season is potentially as important as the overall amount of rain.

Because of the importance of timeliness, particularly in semi-arid areas where soil moisture retention capacity is low, rainfall must be described and analyzed in appropriately short time periods so that prolonged dry periods are not “hidden” within aggregate monthly figures which may indicate that rainfall has been around or even above the average (See Box 2).

Models for different types of crops indicate the moisture requirements for optimum crop growth. In some countries such “moisture satisfaction” models are used to produce forecasts, at different stages within the normal growing season, of crop yield on the basis of rainfall occurring during the cycle. As part of the FAO’s program of strengthening the early warning capacity within countries prone to drought and transitory food insecurity, such agro-meteorological analytical capacity is being developed within Ministries of Agriculture and Meteorological Departments.

The adaptive behavior of farmers

In the face of an intermittent start to a wet season, some farmers may respond with repeated replantings of the same crop variety to take account of the rains when, and if, they finally start, while others may replant using other seed varieties. Some farmers may not have seed reserves of their own or be in a position to purchase replacement seeds for the first, failed planting. In this situation some farmers may experience a crop failure while others in the same area are enjoying a satisfactory harvest. Information on farmer behavior in the face of late or inadequate rains may be available from the Extension Services Departments within Ministries of Agriculture.

Q. Choose a drought that you are familiar with and describe how two of the above factors either increased or decreased drought impacts.

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A standard operational definition of drought?

While it might appear attractive for all agencies to adopt a standard operational definition of drought, such a definition is likely to prove both elusive and unnecessarily restrictive. To determine, for instance, that “an area should qualify as being drought affected when its rainfall falls below 70% of the average for the previous 20 years” is not particularly useful if most of the area’s agriculture is irrigated. Moreover the cut-off chosen is arbitrary and may exclude areas experiencing “classic” drought impacts as a result of average but poorly spaced rainfall. Standardized definitions cannot take account of the wide variations in vulnerability to the effects of drought as a result of wider variations in physical, economic and social conditions between areas and countries.

Standardized definitions cannot take account of the wide variations in vulnerability to the effects of drought as a result of wider variations in physical, economic and social conditions between areas and countries.