(introduction...)

LEARNING OBJECTIVES

After reading and completing the exercises in this module, you will be able to:

· define and distinguish between different types of droughts and their causes
· analyze factors that influence the relationship between rainfall and crop yields
· identify and predict the likely impacts of droughts

Definitions

The climate and weather systems of the earth are constantly changing. As part of these dynamic processes, extremes of temperature, rainfall, and air movement will naturally occur. Periods of unusual dryness, i.e. droughts, are therefore a normal feature of climate and weather systems in all countries, including those generally regarded as being “wet” and “cold” as well as those areas usually associated with the term “drought” - the semi-arid areas of the tropics. While droughts may be regarded as unusual in that they do not occur all the time, or in some areas for most of the time, droughts should not be regarded as being “abnormal” and, in fact, should be planned for in all countries.

While droughts may be regarded as unusual in that they do not occur all the time, or in some areas for most of the time, droughts should not be regarded as being “abnormal” and, in fact, should be planned for in all countries.

Drought is notoriously difficult to define and different definitions abound. Nevertheless, it is important that those involved in drought preparedness, mitigation, and declaration activities share a common understanding of the ways in which drought may be defined and the assumptions and constraints involved in using particular definitions.

Among the factors contributing to the difficulties involved in defining drought are:

· In most cases the drought phenomenon is temporary. A “drought” lasting a month may occur in an area which is known to experience cycles of alternating wet and dry periods (say every 5 years) and which is also known to be experiencing an increasingly drier climate for the past 50 years. (See Box 1.). Defining a temporary reduction of water/moisture availability as a drought given such dynamic processes is extremely difficult as much depends upon the length of the time period being considered.

· Droughts of similar severity may have dramatically different impacts on different societies as a result of ecological, socio-economic and cultural differences. This, in turn, affects how drought is perceived and how the term is used. Thus, it is difficult to define the term solely with regard to the physical event, i.e. the reduction in water/moisture availability. Invariably the definition has to take some account of how the physical event impacts upon society.

A generally accepted definition of drought is a temporary reduction in water or moisture availability significantly below the normal or expected amount (norm) for a specified period.

The key assumptions of such a definition are:

1. the reduction is temporary (if the reduction were permanent then terms such as “dry” and “arid” would be more appropriate)

2. the reduction is significant

3. the reduction is defined in relation to a “norm” i.e. normal expectation

4. the period taken as the basis for the norm is specified.

How the “norm” is defined is of critical importance. Assumptions 3 and 4, therefore, require more detailed clarification. The “norm” may be defined either:

· technically - a reduction of water availability might qualify as a “drought” when it falls below about 80% of the average availability over the preceding 20 years. However, the period selected as the basis for estimating the average may prove misleading. (See Box 1) or

· culturally - in terms of the level of water availability the society has come to expect. Thus, after a run of ten years with above average rainfall a society may have become used to the wetter state and perceive the first year of average rainfall as a drought.

Box 1 RAINFALL FLUCTUATIONS SINCE 1900 IN THE SAHEL The importance of specifying the “norm” or base period when defining and discussing the incidence of drought may be illustrated by a case from the Sahel region of Africa. The graph below shows the national annual rainfall series for Niger in the West African Sahel from the start of the century to 1987. The smooth line shows the overall trend based on the actual rainfall during the period 1941-70. Figure The extended dry period around the early part of the century is clearly visible. The driest year coincided with the widespread famine which affected the Sahel around 1913. The period 1920-60 was comparatively wet and there was a significant increase in both the human and livestock populations. Since the mid-1960s the country has (in common with the rest of the Sahelian countries) been experiencing a run of dry years which is the most prolonged since detailed records began. 1951 is an example of a “drought year” during a period of generally above average rainfall. Under a system established by the World Meteorological Office (WMO) a 30 year international meteorological standard is used as a base period. Until 1991 the 30 year base period covered the years 1931 -60. For the Sahel this included the predominantly wet years of the 1950s and early 1960s. During 1991 the international base period was changed to cover the period 1961-90. So far as the Sahel is concerned, this includes virtually all the declining rainfall periods since the late 1960s. Thus, an agricultural planner utilizing the internationally established methods of estimating “normal” rainfall in relation to a particular project area in 1992 would find a much drier “norm” than if he/she had referred to the data in 1990. What might previously have been considered a drought year in 1960 might be considered an average year in 1992.

Q. Choose a drought that you are personally familiar with and briefly describe it using each of the key assumptions described above.

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Figure 1 Schematic presentation of fluctuations occurring on different time scales.

Causes of reduced rainfall

When discussing the causes of drought it is helpful to distinguish between short drought episodes lasting 1-3 years and long dry regimes of predominantly subnormal rainfall spanning about a decade or more and, which may, include several intense drought episodes.

The proximate or immediate cause of a rainfall shortage may be due to one or more factors including an absence of available moisture in the atmosphere; large scale subsidence (downward movement of air within the atmosphere) which suppresses convective activity; and the absence or non-arrival of rain-bearing systems. Changes in such factors involve changes in weather systems on many spatial scales ranging from local to regional to global. While it may be possible to indicate the immediate cause of a meteorological drought occurring in any particular location, it is often not possible to indicate the underlying cause.

Short term episodes can often be linked to global-scale fluctuations in the atmosphere and oceans elsewhere in the world. For example, the El Niouthern Oscillation (ENSO) phenomenon, which involves the periodic invasion of warm surface water into the normally colder waters off the Pacific coast of South America, affects the levels of rainfall in many different parts of the world, including south-eastern Africa. However, knowledge about causes of the invasion of the warmer currents is presently incomplete.

On a larger scale, the link between sea surface temperatures and rainfall has been suggested as a possible cause of long dry regimes. Thus it has been suggested that the fact that the southern Atlantic has been consistently warmer than the northern Atlantic since around 1970 is related to the predominantly dry period in the Sahel since the mid-1960s.

Increasing levels of carbon dioxide and other “greenhouse” gasses have been suggested as causes of rainfall changes in the Sahel and elsewhere. However, as with all the postulated causes there is insufficient understanding of the physical processes involved to state with certainty that the postulations are correct.

Many causes of long dry “regimes” have been postulated. Among the local level causes are human-induced changes resulting from vegetation loss due to overgrazing and deforestation either in the general vicinity or “upwind” of the area along the line of the prevailing, moisture carrying winds. Such changes may involve “biogeophysical” feedback mechanisms, i.e. once they start, they feed back on themselves and perpetuate the drought conditions.

However, one of the main problems with the postulations involving human induced change is that of distinguishing human induced change from natural long term fluctuations. For instance, there would seem to be a 50 year fluctuation in rainfall in the western Sahel with the predominantly dry period since the mid-1960s being part of such a cycle. However, reliable rainfall data for the Sahel and many other parts of the world are available only for the last 80-90 years, and this is too short a period to support the assertion that there is such a rainfall cycle in the area. The World Meteorological Office believes that five or six such cycles are necessary to confidently predict trends (WMO 1966). Clearly however, land use patterns which result in environmental degradation, desertification, and deforestation are inextricably linked in a causal way to drought.

Q. Why is it important to specify and review the base period chosen to establish overall rainfall expectations?

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ANSWER The base period is used to define the expected level of rainfall. Because rainfall varies over multi-year periods and because individuals tend to expect the most recent events to repeat themselves, it is possible for individuals to expect rainfall amounts which are not realistic, thereby making planting and investment decisions based on incorrect or misleading information.

Types of drought

It is conventional practice to distinguish between three different types of drought, namely meteorological, hydrological and agricultural. Particularly in the case of meteorological and agricultural droughts, these types are frequently, but wrongly, seen as being synonymous.

Of the three types of drought, the first two describe the physical event whereas the third describes the particular impact of the first two on agricultural production. It is necessary to carefully distinguish between these types and clarify where and how they overlap.

Water used in support of human activity is derived from either direct rainfall or previous rainfall which is temporarily “stored” in rivers, lakes, groundwater aquifers and snowfields/glaciers.¹ In the case of some aquifers and glaciers, such “stores” may contain rain that fell decades or even centuries before. A temporary reduction of either of these two main types of water source may cause a drought.

Meteorological drought describes a situation where there is a reduction in rainfall for a specified period (day, month, season, or year) below a specified amount - usually defined as some proportion of the long term average for the specified time period. Its definition involves only precipitation statistics. As indicated in Box 2, care needs to be taken in utilizing and aggregating rainfall data.

Box 2 PROBLEMS IN THE USE AND INTERPRETATION OF RAINFALL DATA AS AN INDICATOR OF DROUGHT When using precipitation data as an indication of the existence and extent of a meteorological drought careful consideration must be given to the problems of measuring rainfall in the area in question and problems associated with aggregating data. Most precipitation in the tropics is associated with convective activity, ie. where currents of warm air rise up into layers of cooler air. Convective rainfall can be heavy and highly localised. Thus immediately adjacent areas may receive very different amounts of rainfall. In contrast, most rainfall in mid-latitude temperate areas is associated with the movement of fronts which causes similar levels of rainfall over large areas. If, as is often the case in developing countries, large regions are covered by only a few rain gauges then it is quite possible that the picture conveyed by the rain gauges may not reflect the actual situation elsewhere in the region. Thus, pockets of drought may exist in an area judged to have received adequate rainfall and vice versa. While rainfall is usually recorded daily (hourly at the most sophisticated stations) the data are often presented to non-meteorologists as monthly totals. While such totals may indicate average or above average rainfall, these may be the result of one or two heavy rainfall episodes. As discussed below, crop yields may be affected by dry periods as short as a week. The increasing practice of using 10-day “decade” periods as the basis for summarizing rainfall performance is preferable.

Hydrological drought involves a reduction in water resources (streamflows, lake levels, groundwater, underground aquifers) below a specified level for a given period of time. Its definition involves data on availability and offtake rates in relation to the normal requirements of the system (domestic, industrial, irrigated agricultural) being supplied.

The distinction between the two types can often be blurred as hydrological droughts may be caused by reductions in precipitation anywhere within the catchment area of the river or aquifer system. Thus, irrigated agricultural areas alongside the River Nile in Egypt may experience a hydrological drought as a result of a meteorological drought in the Ethiopian Highlands regardless of the levels of rainfall within Egypt itself.

In the case of rivers fed by snowmelt, irrigated areas downstream may experience reduced water availability as a result of reduced snowmelt caused by below normal temperatures during the summer months. Areas drawing water from underground aquifers through wells and boreholes may experience hydrological drought as a result of geological changes which cut off parts of the aquifer. Overutilization of the aquifer may also result in its exhaustion.

Agricultural drought is the impact of meteorological and/or hydrological droughts on crop yields. Crops have particular temperature, moisture and nutrient requirements during their growth cycle in order to achieve optimum growth. If moisture availability falls below the required amount during the growth cycle then crop growth will be impaired and yields reduced. However, droughts have different impacts on different crops, e.g. sesame often thrives in dry years. Because of the complexity of the relationships involved, agricultural drought is difficult to measure. A fall in yields may be due to insufficient moisture but it may also stem from, or have been exacerbated by, such factors as the unavailability of fertilizers, lack of weeding, the presence of pests and crop diseases, the lack of labor at critical periods in the growth cycle, and unattractive crop prices. Also these factors can interact with each other and exacerbate conditions. For example, in the 1984 drought in Ethiopia, the drought contributed to army worm infestation which substantially increased the amount of crop damage.

Q. What is the difference between hydrological drought and meteorological drought?

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ANSWER A meteorological drought results directly from a reduction of rainfall in a specific area while a hydrological drought results from the reduced availability of water in holding areas such as streams, lakes, and reservoirs.

Small scale irrigation is part of a rehabilitation program for drought-affected areas in Ethiopia (IFAD/F. Mattioli UNDRO News, May/June 1986.)

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.

The impact of droughts

Of all the natural hazards, droughts are potentially those having the greatest economic impact and affecting the greatest number of people. Earthquakes and cyclones may be of enormous physical intensity but are invariably of short duration and geographically limited. By contrast droughts affect large geographical areas, often covering whole countries or parts of continents and may last for several months and, in some cases, several years.

Of the main natural disasters, droughts are unique in terms of the length of time between the first indications from, for example, rainfall monitoring/that a drought is developing and the point at which it begins to impact significantly upon the population of the affected area. The length of such “warning time” varies significantly between societies.

In many countries the warning time is on the order of several months. In others, for instance those with a high proportion of landless agricultural laborers, the warning time may be much less, perhaps only a few weeks. Whatever me period, a warning time allows for a potential response to mitigate the impacts of the drought before they become significant. In the case of countries where the lead time is on the order of months there is, potentially at least, sufficient time for relief assistance, including food aid, from the international community to be mobilized.

It has become possible to prevent excess mortality resulting from food shortages caused by drought alone.

Thus, by virtue of modern meteorological monitoring and telecommunication systems it has become possible to prevent excess mortality resulting from food shortages caused by drought alone. While droughts may continue to be a contributory cause of famines, other factors such as armed conflict and international politics, are now invariably responsible for propelling a situation of economic hardship caused by drought into a famine.

Droughts, almost or virtually always, have a direct and significant impact on food production and the overall economy. The impact on a particular population is related to the severity and nature of the drought, but equally, and occasionally more importantly, to the nature of me economy and society in the affected area.² Box 3 describes the effects of drought on Niger in 1984.

Box 3 THE ECONOMIC IMPACT OF THE 1984 DROUGHT ON NIGER Niger’s main exports are uranium, livestock, and cowpeas. Agriculture accounts for 36% of the Gross Domestic Product (GDP) and provides employment for approximately 90% of the labor force. As a result of the important role of uranium exports in the economy Niger is less dependent on agricultural exports than many other African countries. Nevertheless, in 1984 total agricultural production fell by 19% when compared to the average for 1981-83 and cereal production fell by 37%. Food aid imports increased by 450% but total cereal availability was still down 37% when compared to the average for the previous three years. The economic impact of droughts extends well beyond the single year losses indicated above. Average GNP growth for the 1965 to 1988 period was only 0.4%/year. In a simulation exercise in which the impact of the 1972-73 and 1983-84 droughts on output were reduced by half, the long term growth rate for the period would have increased to 2.0%/year. Reducing the impact on output by half is not an unrealistic scenario and could be achieved through a combination of effective internal measures to stimulate economic activity and external efforts to meet import requirements. Source: World Bank 1991.

First consider an extreme example. Two semi-arid countries experience a 50% reduction in annual rainfall. Country A is a high income economy where agriculture contributes only 10% of the total Gross Domestic Product (GDP) and water for industry, agriculture and domestic use is drawn from reliable underground aquifers. Country B is a low income economy where agriculture is primarily rainfed and contributes 50% of total GDP. Clearly the drought impacts on Country A are likely to be negligible whereas for Country B they are likely to be severe.

Emergency aid airlifted from Europe to Ethiopia (UNHCR/B. Press)

The scenario becomes more interesting if the assumption about the reliability of the underground aquifers in Country A is altered so that water is drawn from rivers and reservoirs whose catchment area is affected by the reduction in rainfall - a hydrological drought. Likely impacts include reduced agricultural and industrial production, increased unemployment, domestic water rationing, increased cost of living, increased investment in the exploitation of alternative water resources and so on.

However, by virtue of its greater wealth Country A will still probably be able to cope by purchasing imported food to make up for any domestic production losses and providing compensation to those most seriously affected. Thus, the level and distribution of wealth and how the economy is structured are central to any consideration of the nature and severity of drought impacts.

Social and organizational considerations are also important. Two countries with similarly structured economies and levels of wealth may be affected quite differently by droughts of similar severity as a result of:

1. The extent to which the drought hazard is taken account of in resource allocation decisions by

· households - planting and consumption/saving decisions

· communities - construction of dams with reserve capacity, organization of local food stores, and investment in alternative water sources

· governments - encouragement of research on drought-resistant crop varieties, creation of administrative procedures to respond to drought, and investment in additional transport and storage capacity.

2. The extent to which social and political institutions encourage the recognition of the hazard and a rapid and effective response to it when a drought does occur is also critical. For instance, the existence of a free press, accessible administrative systems, open public debate and representative political systems are increasingly recognized to be important factors determining the effectiveness of the response to droughts by national governments.

Bearing in mind the variations between economies and societies noted above, the following list of potential economic and social impacts of an agricultural drought may be helpful. The list is neither exhaustive nor comprehensive but is intended to illustrate some of the possible outcomes of drought. Potential information sources on the impacts are noted in brackets.

· Reduced income for farmers and agricultural laborers (household income and expenditure surveys undertaken by statistics departments, ministries of agriculture, university researchers, employment offices).

· Reduced spending locally on agricultural inputs and equipment and non-agricultural items and services like travel and non-subsistence foods (household income and expenditure surveys, interviews with parastatal and private sector suppliers of agricultural inputs surveys of business activity undertaken by ministries of commerce, central and commercial banks, town councils, local chambers of commerce, etc.).

· Decrease in price of livestock as farmers are forced to sell because of increases in the cost of pasture and purchased feeds (ministry of agriculture, early warning units, NGOs, interviews with livestock traders, district officials, and market clerks).

· Increased price of staple foods (price monitoring units within ministries of commerce, agriculture, district administration offices, NGOs, etc.).

· Inability of certain groups within the population to afford increased food prices results in their:

- switch to cheaper and sometimes less preferred foods
- reduction in overall food intake
- borrowing to maintain food intake
- selling assets to raise funds
- engaging in alternative income earning activities locally
- migrating in search of employment opportunities
- migration to where relief food is being distributed (monitoring undertaken by Early Warning Units, specially organized surveys, interviews with district officials, NGOs, local leaders, etc.)

· Reduced food intake leads to deterioration of nutritional status and reduction in ability to resist infection (nutritional and epidemiological surveillance units with ministries of health, NGOs).

· Difficult and scarce availability of water results in a general increase in diarrhoeas and other water/hygiene-related illnesses (e.g. typhoid, dysentery, eye infections, etc.) Shortage of water and increased mobility of population increase the opportunities for transmission of epidemic diseases such as cholera; dry climatic conditions also facilitate the transmission of meningoccoccal meningitis. (ministry of health statistics, hospital admittance records).

· Increased stress and morbidity (disease and illness) results from migration journey (ministry of health, NGOs, interviews with migrants).

· Drying-up of water sources leads to reduction in water quality, the need to travel further to collect water and possibly migration to better water sources (rural water authorities, district officials, NGOs, specially organized surveys).

· Increased competition for access to remaining water sources may lead to increased incidence of local disputes/conflict (local police, district officials, local leaders).

· Loss of education due to reduction in school attendances by children lacking energy and/or money for fees, plus the need for them to assist other family members in water collection and income generating activities (ministry of education, school attendance and prison records).

· Social costs of migration, e.g. break-up of communities and families.

· Reduced demand within the economy generally (ministries of finance, central and commercial banks, IMF and World Bank country representatives).

· Increased defaults on loans in rural sector (central and commercial banks).

· Loss of basic services, such as rural area health posts and schools which stop functioning because of lack of water.

· Reduced government revenues and foreign exchange earnings as a result of a decline in agricultural exports (ministries of trade, finance, central banks, IMF and World Bank country representatives).

· Increased inflation rate within the economy (ministries of finance, commerce, central banks, etc.).

· Costs to government of organizing relief measures (lead ministries in relief operations, UN Disaster Management Team).

· Costs to international community of assisting governments relief efforts (UN Disaster Management Team, DHA).

· Opportunity costs of funds used in relief efforts.

Q. Choose a drought that you are familiar with and describe its major impacts drawing from the list of impacts provided above or others with which you are familiar.

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Reducing the impact of drought

There are a number of actions that can be taken to counter drought conditions or lessen their impact. They include:

· Getting seeds in place for when the rains begin

· Improving rangeland management

· Improving water resource management

- dig new wells
- improve existing wells
- construct retention dams or catch dams
- construct subsurface dams to trap water in sandy riverbeds
- recharge the aquifer with catchments which trap water and allow it to seep quickly down into the water carrying strata

· Planting drought-resistant crops, such as sorghum and millet instead of hybrid maize

· Implementing counter-desertification measures such as

- erosion control
- tree planting
- construction of wind breaks and barriers

· Gearing up all government sectors to meet the new needs

Summary

This section of the module has focused on drought because drought is a significant and frequent contributing cause of famine. In order to increase your understanding of the drought phenomenon, the module identified three types of drought - hydrological, meteorological and agricultural. Although scientists have postulated many underlying causes of rainfall declines, knowledge is still incomplete. Even so, rainfall tends to vary in multi-year cycles. This creates expectations both on the part of human populations and the scientific community regarding yearly rainfall levels. It is important in determining whether rainfall is below the normal levels to specify how the normal expectation is determined.

Even though areas experience significant reductions in available water supply, factors, which include the planting behavior of farmers, soil types, spacing of rainfall, and extent of irrigation, cause variations in the extent to which reduced rainfall results in reduced crop yields. Other social and economic factors also influence how communities and individuals are affected by drought. Nevertheless, droughts invariably have widespread impact on individuals, households, communities, and countries.