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.)