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DROUGHT DEFINITIONS
Defining what drought is – is the fascinating arena of speculation. Drought has many facets in any single region: it always starts with the lack of precipitation, but may (or may not, depending on how long and severe it is) affect soil moisture, streams and groundwater. Definitions vary from region to region and may depend upon the dominating perception, and the task for which it is defined (academic study or a drought relief plan). Drought should generally be defined relative to some long-term average condition (e.g. precipitation, balance between precipitation and evapotranspiration etc.). This is normally reflected in most of the general drought definitions the examples of which are given below.
Drought is a period of abnormally dry weather sufficiently prolonged for the lack of precipitation to cause a serious hydrological imbalance and carries connotations of a moisture deficiency with respect to man’s usage of water (McMahon and Diaz Arenas, 1982).
The chief characteristic of a drought is a decrease of water availability in a particular period and over a particular area (Beran and Rodier, 1985).
Drought is an interval of time, generally of the order of months or years in duration, during which the actual moisture supply at a given place rather consistently falls short of the climatically expected or climatically appropriate moisture supply (Palmer, 1965).
Drought as a severe shortage in the appearance of natural waters with respect to normal (Ben-Zvi, 1987)
Drought is a condition whenever the amount of water which has been expected and relied upon for use in any of man’s activities cannot be met for some reason (Takeuchi, 1974).
These definitions are normally vague, do not provide quantitative answers to “when”, “how long” or “how severe” a drought is and are often used as a start-up in scientific papers and reports. Some sources (National Drought Mitigation Center,USA) refer to them as “conceptual definitions” and differentiate between conceptual and operational definitions of drought. Operational definitions identify the beginning, end, spatial extent and severity of a drought. They are often region specific and are based on scientific reasoning, which follows the analysis of certain amounts of hydro-meteorological information. They are beneficial in developing drought policies, monitoring systems, mitigation strategies and preparedness plans. Operational definitions are formulated in terms of drought indices.
DROUGHTS AND ARIDITY
Two terms: drought and aridity should be explicitly separated. This could help to eliminate perceptions like “we are living in a permanent drought” or that “we had the drought for the last almost 60 years”. Drought is a recurrent natural climatic event, which stems from the lack of precipitation over an extended period of time (e.g. a season or several years). It occurs in all the geographical zones, but its characteristics vary significantly from one region to another.
Drought is a temporary anomaly and as such it differs from aridity, which is a permanent feature of climate, associated with low rainfall regions. The aridity measures suggested to date normally represent a ratio of precipitation to temperature characteristics during a warm season of the year. Aridity may also be indexed by the ratio of a region's mean annual potential evaporation (MAE) to its mean annual precipitation (MAP). Aridity is usually taken as a situation in which MAP is less than half the value of potential evaporation. Finally, the MAP itself may also serve as an approximate measure of aridity. An arid region is arbitrary defined as the one normally receiving less then 250 mm of precipitation per year. Regions with MAPs in the range of 250 to 700 mm a year may be classified as semi-arid. From this angle, large parts of Southwest Asia may be classified as arid (e.g. Baluchistan, Sindh provinces in Pakistan, western and southern lowlands of Afghanistan) or semi-arid (e.g. parts of Rajastan and Gujarat states in India). This does not however imply that these regions are in permanent drought, although they do experience abnormal (to such arid regions) periodic water shortages as well.
DROUGHT TYPES
A deficit of precipitation has different impacts on different components of hydrological cycle (river flow, groundwater) and components of biosphere (ecosystems, humans). For example, soil moisture conditions respond to precipitation anomalies on a relatively short scale. Groundwater, river flow and reservoir storage reflect the longer-term precipitation anomalies. These anomalies allow different drought types to be defined conceptually and to be described in terms of various drought indices. The distinction between these types are, however, rather arbitrary as different types of drought may happen simultaneously.
Meteorological drought
Every drought event effectively results from the lack of precipitation. Therefore, every drought is a meteorological drought. The real issue is what measures to use to define it in different regions. However, as discussed above, several standardized indices have been developed to date (e.g. SPI, EDI, SWSI), which allow droughts in different regions to be compared regardless of their climatic differences. Depending on its duration, meteorological drought may or may not develop into agricultural or hydrological droughts.
Agro-meteorological (agricultural) drought
Agro-meteorological (agricultural) drought is effectively also just a concept, which reflects the perspective of an agricultural sector on water shortages. This concept links the lack of precipitation (or other sources of water supply - river flow, groundwater) to agricultural impacts. Crop water requirements depend on local weather conditions, soil and plants' characteristics and plant stage of growth. The extremity of an agricultural drought should therefore ideally be defined in terms of its impact on specific plant on a specific soil in a specific area, which makes it a difficult task to accomplish. In more general terms, agricultural drought exists when root-zone soil moisture is insufficient to sustain crops between rainfall events. In this context, the status of soil water deficit in the top meter of a soil profile may be used as a drought measure. In practical terms, some of the indices referred to above are used to monitor the developing deficits of water availability to crops (e.g. CMI and other indices of soil moisture condition).
Hydrological (flow and groundwater) droughts
This concept is used to refer to the impacts that the lack of precipitation has on hydrological systems. In some cases, the impact on both rivers and aquifers are included under this "type". In other cases "a flow drought" is distinguished" from "groundwater drought". In both cases, a hydrological drought usually lags behind the deficient precipitation. It takes longer for precipitation deficiencies to manifest themselves through the river flow or through the groundwater level.
Groundwater drought is a rather subjective and vague concept. The inherent lack of long-term data on groundwater levels or other geo-hydrological variables is often substituted by perceptions. It may be defined in terms of lower than average annual recharge for more than one year. Shallow aquifers with small storage are likely to be more sensitive to groundwater 'drought', while large confined aquifers with big storage may be less affected by low annual recharge.
The drought type terminology is effectively an academic exercise. One thing, however, is worth stressing. Different drought "types" briefly discussed above, are effectively different stages (different extremes) of the same natural and recurring process. The deficiency of rainfall effectively triggers a drought. The longer and the more spatially extensive this deficiency is - the more likely that other types of droughts will occur as the result.
The agricultural sector is usually the first to be affected in drought because of its dependence on soil water storage and the need to replenish it to ensure crop growth. During every extended dry period, soil storage gets emptied first. If precipitation deficiency persists, other "storages" get affected, and sectors dependent on them may be hit. Those users who rely primarily on direct abstractions from a river will be affected next, those which are supplied from reservoirs and aquifers will be last who will feel water shortages. It is therefore the duration of a dry period, which has to be taken into account in drought management. A 3-month long rainless period may have an impact on rainfed agriculture, but little or no impact on domestic water supply, depending, of course, on the existing water supply infrastructure.
When drought ends and precipitation returns to normal or exceeds it, this sequence is repeated. Soil storage is replenished first, followed by flow increase (or floods), filling up of reservoirs and increased ground water recharge. After extreme droughts, agricultural sector may, in principle, recover quickly. But water restrictions may persist in other sectors for months. The length of the recovery period in different sectors is a function of the drought intensity and duration and the amounts of subsequent precipitation.
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