Drought: Definition and Classification

Drought is a natural calamity that adversely affects agriculture, surface, and groundwater resources. It is a temporary condition characterized by insufficient precipitation to meet the needs of vegetation, agriculture, river flow, and water supplies. Droughts can occur in almost every region of the world, although their frequency, duration, and severity vary. In some areas, droughts are rare, while in others, they are frequent and prolonged.

In agriculture, drought is defined as a climatic anomaly that leads to moisture deficiency due to subnormal rainfall, erratic distribution of rainfall, increased water demand, or a combination of these factors.

Definition of Drought

• General Definition: Drought is a temporary condition of water shortage resulting from insufficient rainfall, which adversely affects vegetation, agriculture, water supply, and ecosystems.
• Agricultural Definition: A climatic anomaly where moisture supply is insufficient for crop growth due to subnormal rainfall, erratic distribution, high water demand, or a combination of these factors.
• World Meteorological Organization (WMO): Drought is a sustained, extended deficiency in precipitation compared to the average for a region, resulting in water scarcity.
• Indian Meteorological Department (IMD):
  • Drought Week: Seven consecutive days with less than half the normal rainfall.
  • Drought Month: Rainfall deficiency exceeding 50% of the monthly average.
  • Drought Year: Annual rainfall deficiency of 20-60% of the average. If the deficiency exceeds 60%, it is termed a Scanty Drought Year.

Definitions of Drought Across the World

Differences Between Aridity and Drought

• Aridity is a permanent climatic feature characterized by long-term water scarcity, whereas Drought is a temporary condition caused by short-term rainfall deficiency.

Dictionary Meaning Drought refers to dryness, lack of rain, or a prolonged dry spell.

Drought Definitions in India

• Drought Week: Defined by Ramdas and Mallik (1948) as a period of seven days where actual rainfall is equal to or less than half of the normal amount. If four consecutive drought weeks occur between May and October, it is classified as Agricultural Drought (IMD).
• Drought Month: If monthly rainfall is deficient by more than 50% of the average, it is considered a Drought Month (Sharma et al., 1979).
• Drought Season: Occurs when actual rainfall is less than twice the mean deviation of the seasonal average (Ramdas, 1960).
• Drought Year: If yearly rainfall is deficient by 20-60% of the average, it is classified as a Drought Year. If the deficiency exceeds 60%, it is termed a Scanty Drought Year (Dhar et al., 1979).

Classification of Drought

Drought is generally defined as a condition where the demand for water (needed for processes like transpiration and evaporation) exceeds the available moisture in the soil. The classification of droughts can be done based on several criteria, including the cause, duration, and the impact on agricultural and hydrological systems.

1. Thornthwaite’s Classification (1948)

Thornthwaite defined drought as a situation where the water required for transpiration and direct evaporation exceeds the water available in the soil. He identified three main types of drought:

• Permanent Droughts of Desert Climates: Occur in desert regions where vegetation is sparse and specifically adapted to extreme water scarcity. Agriculture is possible only through irrigation throughout the crop season.
• Seasonal Droughts: Occur in climates with distinct rainy and dry seasons. Common in arid and semi-arid zones. The duration of the growing season and crop varieties should be selected so that they coincide with the rainy season to avoid moisture stress.
• Contingent Droughts: Caused by abnormal or irregular rainfall patterns, leading to temporary water shortages. Can occur almost anywhere, but are typical of sub-humid and humid climates. Usually brief, irregular, and localized but can be intense, impacting crops during critical growth stages.
• Invisible Drought (Thornthwaite and Mather, 1955): Occurs when rainfall appears adequate but does not meet the consistent water demand of the region. Often observed in humid regions with frequent but insufficient rainfall, leading to borderline soil water deficiency and sub-optimal crop yields.

2. Classification Based on Duration

This classification is based on the length and timing of water scarcity:

• Permanent Drought: Characteristic of desert climates where rainfall is extremely low and erratic. Sparse vegetation grows in these regions, adapted to prolonged drought conditions. Agriculture is only feasible with irrigation throughout the crop cycle.
• Seasonal Drought: Occurs in areas with well-defined rainy and dry seasons, common in arid and semi-arid regions. Agricultural practices must be adjusted to match the cropping season with the rainy period to avoid drought stress.
• Contingent Drought: Results from unexpected or abnormal rainfall failure. Can happen in any climatic region but is more common in humid or sub-humid climates. Generally short-lived and localized but can significantly impact crops if occurring during critical growth stages.
• Invisible Drought: Occurs when frequent rainfall is inadequate to meet the evapo-transpiration demands of the crop. Results in borderline soil water deficiency and less than optimum crop yields. Commonly observed in humid regions.

3. National Commission on Agriculture Classification (1976, India)

This classification is widely accepted in India and is based on the impact on various sectors:

• Meteorological Drought: Defined by significantly below-average rainfall, typically less than 75% of the normal level over a wide area. Indian Meteorological Department (IMD) categorizes droughts as:
  • Slight: Rainfall deficit of -11% to -25%
  • Moderate: Rainfall deficit of -26% to -50%
  • Severe: Rainfall deficit below -50% It serves as the basis for planning cropping patterns and agricultural practices.

• Hydrological Drought: Occurs when water sources like rivers, lakes, reservoirs, and wells dry up due to prolonged meteorological drought. Leads to reduced water availability for domestic, agricultural, and industrial use. Affects power generation, irrigation, and other income-generating activities.
• Agricultural Drought: Occurs when soil moisture is insufficient to meet the crop water requirements during its growth cycle. Directly impacts crop growth, leading to reduced yields. Closely linked to meteorological and hydrological droughts but specifically refers to the impact on agriculture.
• Atmospheric Drought: Results from low air humidity and high temperatures, often accompanied by dry winds. Can occur even when soil moisture is adequate. Leads to temporary wilting of plants during the day when transpiration exceeds water absorption. Plants recover when transpiration slows down or absorption catches up.

4. Classification Based on Time of Occurrence (Singh and Raman Rao, 1988)

This classification focuses on the timing and impact on crop growth stages:

• Early Season Drought: Occurs due to delayed onset of monsoon or sowing rains. Early rains may encourage farmers to sow, but a subsequent dry spell can cause seedling death and poor crop establishment.
• Mid-Season Drought: Occurs due to long gaps between rainfall events during the crop growth period. Stored soil moisture becomes insufficient, impacting crop growth and productivity. Often linked to inadequate rainfall during the vegetative growth phase.
• Late Season Drought: Occurs during the reproductive or maturity stage due to early cessation of rains. High temperatures during this phase can hasten crop maturity, leading to forced ripening and lower yields.
• Apparent/Relative Drought: Occurs when rainfall is adequate for one crop but insufficient for another. This is often due to the mismatch between the cropping pattern and the rainfall/moisture availability pattern. For example, in sub-humid regions of India, rainfall may be inadequate for paddy but sufficient for maize or sorghum.
• Permanent Drought: Associated with consistently low rainfall or soil moisture, affecting crop growth in most years. Usually occurs in arid regions where moisture availability is inadequate throughout the crop season.
• Physiological Drought: Occurs when plants are unable to absorb water from the soil despite its availability. Caused by high osmotic pressure of the soil solution due to high salt concentrations (e.g., in saline or alkaline soils). Not due to water shortage but due to the inability of plants to uptake water under high osmotic stress.