Drought: meteorological, agricultural, hydrological
Types of drought and how they are measured: rainfall deficiency classes, SPI, vegetation indices, India's drought manual and crop contingency plans.
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Drought is not the same as aridity. A desert is arid: low rainfall is its normal state. Drought is a temporary shortfall from what is normal for a place. Its impact grows with time and spreads through the water system, from the soil to rivers, reservoirs and aquifers, and then to food prices, fodder supply and livelihoods.
India is one of the most drought-exposed countries in the world. About two-thirds of its sown area depends wholly or partly on rainfall, and the monsoon fails or falters in a significant part of the country in many years. Severe droughts such as those of 1918, 1972, 1987, 2002 and 2009 shaped farm policy, food reserves and irrigation investment. Knowing which kind of drought is under way, and how severe it is, decides the right response.
Four types of drought
The widely used classification by Wilhite and Glantz (1985) recognises four types.
Meteorological drought
Meteorological drought is a rainfall deficiency compared with the long-term normal over a period, such as a month, a season or a year. It is the starting point of all droughts.
The India Meteorological Department classifies seasonal or periodic rainfall departure from the normal as follows:
| Departure from normal | Category |
|---|---|
| +20% or more | Large excess |
| +19% to −19% | Normal |
| −20% to −59% | Deficient |
| −60% to −99% | Large deficient |
| −100% | No rain |
Under IMD's drought definitions, a deficiency of more than 25% is a meteorological drought: moderate at 26–50% and severe above 50%. At the national level, an all-India drought year is one in which monsoon (June–September) rainfall is more than 10% below the long-period average and more than 20% of the country's area is affected.
Agricultural drought
Agricultural drought occurs when soil moisture is too low to meet crop needs during the growing season, which reduces growth and yield. It depends not only on rainfall but on its timing, the soil's water-holding capacity, evaporation demand and the crop stage. A two-week dry spell at flowering can do more harm than a larger deficit early in the season. Agricultural drought can therefore occur in a year with near-normal total rainfall, if the rain comes at the wrong times.
Hydrological drought
Hydrological drought is a prolonged deficit in streamflow, reservoir storage, lake levels and groundwater. It lags behind rainfall by months or years, and can persist after rains return. It hits irrigation, drinking water, hydropower and industry.
Socio-economic drought
Socio-economic drought occurs when the shortage of water, food, fodder or power affects people and the economy: when demand exceeds supply. Rising demand, for example from expanding irrigation of water-hungry crops, can turn a moderate meteorological deficit into a severe socio-economic drought.
Measuring drought
Rainfall-based indices
The Standardised Precipitation Index (SPI) of McKee and colleagues (1993) fits a probability distribution to long-term rainfall for a place and period, then converts the observed total to a standard normal score. Values from −1.0 to −1.49 are moderately dry, −1.5 to −1.99 severely dry, and −2.0 or below extremely dry. Because it is standardised, SPI can compare drought in wet and dry regions, and over different time scales, such as 1-month for soil moisture and 12-month for groundwater.
Classifying a monsoon
A district's normal June–September rainfall is 800 mm, and this year it received 520 mm. The departure is (520 − 800) ⁄ 800 × 100 = −35%. That falls in IMD's deficient category and counts as a moderate meteorological drought (26–50% deficit). Whether it becomes a severe agricultural drought depends on the distribution of rain. If most of the 520 mm fell in June and a five-week dry spell hit during flowering of soybean or cotton, crop losses could be large.
Soil moisture and water balance
The moisture adequacy index (MAI) is the ratio of actual to potential evapotranspiration, expressed as a percentage, from a water balance model. Field moisture sensors and satellite soil moisture products are increasingly used alongside it. The Palmer Drought Severity Index (PDSI, 1965) uses a water balance of precipitation, temperature and soil water, and is widely used in the USA.
Vegetation and remote sensing
The Normalised Difference Vegetation Index (NDVI), measured from satellites, and the Vegetation Condition Index (VCI), which compares current NDVI with its historical range, show crop stress over large areas. India's National Agricultural Drought Assessment and Monitoring System (NADAMS) and its successors use these indices.
Declaring drought in India
The Manual for Drought Management (Ministry of Agriculture, 2016, updated 2020) sets out a scientific, step-by-step process:
- Mandatory indicators: rainfall deviation, SPI, and dry spells.
- Impact indicators, of which at least some must be triggered: remote sensing vegetation indices, sown area, soil moisture, and hydrological indicators such as reservoir storage and groundwater.
- Ground truthing by crop-cutting or field verification in sample villages.
States declare drought as moderate or severe for districts or tehsils based on these. This opens relief under the State Disaster Response Fund and the National Disaster Response Fund, input subsidies, additional work under MGNREGA, and crop insurance claims.
Managing drought in agriculture
- Crop contingency plans: ICAR–CRIDA has prepared district plans with alternative crops and varieties for delayed monsoon or dry spells. Examples are switching from groundnut to pearl millet or pulses, or using short-duration varieties.
- In-situ moisture conservation: ridges and furrows, conservation furrows, mulching, compartmental bunding and tillage.
- Water harvesting: farm ponds, check dams and watershed development, with life-saving irrigation at critical stages.
- Drought-tolerant crops and varieties: millets, sorghum, pulses, and varieties with drought-yield QTLs.
- Fodder banks, livestock camps and water supply in severe droughts.
- Insurance and early warning: PMFBY, weather-based insurance and agromet advisories.
Key points
- Drought is a temporary deficit relative to normal; aridity is a permanent climatic condition.
- The four types are meteorological (rainfall), agricultural (soil moisture and crops), hydrological (surface and groundwater) and socio-economic (supply and demand). Impacts lag from weeks to years.
- In IMD's drought definitions, a deficit of more than 25% is a drought: moderate at 26–50%, severe above 50%. Seasonal rainfall categories range from large excess (+20% or more) to no rain.
- SPI is a standardised rainfall index: −1.5 to −1.99 is severely dry, −2 or below extremely dry. Other indices include MAI, PDSI, NDVI and VCI.
- India's drought manual combines mandatory rainfall indicators with impact indicators and field verification.
- Management uses contingency plans, moisture conservation, water harvesting, tolerant crops and insurance.
What the exams ask
- Define and distinguish the four types of drought.
- State IMD's rainfall departure categories and drought intensity classes.
- Calculate rainfall departure and classify it.
- Explain SPI, MAI, NDVI and VCI.
- Describe crop contingency planning and drought-proofing measures.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can a district be in agricultural drought with normal rainfall?
Yes. If rain falls in a few heavy storms with long dry spells between them, or ends early, crops can suffer moisture stress at critical stages even when the seasonal total is near normal. This is why the drought manual includes dry spells and impact indicators, not just total rainfall.
Is drought becoming more frequent in India?
The evidence varies by region. Studies show an increase in dry spells and in the variability of monsoon rainfall in parts of central and northern India, and rising temperatures increase evaporation demand, which intensifies agricultural drought. Groundwater depletion also makes hydrological drought more severe even when rainfall is unchanged.
What is the difference between drought and desertification?
Drought is a temporary climatic event. Desertification is land degradation in drylands: lasting loss of soil, vegetation and productivity caused by climate variation and human activities such as overgrazing and poor cultivation. Repeated droughts on poorly managed land can accelerate desertification.
Sources
- Wilhite, D. A. and Glantz, M. H. "Understanding the drought phenomenon: the role of definitions." Water International 10, 111–120 (1985).
- McKee, T. B., Doesken, N. J. and Kleist, J. "The relationship of drought frequency and duration to time scales." Proceedings of the 8th Conference on Applied Climatology, American Meteorological Society, 179–184 (1993).
- Ministry of Agriculture and Farmers Welfare. Manual for Drought Management. Government of India, 2016 (updated 2020).
- World Meteorological Organization and Global Water Partnership. Handbook of Drought Indicators and Indices. WMO-No. 1173, Geneva, 2016.
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