Hydrological drought

Topic: Natural Hazards and Disaster Management in India · NCERT: Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"

Meaning

Hydrological drought is a drought in which the water stored in aquifers, lakes, rivers and reservoirs falls below the level that rain can refill. It usually comes after a long meteorological drought (a long shortage of rain).

It matters because it decides whether towns, farms and cattle get drinking and irrigation water. It also takes the longest to build up and to end. In India it is made worse when groundwater is pumped out faster than rain can put it back.

Explanation

How it forms (cause → process → result)

• Cause: the rain fails, or stored water is overused.
• Too little rain falls for a long time. This is meteorological drought.
• High evaporation: heat turns surface water in tanks and lakes into vapour.

• Water stored in reservoirs and underground (groundwater) is drawn out faster than it is refilled.

• Process: the stores drain slowly.

• Less rain means less runoff (rainwater that flows off the land into streams). So rivers and reservoirs get less water.
• Less rain also soaks into the ground, so aquifers are not recharged. An aquifer is a layer of rock or sand underground that holds water and lets it flow.

• Pumping and evaporation keep taking water out at the same time.

• Result: the water bodies run low.

• Stream flow drops. Reservoir levels fall. Groundwater levels go down and wells dry up.
• This brings on jalkal, the traditional word for a shortage of water.

Where it sits in the drought chain

• Drought is a slow-onset hazard: it builds up over weeks and months. Hydrological drought is the slowest stage.
• Meteorological drought comes first: the rain fails.
• Agricultural drought comes next: the soil dries and the crops wilt.
• Hydrological drought comes after that: over months, rivers, tanks and wells run low.

• Ecological drought comes last: forests, grasslands and wetlands suffer.

• It lags behind the rain. Stored water takes months to drain. It also takes time to refill. So hydrological drought starts late and can go on after the rain has started to improve.

How it is measured in India

• It is not judged by rainfall. It is judged by falling reservoir levels, groundwater levels and stream flow.
• Under the Manual for Drought Management (2016, updated 2020), hydrological indices are one of four groups of impact indicators (Trigger-2). The state picks any 3 of the 4 [2]:
• SFDI: an index of stream flow [2]
• GWDI: an index of groundwater [2]

• RSI: an index of reservoir storage [2]

• Who supplies the data [2]:

• stream flow: CWC (Central Water Commission) and India-WRIS
• groundwater: CGWB (Central Ground Water Board)

• reservoirs: CWC and the state water departments

• How severity is graded: drought is severe if 2 of the 3 chosen impact indicators are in the severe category. It is moderate if 2 of the 3 are in the moderate category [2].

Conditions that make it likely

• An unreliable monsoon, so one bad season can drain the stores.
• Hard rock and thin soils, which hold little groundwater (for example, Bundelkhand).
• Heavy groundwater pumping for irrigation and towns.
• Low and uncertain rain, as in rain-shadow areas and at the monsoon margins.

Where it occurs

• Marathwada (central Maharashtra): a chronic drought region in the rain shadow of the Western Ghats. A rain shadow is the dry side of a mountain, away from the moist wind. The Ghats take most of the south-west monsoon rain, so the interior Deccan stays dry, and tanks and wells there run low again and again.
• Bundelkhand (southern Uttar Pradesh and northern Madhya Pradesh): a chronic drought region. Its hard rock base, thin soils and uncertain rain mean little water gets stored underground.
• Rajasthan and Gujarat (monsoon margins): these lie at the outer edge of the monsoon's reach, where rain is low and very uncertain.
• The extreme zone covers most of Rajasthan west of the Aravalis: the Marusthali (Thar desert) and Kachchh (Gujarat).

• Jaisalmer and Barmer get under 90 mm of rain a year on average.

• Other severe-zone areas: eastern Rajasthan, most of Madhya Pradesh, eastern Maharashtra, interior Andhra Pradesh and the Karnataka plateau, northern interior Tamil Nadu, southern Jharkhand and interior Odisha.

• Scale in India (NCERT): about 30% of the area is drought-prone, affecting about 50 million people.
• In the world: it can happen anywhere a long shortage of rain, high evaporation or overuse drains rivers, lakes and aquifers. It is most common in dry and semi-arid lands.

Don't confuse with

• Meteorological drought: a shortage of rain (IMD: seasonal rain more than 25% below normal). It comes first. Hydrological drought is a shortage of stored water and comes after it.
• Agricultural drought: a shortage of soil moisture, so crops fail. It is linked to the crop season. Hydrological drought is about rivers, reservoirs and groundwater, not the soil.
• Ecological drought: water stress makes a natural ecosystem stop being productive. It is the last stage of the chain and can last after the rain returns.
• Jalkal vs trikal: jalkal is a shortage of water only. Trikal is a shortage of grain, fodder and water all together, and is the worst.

Prelims Hooks

• Hydrological drought comes after meteorological drought. It does not happen at the same time.
• It is shown by falling reservoir levels, groundwater levels and stream flow, not by the rainfall figure itself.
• Drought Manual hydrological indices: SFDI = stream flow, GWDI = groundwater, RSI = reservoir storage [2].
• Data sources: stream flow from CWC and India-WRIS; groundwater from CGWB; reservoirs from CWC and the state water departments [2].
• Order of the drought chain: meteorological → agricultural → hydrological → ecological.
• Trap: the hydrological indices are impact indicators (Trigger-2). The mandatory indicators (Trigger-1) are rainfall deviation or SPI, plus dry spell [2].

Mains Points

• Why short rain shortages quickly turn into hydrological drought in India:
• the monsoon is uncertain, and the rain-shadow interior and the monsoon margins get little rain
• groundwater is pumped out faster than rain refills it

• so Marathwada, Bundelkhand and Rajasthan face drying wells, tanks and rivers again and again

• Impacts (GS-I/III):

• jalkal hits drinking water, irrigation and livestock, and pushes people to migrate in distress
• people are forced to use contaminated water, which spreads gastroenteritis, cholera and hepatitis

• the poor and landless suffer most

• Policy responses (GS-III):

• Better declaration: the 2016/2020 Drought Manual uses hydrological data (SFDI, GWDI, RSI) along with satellite and soil-moisture data. This makes declarations faster and fairer and links them to NDRF or SDRF funds [2].
• Refilling the stores: finding new aquifers, building reservoirs, satellite mapping of river basins and groundwater, and rainwater harvesting.
• Jal Shakti Abhiyan: Catch the Rain: watershed development, renovation of traditional water bodies, and reuse and recharge structures [3].
• MGNREGS: its works build ponds and check dams.
• River interlinking and inter-basin transfer: these move water from surplus areas to short areas, but their ecological and legal costs need a careful check.

Related concepts

• Drought
• Meteorological drought
• Agricultural drought
• Ecological drought
• Drought-prone areas of India
• Akal, trinkal, jalkal and trikal
• Drought mitigation

Read more

• Droughts: types, drought-prone India and mitigation