India's water endowment: precipitation, surface water and lagoons

Detail

Water as a resource: basic nature

• Water is a cyclic resource. "Cyclic" means the same water keeps coming back through the hydrological cycle (water cycle).
• Sun heats seas and land → water evaporates and rises as vapour.
• Vapour cools and condenses → it falls back as rain or snow (precipitation).

• Water flows into rivers, lakes and the ground → it returns to the sea, and the cycle starts again.

• So water is a renewable resource (Class 10). Nature refills it again and again. But it can still run short if we use it faster than it is refilled, or if we pollute it.

• Water is an abiotic resource. "Abiotic" means it is not living. Soil, minerals and air are other abiotic resources. Plants and animals are biotic (living) resources.
• Global share:
• Water covers about 71% of the earth's surface.
• Only about 3% of all water is freshwater (water that is not salty).
• Most of this freshwater is locked in ice sheets, glaciers and deep groundwater. Only a small part is easy to reach in rivers and lakes.

India's share of the world

Item	India's share of world total
Area	~2.45%
Water resources	~4%
Population	>17% (NCERT); now about 18%

• Mismatch: about 18% of the world's people depend on only 4% of the world's water. This is the root of India's water stress (water stress: when demand for water comes close to, or goes beyond, the supply that can be used).

From rainfall to usable water: the funnel

• Step 1: Precipitation (input)
• NCERT: rain and snow bring about 4,000 cubic km a year.

• CWC's 2019 reassessment (30-year study period 1985–2015): mean annual rainfall over India's river basins was 3,880 BCM [2]. (BCM = billion cubic metres. 1 BCM = 1 cubic km.)

• Step 2: Total water resource (what reaches rivers and aquifers)

• A large part of the rain is lost. It evaporates, or plants take it up and release it (evapotranspiration: water lost to the air by evaporation plus release from leaves).
• NCERT: surface water plus replenishable groundwater (groundwater refilled each year by rain soaking into the ground) = 1,869 cubic km.

• CWC 2019: average annual water resource of India's river basins = 1,999.20 BCM (period 1985–2015) [2]. (NCERT: 1,869 cubic km, older CWC estimate.)

• Step 3: Utilisable water resource (what we can actually use)

• Utilisable water: the share we can take out in practice with dams, canals and wells.
• The rest is lost to floods, flows to the sea, or sits in places where we cannot store it.
• NCERT: only about 60% can be used = 1,122 cubic km.
• CWC 2019: utilisable water by the conventional approach = 1,137 BCM, made up of 690 BCM surface water + 447 BCM replenishable groundwater [2]. (NCERT: 1,122 cubic km = 690 surface + 432 groundwater.)

Stage	NCERT (older CWC)	CWC reassessment 2019 [2]
Precipitation	~4,000 km³	3,880 BCM (mean, 1985–2015)
Total water resource	1,869 km³	1,999.20 BCM
Utilisable surface water	690 km³	690 BCM
Utilisable groundwater	432 km³	447 BCM
Total utilisable	1,122 km³	1,137 BCM

• Key point: the surface-water usable figure (690) stayed the same. The rise in the total usable figure came from groundwater (432 → 447) [2].

Surface water resources

• Surface water resources: water found on top of the ground. Its four main sources are rivers, lakes, ponds and tanks. (A tank is a small reservoir made by building an earthen bund across a slope or stream. Tanks are common in peninsular India.)
• Extent of the river network:
• India has about 10,360 rivers and tributaries that are longer than 1.6 km each.

• Their mean annual flow in all basins together is about 1,869 cubic km.

• Only about 690 cubic km (32%, NCERT) can be used. Three reasons:

• Topography (relief): steep Himalayan slopes and narrow gorges make large storage hard in many places. Flat deltas have few good dam sites.
• Hydrology: much of the flow comes in sudden floods and rushes to the sea before it can be stored.

• Monsoon concentration: most rain falls in about four months (June–September). So rivers carry huge flows for a short time and little water in the dry season.

• What decides a river's flow:

• The size of its catchment area (catchment area: all the land that drains into a river).
• The amount of rainfall in that catchment.
• Large catchment + heavy rain → large flow (for example, the Brahmaputra).

Uneven distribution across basins

Basin group	Share of India's area	Share of surface water	How much of the flow is already used
Ganga + Brahmaputra + Barak	~one-third	~60%	Ganga and Brahmaputra: largely untapped
Godavari, Krishna, Kaveri (peninsular)	Smaller	Much lower	Most of the flow already harnessed

• Why the north-east and the Ganga plain are water-rich:
• The rivers are perennial (they flow all year): they are fed by monsoon rain and by Himalayan snow and glaciers.

• The Brahmaputra and Barak basins in north-east India get very heavy monsoon rain.

• Why the Peninsular basins are stressed:

• The rivers depend on rain alone, so they are seasonal.
• Their catchments are smaller and many lie in rain-shadow areas (the dry side of a mountain, away from the wind; for example, east of the Western Ghats).

• Many dams and canals have been built, so little extra water is left. This also leads to inter-state water disputes (for example, over the Kaveri).

• Basin map: see Fig. 4.1 of the NCERT chapter. For river origins, courses and tributaries, see india-drainage-rivers.

Lagoons and backwaters

• Lagoon: a shallow body of coastal water that is cut off from the sea by a sandbar or spit (a long ridge of sand) but still linked to it by one or more narrow openings.
• Backwaters: a connected network of lagoons, lakes and canals running parallel to the coast. They are best known in Kerala.
• How they form:
• Waves and currents carry sand along the coast.
• The sand builds bars or spits across bays and river mouths.
• Sea water and river water get trapped behind the bar → a lagoon forms.

• Such coasts are indented (the coastline has many inlets and curves).

• Main lagoons and backwaters:

Water body	State	Coast	Note
Vembanad	Kerala	West (Malabar) coast	Largest lake of Kerala, near Kochi and Alappuzha; borders Kuttanad
Ashtamudi	Kerala	West coast	Near Kollam
Chilika	Odisha	East coast	South-west of the Mahanadi delta; India's largest coastal lagoon
Pulicat	Andhra Pradesh–Tamil Nadu border	East (Coromandel) coast	North of Chennai
Lagoons and lakes of West Bengal	West Bengal	East coast	In the Ganga delta area

• Water quality: the water is brackish (a mix of salt water and fresh water, so it is less salty than the sea).
• Uses:
• Fishing: fish, prawns and crabs.

• Irrigation of certain salt-tolerant paddy varieties and coconut. Kuttanad, near Vembanad, grows paddy below sea level (see land-use-farming-systems).

• Limit: brackish water cannot be used for drinking or for most crops. So this "water endowment" adds little to the utilisable freshwater total.

Census of Water Bodies (first edition, 2023)

• Census of Water Bodies: the first nationwide count of India's water bodies, done by the Ministry of Jal Shakti and released in 2023. It counted ponds, tanks, lakes and reservoirs and recorded each one's use and condition.
• Main results (2023):
• About 24.2 lakh water bodies were counted.
• Most are ponds in rural areas.
• West Bengal has the largest number.

• A share of them is encroached (illegally occupied by buildings, farms or roads) or no longer in use (dried up, silted or polluted).

• Why it matters: it gives the first baseline (a starting record to measure change against) for planning tank and pond revival under water-conservation drives such as the Jal Shakti Abhiyan (see Section 12).

Prelims Hooks

• India has ~2.45% of the world's area, ~4% of its water and ~18% of its people (NCERT: >17%).
• Water is a cyclic, renewable, abiotic resource. Only about 3% of the earth's water is freshwater.
• CWC 2019 reassessment: average annual water resource = 1,999.20 BCM (1985–2015). Utilisable = 1,137 BCM (690 surface + 447 groundwater) [2]. (NCERT: 1,869 total; 1,122 utilisable = 690 + 432.)
• Trap: the usable surface-water figure is 690 in both the old and the 2019 estimates. Only the groundwater figure changed [2].
• About 10,360 rivers longer than 1.6 km. Mean annual flow is about 1,869 cubic km, but only about 32% (690 cubic km) is usable (NCERT).
• The Ganga–Brahmaputra–Barak basins cover about one-third of the area but hold about 60% of surface water.
• Already heavily used: Godavari, Krishna, Kaveri. Largely untapped: Ganga, Brahmaputra.
• Lagoon–state pairs: Vembanad, Ashtamudi → Kerala; Chilika → Odisha; Pulicat → Andhra Pradesh–Tamil Nadu border.
• Lagoon water is brackish. It is used for fishing, certain paddy varieties and coconut, not for drinking.
• First Census of Water Bodies (2023, Ministry of Jal Shakti): about 24.2 lakh water bodies. Most are rural ponds, and West Bengal has the most.

Mains Points

• Paradox of plenty (GS-III): about 2,000 BCM of water each year, but only about 1,137 BCM is usable [2].
• Rain is packed into the monsoon months, and relief limits where water can be stored.

• So India is short of water in space and time even though its total supply looks large. More storage, groundwater recharge and rainwater harvesting are the answer.

• Spatial imbalance (GS-I): about 60% of surface water is in the Ganga–Brahmaputra–Barak basins, while the peninsular basins are already over-used.

• This explains floods in the north-east, shortages and Kaveri-type disputes in the south, and calls for inter-basin water transfer (the river-linking debate).

• Brackish coastal water bodies (GS-I/III): Vembanad, Chilika and Pulicat support fishing and special farming systems (such as Kuttanad).

• They are threatened by siltation, encroachment and salt-water intrusion, so they need wetland-based management.

• Traditional water bodies (GS-III): the 2023 Census of Water Bodies shows that most of India's 24.2 lakh water bodies are rural ponds and that some are encroached or unused.

• Reviving them through community-led programmes, using the Census as the baseline, is a low-cost way to recharge groundwater.