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Every year, the Central Ground Water Board (CGWB) and the state governments work out how much groundwater India's aquifers take in and how much is pumped out. The 2025 assessment put annual recharge at 448.52 billion cubic metres and annual extraction at 247.22 billion cubic metres (Ministry of Jal Shakti via PIB, 29 January 2026). Working out numbers like these, and turning them into water plans, is the kind of work hydrologists and hydrogeologists do.
What does a hydrologist actually do?
A hydrologist studies how water moves through rain, rivers, soil and rock. A hydrogeologist specialises in water underground, held in layers of rock and sediment called aquifers. The work mixes field readings (well levels, river flow), water chemistry and computer models.
IIT Roorkee's Department of Hydrology shows the two main sides of the job. Its surface-water specialisation covers channel and fluvial hydraulics and hydrometeorology; its groundwater specialisation covers groundwater systems analysis, geophysical investigations and contamination modelling (IIT Roorkee admission brochure 2025-26).
The key number in Indian groundwater planning is the Stage of Ground Water Extraction: a year's extraction divided by the groundwater that can be extracted. For India as a whole it was 60.63% in 2025 (PIB). Each assessment unit, usually a block, gets a category from its own figure: below 70% is safe, 70 to 90% semi-critical, 90 to 100% critical, and above 100% over-exploited (same release).
These categories help decide where money goes. Under a September 2025 decision, at least 65% of MGNREGS funds in over-exploited and critical blocks, and 40% in semi-critical blocks, are to be spent on water conservation works (PIB). For the causes, read why groundwater levels are falling in many parts of India.
How is a block's groundwater status worked out?
Here is the ratio with made-up numbers for an imaginary block. It can extract 50 million cubic metres (MCM) a year. Farms pump 39 MCM, homes 5 and industry 2, so the total is 46 MCM.
Normal year: 46 / 50 = 92%. Critical.
A strong monsoon raises extractable water to 55 MCM. Same pumping: 46 / 55 = 83.6%. Semi-critical.
A drought cuts it to 42 MCM. Same pumping: 46 / 42 = 109.5%. Over-exploited.
In this example the pumping did not change; only the rain did. So drought planning means cutting demand (micro-irrigation, less thirsty crops) as well as adding supply (recharge structures, rainwater harvesting).
How water flows through an aquifer in the first place is the story of Henry Darcy and the 1856 discovery behind groundwater flow.
Earth science or civil engineering: which route should you take?
Both lead to water work. They differ in your first degree and the government exam you take.
On the earth science route:
Take physics, chemistry and maths in Class 11 and 12. That keeps both routes open.
Do a BSc with geology.
Do a master's in Geology, Applied Geology, Marine Geology or Hydrogeology. UPSC lists these subjects for the Central Ground Water Board's Scientist 'B' (Hydrogeology) and Assistant Hydrogeologist posts (UPSC Combined Geo-Scientist Examination 2027 notice).
Sit the Combined Geo-Scientist Examination: a preliminary exam, a main exam and an interview. For 2027, candidates must be 21 to 32 years old on 1 January 2027, with relaxations for some categories (same notice).
Vacancies are few. The 2027 notice lists a tentative 5 Scientist 'B' (Hydrogeology) posts and 7 Assistant Hydrogeologist posts (UPSC). Keep a second option open, such as research, state groundwater departments or consulting.
On the civil engineering route:
Do a BTech in civil engineering.
Take the Engineering Services Examination. Its civil category includes the Central Water Engineering (Group 'A') Service (UPSC Engineering Services Examination 2027 notice).
Or specialise with an M.Tech in Hydrology. IIT Roorkee's programme accepts civil, agricultural or environmental engineering graduates, and MSc graduates in subjects such as geology, physics or meteorology who studied maths in their BSc. Its PG Diploma and M.Tech share 59 seats, of which 29 are earmarked for candidates with a valid GATE score (IIT Roorkee brochure).
For dams, canals and floods, read water resources engineering in civil: dams, floods and jobs.
What should you study in Class 11 and 12 for hydrology?
Physics: pressure and fluid flow, the base of hydraulics.
Maths: rates of change, logarithms and statistics. Floods and droughts are probability questions.
Chemistry: solubility and pH, which you need to judge water quality.
Geography: the monsoon and rock types, if your school offers it.
If fluid pressure or flow rate feels shaky, the Fermi Concept Library gives each physics and maths concept its own lesson and its place in the map. When a numerical goes wrong, the Fermi Problem Library works through it step by step and explains the reasoning, so you can see which step broke.
This week, put a straight-sided jar outside as a rain gauge and log the rainfall each day. Careful measurement of water is at the heart of the job.
