Hydrological Cycle
This domain tests the water balance and the processes that move rain into soil, streams and groundwater. Questions often ask where water goes after monsoon rain and how recharge differs from runoff.
Core rules
- The catchment water balance is P = ET + R + I + ΔS, where P is precipitation, ET evapotranspiration, R runoff, I infiltration and ΔS storage change.
- In Maharashtra the monsoon delivers most annual rain in a few intense spells. High-intensity rain favours runoff; gentle rain and vegetated soils favour infiltration.
- Groundwater recharge is slow and lagged; it is not the same as infiltration during one storm. Streamflow includes baseflow from groundwater.
Worked example
A 1 km2 catchment receives 800 mm rain. ET is 500 mm, runoff 150 mm and infiltration 150 mm. In mm over the catchment: 800 = 500 + 150 + 150 + ΔS, so ΔS = 0. If ET rises to 550 mm with the same rain and runoff, infiltration or storage must fall by 50 mm unless runoff increases. That is 50 mm × 1,000,000 m2 = 50,000 m3 less water entering storage.
Traps
- Treating recharge as instantaneous after rain.
- Ignoring evapotranspiration in the balance.
- Confusing runoff with streamflow, which includes baseflow.
- Assuming all infiltration reaches groundwater; some is held in soil and later evaporates.