Last reviewed 16 Sept 2026 · 8 min read
Hydrology and the hydrological cycle
Hydrology is the science of the occurrence, circulation and distribution of water on the earth. Engineering hydrology applies it to the design and operation of water-resources projects — estimating floods, yields, storage and groundwater.
The hydrological cycle is the continuous circulation of water between the oceans, the atmosphere and the land, driven by solar energy and gravity:
- Evaporation from oceans and water bodies, and transpiration from plants.
- Condensation into clouds and transport by winds.
- Precipitation on land and sea.
- On land: interception, depression storage, infiltration into the soil, percolation to groundwater, surface runoff, interflow and baseflow back to streams and oceans.
The cycle has no beginning or end, and the total quantity of water is essentially constant.
For a catchment over a time interval:
= precipitation, = surface runoff, = net groundwater outflow, = evaporation, = transpiration, = change in storage. Simply: Inflow − Outflow = Change in storage.
Precipitation
Forms
| Form | Description |
|---|---|
| Rain | Water drops mostly larger than 0.5 mm |
| Drizzle | Fine drops smaller than 0.5 mm, low intensity (below about 1 mm/h) |
| Snow | Ice crystals, usually combined as flakes |
| Glaze | Rain or drizzle freezing on contact with cold ground |
| Sleet | Frozen raindrops (falling through air below 0 °C) |
| Hail | Lumps of ice (> 8 mm) formed in violent convective clouds |
| Dew, frost, fog, mist | Condensation forms at or near the ground |
Rainfall intensity is commonly classed as light (up to about 2.5 mm/h), moderate (about 2.5–7.5 mm/h) and heavy (above about 7.5 mm/h).
Types of precipitation (by the cause of air lifting)
- Convective — heated air near the ground rises, cools and condenses; intense, short-duration, local storms (common in tropical summers).
- Orographic — moist air forced up by mountain barriers; heavy rain on the windward side and a rain shadow on the leeward side (Western Ghats, Himalayan foothills).
- Cyclonic — lifting due to low-pressure systems: frontal (warm and cold fronts) and non-frontal (convergence into a low, e.g. tropical cyclones and monsoon depressions).
In India the south-west monsoon (June–September) brings the major part of the annual rainfall for most of the country; Tamil Nadu receives much of its rainfall from the north-east monsoon (October–December).
Measurement of rainfall
| Rain gauge | Type | Features |
|---|---|---|
| Symons' gauge | Non-recording | Funnel over a receiving bottle; measured once a day (in India at 8:30 a.m. IST) with a graduated glass |
| Standard (FRP) rain gauge | Non-recording | Used by IMD; collector and bottle measured daily |
| Tipping-bucket gauge | Recording | A pair of small buckets tips for each fixed rainfall increment (commonly 0.25 mm); gives intensity; suited to telemetry |
| Weighing-bucket gauge | Recording | Weight of collected water plotted against time — mass curve; records snow and hail too |
| Natural siphon (float) gauge | Recording | Float rises with collected water, pen records on a drum chart; empties by siphon — the type widely used in India |
Other methods: radar (areal coverage), satellite estimation.
Siting a rain gauge: on level open ground; the distance from any obstruction should be at least about twice its height; the rim kept horizontal and a little above ground; enclosure protected by a fence.
Rain gauge network
The World Meteorological Organization recommends roughly: flat regions of temperate, Mediterranean and tropical zones — 1 station per 600–900 km² (ideal); mountainous regions — 1 per 100–250 km²; arid and polar zones — 1 per 1500–10 000 km². At least about 10% of gauges should be self-recording.
= allowable percentage error in mean rainfall (usually 10%); from the existing gauges.
Preparation of rainfall data
Estimating missing data
For station X with missing record and neighbouring stations A, B, C (with normal annual precipitation ):
- Arithmetic mean method — if the normal annual precipitations of the neighbouring stations are within 10% of that of X: .
- Normal ratio method — otherwise:
Consistency — double mass curve
A change in gauge location, exposure or observation method makes the record inconsistent. The accumulated annual rainfall of the station is plotted against the accumulated mean of a group of base stations. A break in slope reveals the inconsistency; the record is corrected by
( = corrected slope, i.e. the slope of the period to be retained; = original slope of the period being corrected.)