Part 1 of 2
Evaporation, Evapotranspiration & Infiltration
Last reviewed 16 Sept 2026 · 7 min read
Abstractions
Abstractions are the parts of precipitation that do not become direct runoff: evaporation, transpiration, interception, depression storage and infiltration. Rainfall minus abstractions gives effective (excess) rainfall.
Evaporation
Evaporation is the process by which water changes from liquid to vapour at a free water surface below its boiling point. It is a cooling process (latent heat is absorbed).
Factors affecting evaporation
| Factor | Effect |
|---|---|
| Vapour pressure difference | Evaporation ∝ difference between saturation vapour pressure at water temperature and actual vapour pressure of air |
| Temperature | Higher water and air temperature → more evaporation |
| Wind speed | Removes saturated air → increases evaporation (up to a critical speed) |
| Atmospheric pressure | Lower pressure (high altitude) → more evaporation |
| Dissolved salts | Reduce evaporation (roughly 1% less for each 1% increase in specific gravity) |
| Depth and area of water body | Deep water bodies store heat, shifting evaporation to colder months |
Dalton's law:
Meyer's formula (lake evaporation, mm/day):
, in mm of mercury; = monthly mean wind velocity (km/h) at about 9 m above ground; ≈ 0.36 for large deep waters and 0.50 for small shallow waters.
Wind speed at another height (1/7th power law):
Other approaches: water budget method, energy budget method and mass transfer method.
Evaporation pans
| Pan | Pan coefficient (average) |
|---|---|
| Class A land pan (US) | 0.70 |
| ISI standard pan (modified Class A) | 0.80 |
| Colorado sunken pan | 0.78 |
| USGS floating pan | 0.80 |
Lake evaporation = × pan evaporation. Pans evaporate more than lakes because of their small size, heat exchange through the sides and differences in exposure.
The ISI (IMD) standard pan is a copper pan 1220 mm in diameter and 255 mm deep, painted white, covered with a wire mesh and mounted on a wooden stand.
Reduction of reservoir evaporation
- Reduction of surface area — deep reservoirs rather than wide, shallow ones.
- Mechanical covers — only for small tanks.
- Chemical films — monomolecular layers of cetyl alcohol (hexadecanol) or stearyl alcohol, which reduce evaporation without harming aquatic life.
- Wind breaks — tree lines on the windward side.
- Removing water-loving vegetation (phreatophytes) along the periphery.
Transpiration and evapotranspiration
- Transpiration — water taken up by plant roots and released as vapour through leaf stomata; measured on a single plant by a phytometer.
- Evapotranspiration (ET, consumptive use) — total evaporation from soil and water surfaces plus transpiration from vegetation.
- Potential evapotranspiration (PET) — ET when water supply to plants is unlimited.
- Actual evapotranspiration (AET) — ET under actual soil-moisture conditions; AET = PET only when soil moisture is at field capacity.
- Reference crop ET () — PET of a standard grass (or alfalfa) surface; crop ET .
Measurement and estimation
- Lysimeter — a tank of soil with vegetation, isolated from the surroundings; ET found from the water balance (weighing or non-weighing).
- Field plots — water balance of an actual field.
- Blaney–Criddle (monthly consumptive use, cm):
( = crop coefficient, = monthly percentage of annual daytime hours, = mean monthly temperature in °F.)
- Thornthwaite — temperature-based monthly PET.
- Penman (Penman–Monteith) — combines energy balance and aerodynamic terms; the most reliable physically based method (FAO recommended).
- Hargreaves — radiation and temperature based.
Interception and depression storage
- Interception — rainfall caught by vegetation and later evaporated; significant in forests and for light, short storms.
- Depression storage — water held in small surface depressions, later evaporating or infiltrating; must be filled before overland flow starts.
Infiltration
Infiltration is the entry of water into the soil surface; percolation is its downward movement through the soil towards the water table.
- Infiltration capacity — maximum rate at which a soil can absorb water at a given time.
- Actual infiltration rate if rainfall intensity ; if .
Factors affecting infiltration capacity
Soil texture and structure; initial moisture content (dry soil infiltrates faster); vegetative cover (increases); compaction by rain impact, traffic and grazing (decreases); entrapped air; water temperature (viscosity); quality of water (suspended silt clogs pores); land use and surface crusting.
= initial infiltration capacity, = final steady capacity, = decay constant.
Cumulative infiltration from 0 to :
Other models: Philip ; Kostiakov ; Green–Ampt (physically based).
Measurement of infiltration
- Flooding-type infiltrometer — a ring (about 30 cm diameter) driven into the soil; water added to keep a constant head; rate of addition gives infiltration. The double-ring infiltrometer (inner about 30 cm, outer about 60 cm) reduces lateral-spread error — readings are taken in the inner ring.
- Rainfall simulator — sprinklers apply artificial rain over a small plot; infiltration = rain − runoff; represents rain-impact effects better.
Infiltration indices
- φ-index — the constant rate of loss above which all rainfall becomes runoff (volume of rainfall above the φ line = volume of runoff):
- W-index — average infiltration rate during the period when rainfall intensity exceeds infiltration capacity, excluding initial losses:
( = duration of rainfall excess). W-index ≤ φ-index.
- — for very wet conditions (initial losses negligible).
The φ-index is widely used for large catchments and flood estimation.