Evaporation, Evapotranspiration & Infiltration Notes

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 $(e_{w} - e_{a})$	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

FormulaEvaporation formulas

Dalton's law: $E_{L} = C (e_{w} - e_{a})$

Meyer's formula (lake evaporation, mm/day):

E_{L} = K_{M} (e_{w} - e_{a}) (1 + \frac{u _{9}}{16})

$e_{w}$ , $e_{a}$ in mm of mercury; $u_{9}$ = monthly mean wind velocity (km/h) at about 9 m above ground; $K_{M}$ ≈ 0.36 for large deep waters and 0.50 for small shallow waters.

Wind speed at another height (1/7th power law): $u_{h} = C h^{1/7}$

Other approaches: water budget method, energy budget method and mass transfer method.

Evaporation pans

Pan	Pan coefficient $C_{p}$ (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 = $C_{p}$ × 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 ( $E T_{0}$ ) — PET of a standard grass (or alfalfa) surface; crop ET $= K_{c} \times E T_{0}$ .

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):

E_{t} = 2.54 K F, F = \sum \frac{P _{h} T _{f}}{100}

( $K$ = crop coefficient, $P_{h}$ = monthly percentage of annual daytime hours, $T_{f}$ = 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 $f_{p}$ — maximum rate at which a soil can absorb water at a given time.
Actual infiltration rate $f = f_{p}$ if rainfall intensity $i \geq f_{p}$ ; $f = i$ if $i < f_{p}$ .

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.

FormulaHorton's equation

f_{p} = f_{c} + (f_{0} - f_{c}) e^{- k_{h} t}

$f_{0}$ = initial infiltration capacity, $f_{c}$ = final steady capacity, $k_{h}$ = decay constant.

Cumulative infiltration from 0 to $t$ :

F = f_{c} t + \frac{f _{0} - f _{c}}{k _{h}} (1 - e^{- k_{h} t})

Other models: Philip $F = s t^{1/2} + K t$ ; Kostiakov $F = a t^{b}$ ; 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

FormulaInfiltration indices

φ-index — the constant rate of loss above which all rainfall becomes runoff (volume of rainfall above the φ line = volume of runoff):

R = \sum (i_{k} - ϕ) Δ t for i_{k} > ϕ

W-index — average infiltration rate during the period when rainfall intensity exceeds infiltration capacity, excluding initial losses:

W = \frac{P - R - I _{a}}{t _{e}}

( $t_{e}$ = duration of rainfall excess). W-index ≤ φ-index.

$W_{min}$ — for very wet conditions (initial losses negligible).

The φ-index is widely used for large catchments and flood estimation.

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Evaporation, Evapotranspiration & Infiltration