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150 Chapter Five

diameter of 1.22 m and a depth of 0.25 m, is made The Penman formula for the estimation of eva-
of unpainted, galvanized metal and is generally poration from meteorological data is based on two
accepted as an international standard. Evaporation requirements which must be met if continuous eva-
tanks tend to give a more accurate measurement of poration is to occur. The first is that there must be a
true evaporation, unlike pans that are affected by supply of energy (radiation) to provide latent heat of
higher losses of vapour caused by heating of their vaporization and, second, there must be an aerody-
exposed walls and shallow water. Pan coefficients namic mechanism (wind and humidity) for removing
(correction factors) can be applied to correct measure- vapour, once produced. The Penman formula for
−1
ments to ‘true evaporation’. Pan coefficients for evaporation from open water, E , in mm day , is
o
the British standard tank and US NWS Class A pan given as:
range between 0.93–1.07 and 0.60–0.80, respectively
(WMO 1994). ∆ H + f u e − e )
()
(
For vegetated surfaces, water is lost by evapora- γ sat act
E = eq. 5.7
o
tion from bare soil and also by transpiration through ∆
+ 1
the leaf stomata of plants. The term evapotranspira- γ
tion is used to describe the combination of these
effects and is a significant process in terms of catch- where H = net radiation balance in mm of water
ment water balances, often being the principal loss equivalent, ∆ = rate of change of saturated vapour
of water from a catchment. pressure with temperature, γ = psychrometric or
The concept of potential evapotranspiration (PE) hygrometric constant (different values depending on
is defined as the amount of water that would be the temperature units and the method of ventilation
removed from a vegetated surface if sufficient water (aspiration) of the wet and dry bulb thermometers),
were available in the soil to meet the water demand. f(u) = aerodynamic coefficient (function of wind
The PE may be met in areas where the soil is satur- speed), e = saturated vapour pressure of air at
sat
ated, for example after a rainfall event or in an area temperature, t, e = actual vapour pressure of air at
act
of high water table, say in a groundwater discharge temperature, t.
area; otherwise, the actual evapotranspiration rate Penman’s formula for evaporation (eq. 5.7) has
will be less than PE. A direct measurement of evapo- been adapted to calculate potential evapotranspira-
transpiration can be made using a lysimeter, a large tion by the application of empirically derived factors.
container holding a monolith of soil and plants that is Penman introduced the empirical formula:
set outdoors. Evapotranspiration is estimated for
the lysimeter from the balance of precipitation and PE = fE eq. 5.8
o
irrigation inputs, change in soil moisture content and
loss of water as soil drainage. Alternatively, field where f is a seasonal correction factor that includes
estimates of soil moisture content can be combined the effects of differing solar insolation intensity, day
with precipitation, river discharge and groundwater length, plant stomatal response and geometry. For
monitoring data to calculate catchment-scale evapo- example, the evaporation rate from a freshly wetted
transpiration losses. bare soil is about 90% of that from an open water
PE is dependent on the evaporative capacity of surface exposed to the same weather conditions.
the atmosphere and can be calculated theoretically For a grassed surface in temperate latitudes the value
using meteorological data. The most commonly used of PE is, on average, about 75% of the open water
methods for calculating PE are those of Blaney and evaporation rate. A more process-based approach
Criddle (1962) and Thornthwaite (1948), which are to calculating PE, following Penman’s method
based on empirical correlations between evapotran- and extended by experimental work, is given by the
spiration and climatic factors, and Penman (1948) and Penman–Monteith formula (eq. 5.9) that incorporates
Penman–Monteith (Monteith 1965, 1985) which are canopy stomatal and aerodynamic resistance effects,
energy-budget approaches requiring further meteo- to calculate evapotranspiration rate in mm day −1 as
rological data. follows: