The Blaney–Criddle equation (named after H. F. Blaney and W. D. Criddle) is a method for estimating reference crop
evapotranspiration
Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the Atmosphere of Earth, atmosphere. It covers both water evaporation (movement of w ...
.
Usage
The Blaney–Criddle equation is a relatively simplistic method for calculating
evapotranspiration
Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the Atmosphere of Earth, atmosphere. It covers both water evaporation (movement of w ...
. When sufficient meteorological data is available the
Penman–Monteith equation is usually preferred. However, the Blaney–Criddle equation is ideal when only air-temperature datasets are available for a site.
Given the coarse accuracy of the Blaney–Criddle equation, it is recommended that it be used to calculate
evapotranspiration
Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the Atmosphere of Earth, atmosphere. It covers both water evaporation (movement of w ...
for periods of one month or greater.
The equation calculates
evapotranspiration
Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the Atmosphere of Earth, atmosphere. It covers both water evaporation (movement of w ...
for a 'reference crop', which is taken as actively growing green grass of 8–15 cm height.
Equation
''ET
o'' = ''p'' ·(0.457·''T
mean'' + 8.128)
Where:
''ET
o'' is the reference evapotranspiration
m day−1(monthly)
''T
mean'' is the mean daily temperature
�Cgiven as ''T
mean = (T
max + T
min )/ 2''
''p'' is the mean daily percentage of annual daytime hours.
Accuracy and bias
Given the limited data input to the equation, the calculated
evapotranspiration
Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the Atmosphere of Earth, atmosphere. It covers both water evaporation (movement of w ...
should be regarded as only broadly accurate. Rather than a precise measure of
evapotranspiration
Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the Atmosphere of Earth, atmosphere. It covers both water evaporation (movement of w ...
, the output of the equation is better thought of as providing an order of magnitude.
The inaccuracy of the equation is exacerbated by extreme variants of weather. In particular
evapotranspiration
Evapotranspiration (ET) refers to the combined processes which move water from the Earth's surface (open water and ice surfaces, bare soil and vegetation) into the Atmosphere of Earth, atmosphere. It covers both water evaporation (movement of w ...
is known to be exaggerated by up to 40% in calm, humid, clouded areas and depreciated by 60% in windy, dry, sunny areas.
See also
*
Jensen–Haise equation (M. E. Jensen and H. R. Haise, 1963)
*
Penman–Monteith equation
External links
Rational Use of the FAO Blaney-Criddle Formula (Allen 1986)
Notes and references
{{DEFAULTSORT:Blaney-Criddle equation
Agronomy
Equations