The Blaney–Criddle equation (named after H. F. Blaney and W. D. Criddle) is a method for estimating reference crop
evapotranspiration
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
.
Usage
The Blaney–Criddle equation is a relatively simplistic method for calculating
evapotranspiration
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
. When sufficient meteorological data is available the
Penman–Monteith equation The Penman–Monteith equation approximates net evapotranspiration (ET) from meteorological data, as a replacement for direct measurement of evapotranspiration. The equation is widely used, and was derived by the United Nations Food and Agriculture ...
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) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
for periods of one month or greater.
The equation calculates
evapotranspiration
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
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) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
should be regarded as only broadly accurate. Rather than a precise measure of
evapotranspiration
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
, 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) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
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 The Penman–Monteith equation approximates net evapotranspiration (ET) from meteorological data, as a replacement for direct measurement of evapotranspiration. The equation is widely used, and was derived by the United Nations Food and Agriculture ...
External links
Rational Use of the FAO Blaney-Criddle Formula (Allen 1986)
Notes and references
{{DEFAULTSORT:Blaney-Criddle equation
Agronomy
Equations