Blaney-Criddle equation
Encyclopedia
The Blaney–Criddle equation (named after H. F. Blaney and W. D. Criddle) is a method for estimating reference crop evapotranspiration
.
. When sufficient meteorological data is available the Penman–Monteith equation is usually preferred. The Blaney–Criddle equation is however ideal when only air temperature data is available for a site.
Given the coarse accuracy of the Blaney–Criddle equation it is recommended that it is used to calculate evapotranspiration
for periods of one month or greaterhttp://www.fao.org/docrep/X0490E/x0490e06.htm.
The equation calculates evapotranspiration
for a 'reference crop', which is taken as actively growing green grass of 8–15 cm heighthttp://www.fao.org/docrep/S2022E/s2022e07.htm.
Where:
ETo is the reference evapotranspiration [mm day−1] (monthly)
Tmean is the mean daily temperature [°C] given as Tmean = (Tmax + Tmin )/ 2
p is the mean daily percentage of annual daytime hours.
should be regarded as only broadly accurate. Rather than a precise measure of evapotranspiration
, the output of the equation is better though of as providing an order of magnitude http://www.fao.org/docrep/S2022E/s2022e07.htm
The inaccuracy of the equation is exacerbated by extreme variants of weather. In particular evapotranspiration
is known to be exaggerated by up to 40% in calm, humid, clouded areas and depreciated by 60% in windy, dry, sunny areashttp://www.fao.org/docrep/S2022E/s2022e07.htm.
Evapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
.
Usage
The Blaney–Criddle equation is a relatively simplistic method for calculating evapotranspirationEvapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
. When sufficient meteorological data is available the Penman–Monteith equation is usually preferred. The Blaney–Criddle equation is however ideal when only air temperature data is available for a site.
Given the coarse accuracy of the Blaney–Criddle equation it is recommended that it is used to calculate evapotranspiration
Evapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
for periods of one month or greaterhttp://www.fao.org/docrep/X0490E/x0490e06.htm.
The equation calculates evapotranspiration
Evapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
for a 'reference crop', which is taken as actively growing green grass of 8–15 cm heighthttp://www.fao.org/docrep/S2022E/s2022e07.htm.
Equation
ETo = p ·(0.46·Tmean + 8)Where:
ETo is the reference evapotranspiration [mm day−1] (monthly)
Tmean is the mean daily temperature [°C] given as Tmean = (Tmax + Tmin )/ 2
p is the mean daily percentage of annual daytime hours.
Accuracy and bias
Given the limited data input to the equation, the calculated evapotranspirationEvapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
should be regarded as only broadly accurate. Rather than a precise measure of evapotranspiration
Evapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
, the output of the equation is better though of as providing an order of magnitude http://www.fao.org/docrep/S2022E/s2022e07.htm
The inaccuracy of the equation is exacerbated by extreme variants of weather. In particular evapotranspiration
Evapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
is known to be exaggerated by up to 40% in calm, humid, clouded areas and depreciated by 60% in windy, dry, sunny areashttp://www.fao.org/docrep/S2022E/s2022e07.htm.
See also
- Jensen–Haise equation (M. E. Jensen and H. R. Haise, 1963)
- Penman–Monteith equation