The concept of potential evapotranspiration (ET) estimates the ability of water lost from the soil and vegetation in an area due to evaporation and transpiration. It assumes that there is always enough water in the ET area to meet the demand for evapotranspiration. However, the characteristics of the ET area, such as whether it is covered with vegetation or bare soil, can affect the amount of evapotranspiration that occurs. In order to accurately estimate potential ET, we need to consider these characteristics.
But we may not always have access to the necessary information or effective methods to do this.
In these cases, we can use a simplified method known as reference ET.
This method defines the ET area using certain fixed characteristics, such as those provided by the evatransPotential_FAO56()
function.
In this situation, we need to provide factors to account for the differences between the actual ET area and the reference ET area.
Usage
evatransPotential_TurcWendling(
atmos_temperature_Cel,
atmos_solarRadiat_MJ,
param_evatrans_tur_k
)
evatransPotential_Linacre(
atmos_temperature_Cel,
atmos_relativeHumidity_1,
land_latitude_Degree,
land_elevation_m,
land_albedo_1
)
evatransPotential_FAO56(
atmos_temperature_Cel,
atmos_vaporPress_hPa,
atmos_saturatVaporPress_hPa,
atmos_netRadiat_MJ,
atmos_windSpeed2m_m_s,
land_elevation_m
)
Arguments
- atmos_temperature_Cel
(Cel) the average air temperature in the time phase
- atmos_solarRadiat_MJ
(MJ/m2/TS) the solar radiation that actually reaches the earths surface
- param_evatrans_tur_k
<0.6, 1> parameter for
evatransPotential_TurcWendling()
, higher value when closer to the sea- atmos_relativeHumidity_1
(0, 1) relative humidity
- land_latitude_Degree
(degree) average latitude
- land_elevation_m
(m) average elevation
- land_albedo_1
<0, 1> albedo of the region
- atmos_vaporPress_hPa
(hPa) actual vapour pressure, can be calculated by
atmos_VaporPress()
- atmos_saturatVaporPress_hPa
(hPa) saturation vapour pressure at
atmos_temperature_Cel
, can be calculated byatmos_SaturatVaporPress()
- atmos_netRadiat_MJ
(MJ/m2/TS) the balance between the energy absorbed, reflected and emitted by the earths surface or the difference between the incoming net shortwave (Rns) and the net outgoing longwave (Rnl) radiation
- atmos_windSpeed2m_m_s
(m/s) wind speed at 2 m above ground surface
Details
_TurcWendling (WENDLING et al. 1991) : consider only the radiation and temperature as the main factors. Ep=(100Rs+3.875thk)⋅(T+22)150(T+123) where
Ep is potential ET,
atmos_potentialEvatrans_mm
Rs is solar radiation,
atmos_solarRadiat_MJ
th is time step in hour,
time_step_h
T is average air temperature,
atmos_temperature_Cel
k is
param_evatrans_tur_k
_Linacre (Linacre 1977) : consider only the temperature as the main factors. Ep=100(0.75−α)(T+0.006z)100−ϕ+15(T−Td)80−T Td=T−20(1−HR) where
α is albedo,
land_albedo_1
z is elevation,
land_elevation_m
Td is dewpoint temperature,
HR is relative humidity,
atmos_relativeHumidity_1
_FAO56 (Allen et al. 1998) : consider not only radiation and temperature but also other variable like wind speed as the main factors. Ep=0.408Δ(Rn−G)+γ900T+273u2(es−ea)Δ+γ(1+0.34u2) where
Δ is slope vapour pressure curve (kPa °C-1)
Rn is net radiation,
atmos_netRadiat_MJ
G is soil heat flux density
u2 is wind speed at 2 m height,
atmos_windSpeed2m_m_s
es is saturation vapour pressure,
atmos_saturatVaporPress_hPa
ea is actual vapour pressure,
atmos_vaporPress_hPa
γ is psychrometric constant
References
Allen RG, Pereira LS, Raes D, Smith M (1998).
“Crop Evapotranspiration. Guidelines for Computing Crop Water Requirements.”
FAO Irrigation and Drainage Paper (FAO).
Linacre ET (1977).
“A Simple Formula for Estimating Evaporation Rates in Various Climates, Using Temperature Data Alone.”
Agricultural Meteorology, 18(6), 409--424.
ISSN 0002-1571, doi:10.1016/0002-1571(77)90007-3
.
WENDLING U, SCHELLIN H, THOMÄ M (1991).
“Bereitstellung von Täglichen Informationen Zum Wasserhaushalt Des Bodens Für Die Zwecke Der Agrarmeteorologischen Beratung.”
Bereitstellung von täglichen Informationen zum Wasserhaushalt des Bodens für die Zwecke der agrarmeteorologischen Beratung, 41(6), 468--475.
ISSN 0084-5361.