evapotranspiration¶
PENMAN potential evaporation and transpiration with CO₂ correction.
Computes Penman reference fluxes for an open-water, bare-soil, and closed canopy reference, then partitions them into potential canopy transpiration and potential soil evaporation according to the fraction of incoming light intercepted by the crop.
References
Penman (1948) with the van Kraalingen modifications used by SIMPLACE
PotentialEvapoTranspiration.java and LintulFunctions.PENMAN.
Outputs
E0— potential evaporation from open water [mm d⁻¹]ES0— potential evaporation from bare soil [mm d⁻¹]ETC— potential transpiration of a closed canopy, CO₂-corrected [mm d⁻¹]
Equations
Canopy transpiration and soil evaporation are split by the fractional light interception \(F_{\text{INT}}\):
\[
P_t = \text{CFET} \cdot \text{ETC} \cdot F_{\text{INT}}
\]
\[
P_s = \text{ES}_0 \cdot (1 - F_{\text{INT}})
\]
where CFET is a crop-specific transpiration correction factor.
PotentialEvapoTranspiration (Module)
¶
PENMAN-based potential evapotranspiration calculation.
Implements the full PENMAN formula per SIMPLACE PotentialEvapoTranspiration.java. Outputs reference ET (E0, ES0, ETC) which are then split into potential transpiration and soil evaporation based on light interception.
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
altitude |
float |
Station altitude [m] (default 0). |
0.0 |
cfet |
float |
Crop-specific correction factor for transpiration (default 1.0). |
1.0 |
co2 |
float |
Atmospheric CO2 concentration [ppm] (default 370). |
370.0 |
Source code in torchcrop/processes/evapotranspiration.py
class PotentialEvapoTranspiration(nn.Module):
"""PENMAN-based potential evapotranspiration calculation.
Implements the full PENMAN formula per SIMPLACE PotentialEvapoTranspiration.java.
Outputs reference ET (E0, ES0, ETC) which are then split into potential
transpiration and soil evaporation based on light interception.
Args:
altitude: Station altitude [m] (default 0).
cfet: Crop-specific correction factor for transpiration (default 1.0).
co2: Atmospheric CO2 concentration [ppm] (default 370).
"""
def __init__(
self,
altitude: float = 0.0,
cfet: float = 1.0,
co2: float = 370.0,
) -> None:
super().__init__()
self.altitude = altitude
self.cfet = cfet
self.co2 = co2
# CO2 correction table from SIMPLACE cET0CorrectionTableCo2 / cET0CorrectionTableFactor.
self.register_buffer(
"co2_table",
torch.tensor(
[
[40.0, 1.05],
[360.0, 1.00],
[720.0, 0.95],
[1000.0, 0.92],
[2000.0, 0.92],
]
),
)
def forward(
self,
tmin: torch.Tensor,
tmax: torch.Tensor,
wind: torch.Tensor,
vap: torch.Tensor,
avrad: torch.Tensor,
atmtr: torch.Tensor,
frac_int: torch.Tensor,
) -> dict[str, torch.Tensor]:
"""Compute PENMAN potential ET and split into canopy/soil fluxes.
Args:
tmin: Minimum daily air temperature [°C], shape ``[B]``.
tmax: Maximum daily air temperature [°C], shape ``[B]``.
wind: Average wind speed [m s⁻¹], shape ``[B]``.
vap: Vapour pressure [kPa], shape ``[B]``.
avrad: Daily total irradiation [J m⁻² d⁻¹], shape ``[B]``.
atmtr: Atmospheric transmission fraction [-], shape ``[B]``.
frac_int: Fractional light interception [-], shape ``[B]``.
Returns:
Dict of ``[B]`` tensors:
* ``e0`` [mm d⁻¹] — Potential evaporation from open water.
* ``es0`` [mm d⁻¹] — Potential evaporation from bare soil.
* ``etc`` [mm d⁻¹] — Potential transpiration (CO2-corrected).
* ``ptran`` [mm d⁻¹] — Potential canopy transpiration (= CFET * ETC * frac_int).
* ``pevap`` [mm d⁻¹] — Potential soil evaporation (= ES0 * (1 - frac_int)).
"""
# Constants from PENMAN formula (SIMPLACE LintulFunctions.PENMAN)
A = 0.20
B = 0.56
REFCFW = 0.05 # Albedo for water
REFCFS = 0.15 # Albedo for soil
REFCFC = 0.25 # Albedo for canopy
LHVAP = 2.45e6 # Latent heat of evaporation [J kg-1]
STBC = 4.9e-3 # Stefan-Boltzmann constant [J m-2 d-1 K-4]
PSYCON = 0.000662 # Psychrometric constant [K-1]
# Convert vapour pressure from kPa to mbar (1 kPa = 10 mbar)
vap_mbar = vap * 10.0
# Average daily temperature
tmpa = (tmin + tmax) / 2.0
# Temperature difference
tdif = tmax - tmin
# Wind function coefficient (depends on temperature range)
bu = 0.54 + 0.35 * torch.clamp((tdif - 12.0) / 4.0, min=0.0, max=1.0)
# Barometric pressure [mbar]
pbar = 1013.0 * torch.exp(-0.034 * self.altitude / (tmpa + 273.0))
# Psychrometric constant [mbar K-1]
gamma = PSYCON * pbar
# Saturated vapour pressure [mbar] per Goudriaan (1977)
svap = 6.11 * torch.exp(17.4 * tmpa / (tmpa + 239.0))
# Measured vapour pressure should not exceed saturated vapour pressure
vap_clamped = torch.clamp(vap_mbar, max=svap)
# Slope of saturation vapour pressure curve [mbar K-1]
delta = 239.0 * 17.4 * svap / torch.clamp((tmpa + 239.0) ** 2, min=1e-6)
# Relative sunshine duration (from Angstrom formula)
relssd = torch.clamp((atmtr - A) / B, min=0.0, max=1.0)
# Net outgoing long-wave radiation [J m-2 d-1]
rb = (
STBC
* (tmpa + 273.0) ** 4
* (0.56 - 0.079 * torch.sqrt(torch.clamp(vap_clamped, min=0.0)))
* (0.1 + 0.9 * relssd)
)
# Net absorbed radiation [J m-2 d-1]
rnw = avrad * (1.0 - REFCFW) - rb
rns = avrad * (1.0 - REFCFS) - rb
rnc = avrad * (1.0 - REFCFC) - rb
# Evaporative demand of atmosphere [mm d-1]
ea = 0.26 * (svap - vap_clamped) * (0.5 + bu * wind)
eac = 0.26 * (svap - vap_clamped) * (1.0 + bu * wind)
# PENMAN formula [mm d-1]
e0 = (delta * (rnw / LHVAP) + gamma * ea) / torch.clamp(delta + gamma, min=1e-6)
es0 = (delta * (rns / LHVAP) + gamma * ea) / torch.clamp(
delta + gamma, min=1e-6
)
et0 = (delta * (rnc / LHVAP) + gamma * eac) / torch.clamp(
delta + gamma, min=1e-6
)
# Ensure non-negative
e0 = torch.clamp(e0, min=0.0)
es0 = torch.clamp(es0, min=0.0)
et0 = torch.clamp(et0, min=0.0)
# CO2 correction for ET0
co2_factor = interpolate(self.co2_table, torch.full_like(e0, self.co2))
etc = et0 * co2_factor
# Potential transpiration and soil evaporation split by light interception
ptran = torch.clamp(self.cfet * etc * frac_int, min=0.0001)
pevap = es0 * (1.0 - frac_int)
return {
"e0": e0,
"es0": es0,
"etc": etc,
"ptran": ptran,
"pevap": pevap,
}
forward(self, tmin, tmax, wind, vap, avrad, atmtr, frac_int)
¶
Compute PENMAN potential ET and split into canopy/soil fluxes.
Parameters:
| Name | Type | Description | Default |
|---|---|---|---|
tmin |
torch.Tensor |
Minimum daily air temperature [°C], shape |
required |
tmax |
torch.Tensor |
Maximum daily air temperature [°C], shape |
required |
wind |
torch.Tensor |
Average wind speed [m s⁻¹], shape |
required |
vap |
torch.Tensor |
Vapour pressure [kPa], shape |
required |
avrad |
torch.Tensor |
Daily total irradiation [J m⁻² d⁻¹], shape |
required |
atmtr |
torch.Tensor |
Atmospheric transmission fraction [-], shape |
required |
frac_int |
torch.Tensor |
Fractional light interception [-], shape |
required |
Returns:
| Type | Description |
|---|---|
Dict of ``[B]`` tensors |
|
Source code in torchcrop/processes/evapotranspiration.py
def forward(
self,
tmin: torch.Tensor,
tmax: torch.Tensor,
wind: torch.Tensor,
vap: torch.Tensor,
avrad: torch.Tensor,
atmtr: torch.Tensor,
frac_int: torch.Tensor,
) -> dict[str, torch.Tensor]:
"""Compute PENMAN potential ET and split into canopy/soil fluxes.
Args:
tmin: Minimum daily air temperature [°C], shape ``[B]``.
tmax: Maximum daily air temperature [°C], shape ``[B]``.
wind: Average wind speed [m s⁻¹], shape ``[B]``.
vap: Vapour pressure [kPa], shape ``[B]``.
avrad: Daily total irradiation [J m⁻² d⁻¹], shape ``[B]``.
atmtr: Atmospheric transmission fraction [-], shape ``[B]``.
frac_int: Fractional light interception [-], shape ``[B]``.
Returns:
Dict of ``[B]`` tensors:
* ``e0`` [mm d⁻¹] — Potential evaporation from open water.
* ``es0`` [mm d⁻¹] — Potential evaporation from bare soil.
* ``etc`` [mm d⁻¹] — Potential transpiration (CO2-corrected).
* ``ptran`` [mm d⁻¹] — Potential canopy transpiration (= CFET * ETC * frac_int).
* ``pevap`` [mm d⁻¹] — Potential soil evaporation (= ES0 * (1 - frac_int)).
"""
# Constants from PENMAN formula (SIMPLACE LintulFunctions.PENMAN)
A = 0.20
B = 0.56
REFCFW = 0.05 # Albedo for water
REFCFS = 0.15 # Albedo for soil
REFCFC = 0.25 # Albedo for canopy
LHVAP = 2.45e6 # Latent heat of evaporation [J kg-1]
STBC = 4.9e-3 # Stefan-Boltzmann constant [J m-2 d-1 K-4]
PSYCON = 0.000662 # Psychrometric constant [K-1]
# Convert vapour pressure from kPa to mbar (1 kPa = 10 mbar)
vap_mbar = vap * 10.0
# Average daily temperature
tmpa = (tmin + tmax) / 2.0
# Temperature difference
tdif = tmax - tmin
# Wind function coefficient (depends on temperature range)
bu = 0.54 + 0.35 * torch.clamp((tdif - 12.0) / 4.0, min=0.0, max=1.0)
# Barometric pressure [mbar]
pbar = 1013.0 * torch.exp(-0.034 * self.altitude / (tmpa + 273.0))
# Psychrometric constant [mbar K-1]
gamma = PSYCON * pbar
# Saturated vapour pressure [mbar] per Goudriaan (1977)
svap = 6.11 * torch.exp(17.4 * tmpa / (tmpa + 239.0))
# Measured vapour pressure should not exceed saturated vapour pressure
vap_clamped = torch.clamp(vap_mbar, max=svap)
# Slope of saturation vapour pressure curve [mbar K-1]
delta = 239.0 * 17.4 * svap / torch.clamp((tmpa + 239.0) ** 2, min=1e-6)
# Relative sunshine duration (from Angstrom formula)
relssd = torch.clamp((atmtr - A) / B, min=0.0, max=1.0)
# Net outgoing long-wave radiation [J m-2 d-1]
rb = (
STBC
* (tmpa + 273.0) ** 4
* (0.56 - 0.079 * torch.sqrt(torch.clamp(vap_clamped, min=0.0)))
* (0.1 + 0.9 * relssd)
)
# Net absorbed radiation [J m-2 d-1]
rnw = avrad * (1.0 - REFCFW) - rb
rns = avrad * (1.0 - REFCFS) - rb
rnc = avrad * (1.0 - REFCFC) - rb
# Evaporative demand of atmosphere [mm d-1]
ea = 0.26 * (svap - vap_clamped) * (0.5 + bu * wind)
eac = 0.26 * (svap - vap_clamped) * (1.0 + bu * wind)
# PENMAN formula [mm d-1]
e0 = (delta * (rnw / LHVAP) + gamma * ea) / torch.clamp(delta + gamma, min=1e-6)
es0 = (delta * (rns / LHVAP) + gamma * ea) / torch.clamp(
delta + gamma, min=1e-6
)
et0 = (delta * (rnc / LHVAP) + gamma * eac) / torch.clamp(
delta + gamma, min=1e-6
)
# Ensure non-negative
e0 = torch.clamp(e0, min=0.0)
es0 = torch.clamp(es0, min=0.0)
et0 = torch.clamp(et0, min=0.0)
# CO2 correction for ET0
co2_factor = interpolate(self.co2_table, torch.full_like(e0, self.co2))
etc = et0 * co2_factor
# Potential transpiration and soil evaporation split by light interception
ptran = torch.clamp(self.cfet * etc * frac_int, min=0.0001)
pevap = es0 * (1.0 - frac_int)
return {
"e0": e0,
"es0": es0,
"etc": etc,
"ptran": ptran,
"pevap": pevap,
}