Adding two leaf irradience model

pyrealm/constants/two_leaf_canopy.py

@@ -0,0 +1,27 @@
+"""To do."""
+
+from dataclasses import dataclass
+
+import numpy as np
+
+from pyrealm.constants import ConstantsClass
+
+
+@dataclass(frozen=True)
+class TwoLeafConst(ConstantsClass):
+    """Pyrealm two leaf canopy model constants dataclass."""
+
+    # two leaf canopy model constants
+
+    k_fa: float = 0.426  # needs citation
+    """scattering coefficient of PAR in the atmosphere, dimensionless"""
+    k_sigma: float = 0.15  # needs citation
+    """leaf scattering coefficient of PAR (relections and transmissivity),
+     dimensionless"""
+    k_rho_cd: float = 0.036  # needs citation
+    """canopy reflection coefficient for diffuse PAR, dimensionless"""
+    k_kd_prime: float = 0.719  # needs citation
+    """diffuse and scattered diffuse PAR extinction coefficient, dimensionless"""
+
+    k_sol_obs_angle: float = np.deg2rad(1)  # needs a citation
+    """ solar obscurity angle, radians"""

pyrealm/pmodel/two_leaf_irradience.py

@@ -0,0 +1,31 @@
+"""To do."""
+
+import numpy as np
+from numpy.typing import NDArray
+
+
+class TwoLeafIrradience:
+    """Calculates two-leaf irradience."""
+
+    def __init__(self, beta_angle: NDArray, PPFD: NDArray, LAI: NDArray, PATM: NDArray):
+
+        self.beta_angle: NDArray = beta_angle
+
+
+def beta_angle(lat: NDArray, d: NDArray, h: NDArray) -> NDArray:
+    """Calculates solar beta angle.
+
+    Calculates solar beta angle using Eq A13 of dePury & Farquhar (1997).
+
+    Args:
+        lat: array of latitudes (rads)
+        d: array of declinations (rads)
+        h: array of hour angle (rads)
+
+    Returns:
+        beta: array of solar beta angles
+    """
+
+    beta = np.sin(lat) * np.sin(d) + np.cos(lat) * np.cos(d) * np.cos(h)
+
+    return beta