Added documentation

Docs/sphinx_doc/index.rst

@@ -57,6 +57,7 @@ In addition to this documentation, there is API documentation for ERF generated
    theory/Forcings.rst
    Particles.rst
    theory/WindFarmModels.rst
+   theory/Forest.rst 
    theory/UnitsAndConstants.rst
 
 .. toctree::

Docs/sphinx_doc/theory/Forest.rst

@@ -0,0 +1,31 @@
+Forest Model
+--------------
+
+The forest model provides an option to include the drag from forested regions to be included in the momentum equation. The 
+drag force is calculated as follows: 
+
+.. math::
+
+   F_i= - C_d L(x,y,z) U_i | U_i |
+
+
+Here :math:`C_d` is the coefficient of drag for the forested region and :math:`L(x,y,z)` is the leaf area density (LAD) for the 
+forested region. A three-dimensional model for the LAD is usually unavailable and is also cumbersome to use if there are thousands
+of trees. Two different models are available as an alternative: 
+
+.. math::
+   L=\frac{LAI}{h}
+
+.. math:: 
+   L(z)=L_m \left(\frac{h - z_m}{h - z}\right)^n  exp\left[n \left(1 -\frac{h - z_m}{h - z}\right )\right]
+
+Here :math:`LAI` is the leaf area index and is available from measurements, :math:`h` is the height of the tree, :math:`z_m` is the location 
+of the maximum LAD, :math:`L_m` is the maximum value of LAD at :math:`z_m` and :math:`n` is a model constant with values  6 (below :math:`z_m`) and 0.5 
+(above :math:`z_m`), respectively. :math:`L_m` is computed by integrating the following equation (see `Lalic and Mihailovic (2004)
+<https://doi.org/10.1175/1520-0450(2004)043<0641:AERDLD>2.0.CO;2>`_): 
+
+.. math::
+   LAI = \int_{0}^{h} L(z) dz 
+
+The simplified model with uniform LAD is recommended for forested regions with no knowledge of the individual trees. LAI values can be used from 
+climate model look-up tables for different regions around the world if no local remote sensing data is available.