Function to calculate the soil water content based on the Groenevelt & Grant (2004) model.
It is based on thermodynamic principles. Therefore, it is appropriate for the case in which
thermodynamic equilibrium has been attained by diffusion of water.
In this case, the water retention curve is given by:
$$\theta = k_1 \exp(-k_0 / x_0^n) - k_1 \exp(-k_0 / x^n)$$
where \(x = log h\) (pore water suction), and h is in units of hPa