Summary:
Natural logarithm.

Syntax:
ln(expr)

Description:
Returns the natural logarithm of the numeric (float or double) expression expr (ln expr). If expr does not evaluate to a float or a double, or if expr evaluates to a negative number, then exp(expr) returns an error.

Example:

...
variable: irrigated_surface
  type: float
  definition:
    irrigated_surface = past_surface * exp(time_step * ln(1 + irrigation_coeff))