def monthly_N_fixation_point(
        precip, annual_precip, baseNdep, epnfs_2, prev_minerl_1_1):
    """Add monthly N fixation to surface mineral N pool.

    Monthly N fixation is calculated from annual N deposition according to
    the ratio of monthly precipitation to annual precipitation.

    Parameters:
        precip (float): input, monthly precipitation
        annual_precip (float): derived, annual precipitation
        baseNdep (float): derived, annual atmospheric N deposition
        epnfs_2 (float): parameter, intercept of regression
            predicting N deposition from annual precipitation
        prev_minerl_1_1 (float): state variable, mineral N in the
            surface layer in previous month

    Returns:
        minerl_1_1, updated mineral N in the surface layer

    """
    wdfxm = (
        baseNdep * (precip / annual_precip) + epnfs_2 *
        min(annual_precip, 100.) * (precip / annual_precip))
    minerl_1_1 = prev_minerl_1_1 + wdfxm
    return minerl_1_1