def runoff_pitt(precip, evaptrans, soil_type, land_use):
    """
    The Pitt Small Storm Hydrology method.  The output is a runoff
    value in inches.
    This uses numpy to make a linear interpolation between tabular values to
    calculate the exact runoff for a given value

    `precip` is the amount of precipitation in inches.
    """

    runoff_ratios = lookup_pitt_runoff(soil_type, land_use)

    runoff_ratio = np.interp(precip, runoff_ratios['precip'], runoff_ratios['Rv'])
    runoff = precip*runoff_ratio

    return min(runoff, precip - evaptrans)