def UC_C(C_kgmm, A_catch):
    """ Convert concentration from units of kg/mm to mg/l.
        Divide answer by 10**6 to convert from mg/mm to mg/l

    Args:
        C_kgmm:  Float. Concentration in kg/mm
        A_catch: Float. Catchment area in km2

    Returns:
        Float. Concentration in mg/l
    """    
    C_mgl = C_kgmm/A_catch
    return C_mgl