def UC_Cinv(C_mgl, A_catch):
    """ Convert concentration from units of mg/l to kg/mm

    Args:
        C_mgl:   Float. Concentration in mg/l
        A_catch: Float. Catchment area in km2

    Returns:
        Float. Concentration in kg/mm
    """ 
    C_kgmm = C_mgl*A_catch
    return C_kgmm