def UC_V(V_mm, A_catch, outUnits):
    """ Convert volume from mm to m^3 or litres

    Args:
        V_mm:     Float. Depth in mm
        outUnits: Str. 'm3' or 'l'
        A_catch:  Float. Catchment area in km2

    Returns:
        Float. Volume in specified units.
    """
    factorDict = {'m3':10**3, 'l':10**6}
    V = V_mm * factorDict[outUnits] * A_catch
    return V