def _calculate_water_demand(elec_generation):
    """Calculates water demand as a function of electricity generation
    This is a stop-gap, until we calculate water demand in the energy supply
    model
    Arguments
    ---------
    elec_generation : float
        Electricity generation in MWh
    Returns
    -------
    float
        Cooling water demand in ML (million litres)
    """
    COOLING_WATER_DEMAND_ML_PER_MWH = 150 / 10**6
    cooling_water_demand = elec_generation * COOLING_WATER_DEMAND_ML_PER_MWH

    return cooling_water_demand