This paper examines factors affecting water availability and hydrological trends of water supply. The relative impacts of the different factors have been assessed on a planning time frame of around 30 years. It is demonstrated that the non-greenhouse processes of multi-decadal climate change and el Niño-la Niña climate change will almost certainly be more significant than greenhouse induced climate change. Further, in developing countries, increased water consumption, population growth, and urbanization are likely to be the dominant factors when considering water availability. The type of responses that a water supply organization can make are discussed.