Quite simply put- an increase in global temperature causes more evaporation and less available water. The water which is available will also be of low quality due to contamenation from extreme events. An interesting statistic is that by 2020 there will be 50% less available water on earth then currently available (and lets face it, countries like Darfur and Somalia have no water at present!)
Lake Chad is one such body of water that clearly shows climate change first hand. In the last 20 years the lake has decreased by 90%. Because Lake Chad is has such a large surface area but is relatively shallow, evaporation of the water is rapid. The scary reality is that 30 million peoples' livlihoods depend on the Lake so Chad is in a dire situation- there could ultimately be 30 million refugees in the next 10 years due to water scarcity.
Water managmement schemes need to be integrated into all sectors. Water is the medium which unites all themes from agriculture to clean energy. How can you expect hydro-elctricity generation as an alternative to coal if there may be no water in the future. This has been witnessed in hydropower schemes in Kenya, Tanzania and Ethiopia where decreased volumes of water have compromised the powerplants.