My tupennyworth of useless information reveals that instructions for the installation of shower/water heating appliances refer to seasonal variations:
"The temperature of the mains cold water supply varies during the year and can be as much as 20°C lower in winter than summer." (location unknown)
"Due to seasonal variations in the temperature of the incoming cold water supply, the flow of water required to deliver a certain temperature will vary."
and, on second thoughts, not quite so useless after all if you're plumbing
P.S. And according to Dr Rosemary Leonard, speaking this morning on Breakfast TV, it really doesn't matter how much (tap)water you drink; your daily intake can be achieved from drinking coffee/tea and eating fruit and vegetables.