There are five factors at work.
1) A definite reduction in average rainfall. (Keep in mind that rainfall patterns vary enormously across the country, from over 100" per annum in parts of the mountainous north west, to barely 20" in the flat, rain shadow lands in the south east.
2) Insufficient investment in new reservoirs. (As noted by Justin.)
3) Excessive transportation loss. (As noted by Justin.)
4) Increased per capita usage.
5) Increased run off because of land use practices and spreading suburbs.
Today it was raining here, in the NE of Scotland, though not persistently and not heavily. The countryside is green and my efforts to preserve endangered weed species is proving fruitful.
