Dunsop Bridge: catching water for Blackburn

We went to Dunsop Bridge in the Forest of Bowland to see the Pallid Harrier. Turns out that the whole valley supplies Blackburn with water. The valley was not suitable for a reservoir so water is taken straight out of the rivers. I was taken aback by the claim of 34 million litres per day. A litre is 1000 cubic centrimetres but 1 cubic metre is 1 million cubic centimetres. So there are 1000 litres in a cubic metre. Blackburn is taking 34000 cubic metres a day. I looked on a map to assess the catchment area of the river. It fills less than a 5km by 5km square on the map. So let's estimate 10 square km. 1 square km is 1 million square metres so we have a catchment of 10 million square metres. 34000/10million = about 3mm depth of water. Now that doesn't sound too bad.