NN0829 : Water Tunnel under Coire Cruachan NN0829 : Water flowing out of the mountainThis tunnel, which appears to collect water from the north side of Ben Cruachan, is part of a network of tunnels and pipelines (19km) which extends the catchment area of the reservoir to 23 sq km. (Data from Scottish Power website)
My estimate of the natural catchment area is about 7 sq km.
The principle is simple - if more water is collected, more power can be generated. Many other hydro-electric schemes have similar tunnels and pipelines.