According to the OS Map, that's Quarrel Burn Reservoir. (If you use multimap rather than Google maps it gives you the option of viewing the OS Map which is generally better for loch names.)
I've never heard of it before but it seems to feed into Eight Mile Burn which is somewhere I've heard of, so maybe that is better known and somebody else here can make a mental connection and give more info.
It seems to be part of the Pentland Hills Regional Park.
http://download.edinburgh.gov.uk/Pen..._New_Guide.pdf
You could try contacting their ranger service to see if they have any info:
Pentland Hills Ranger Service
0131 445 3383
Pentland Hills home page
Neil
---------- Post added at 08:38 AM ---------- Previous post was at 08:37 AM ----------
Just beat me too it!