http://www.greenhorngardening.com/drip-irrigation-how-to-calculate-flow-requirements/
http://forums2.gardenweb.com/forums/load/water/msg0616163214895.html?5
http://www.bradshawsdirect.co.uk/blog/water-gardening-library/pond-filters/filter-types/gravity-fed-filter-systems/
http://www.dripirrigation.com/drip_irrigation_chapters/9/drip_irrigation_pages/40
http://www.rainwater-harvesting.co.uk/rainwater-harvesting-systems/headertank-system.php?system=headertank
http://www.thecenterforrainwaterharvesting.org/5_pressurization.htm
http://www.calctool.org/CALC/eng/civil/hazen-williams_g
http://www.tapstore.com/gravity-fed-water-system
http://www.ebay.co.uk/itm/like/111350906220?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&device=c&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=108&ff19=0
http://www.itacanet.org/itaca/sections/water/gravity-flow-water-systems/