Less surface area & evaporation. Package deal. Float is protected from varmits.
This is one they offer and there is another larger round one that has garden hose barb fittings.
http://www.tractorsupply.com/webapp/wcs/...mp;cm_vc=-10005 http://www.tractorsupply.com/en/store/little-giantreg%3B-duramate-hose--stock-waterer
little giantThis one could be put in hole with Tpost to anchor and support it.
Half bury them in ground and put sackcrete around the trough. to Keep it cooler and protects trough from damage. You Need about 1 foot of head to gravity feed these. Run freeze proof hose under ground and wrap screen wire at expose area to keep varmits from chewing hose fitting pipe into.