Brown Patch

Assuming there isn’t a physical cause (i.e., the wading pool was there for the month of August), this looks like brown patch. Brown patch is caused by a fungal pathogen (Rhizoctonia solani) that affects all cool season turfgrass species. It is a foliar disease that does not affect crowns or roots.
Leaf blades within the patch turn brown after infection, while a gray-white band at the perimeter of active patches can be seen on closely-mown turf. Leaf blades may have individual lesions with brown margins. Manage by reducing summer fertilization and watering in early morning so grass dries rapidly. Often anti-fungal sprays are necessary. More information: https://www.extension.purdue.edu/extmedia/bp/bp-106-w.pdf and http://www.ext.colostate.edu/pubs/Garden/07202.html