Have you noticed little white moths flying in the grass as you water? Perhaps a flock a birds pecking in the lawn? Both of these could be a sign of sod webworms.
These tiny little green worms eat your grass blades down leaving the brown thatch beneath visible giving the appearance of a dead patch. The grass will also be stunted in that area since they have eaten the blades.
Inspect closely and should see irregular bites out of the edges of the blades. Another easy way to check for these worms is to fill a bucket with soapy water and drench the area. The sod webworms will come to the surface and be visible.
An easy organic solution is to spray Spinosad or BT (a.k.a Thuricide) to the lawn. You could also use Cyonara, Triazicide or Broad Spectrum Insect Killer spray.