It is not just the amount of sunlight hit the lawn that counts, although this is very important. Attached is a link to a pdf file from the North Carolina State University explaining turfgrass watering needs http://www.turffiles.ncsu.edu/PDFFiles/000851/ag661.pdf . I realize that you may not live in the Carolinas or, indeed have turfgrass (Tall Fescue in my case). What it does show is a nice set of numbers and an explanation of how to look at soil water retention.