Jay Chan said:

I just come across the following PDF page in Internet:
http://www.espoma.com/pdf/garden/GI_4.pdf

It says that we should deep water the lawn. But it also says this:
"During the summer, much of the root system deteriorates.
Therefore, summer waterings should be light and frequent."

This is new to me. I am under the impression that we should always
water the lawn deep, not "light and frequent".

Can someone help me to understand that suggestion?

It's based on the latest and greatest research by the turf grass experts.

"The idea is to give the turf what it needs and get away from the idea that 1 inch
every Saturday is the only way to irrigate" according to the Extension Service.

From the June 17, 2003 MSU Extension Service CAT Landscape Alert:

If rains cease and you desire a green lawn throughout the summer, there are
two irrigation schemes that can be followed. The first is the traditional irrigation
schedule of deep and infrequent applications in order to discourage shallow
turfgrass rooting, flush salts from the soil profile and reduce weed competition.
For those without in-ground irrigation systems this is most likely the irrigation
schedule you’ll follow. Apply between 0.5 to 1.0 inches of water weekly depending
on precipitation and temperature.

For those with an in-ground irrigation system, research from Michigan State
University has shown that light, frequent water applications reduce damage from
certain diseases and insects. Hot, dry weather will naturally result in a shorter
root system so it could be argued that the majority of a heavy, deep irrigation
application would move past this shallow root system and not be available for
the plant. If you want to give the daily or maybe every other day irrigation scheme
a whirl here’s what you need to do. First of all I think it will take some tweaking
based on what soil type you have, shady or sunny site, and turf species. The
research project at MSU applied 0.1 inch of water daily at about 1:00 PM. This
application serves a couple purposes: it cools the turf in the heat of the day and
also applies just enough water to get it through. In several areas water restrictions
would certainly prevent this type of irrigation scheme, if that’s the case consider
irrigating every second or third day with slightly higher amounts. The idea is to
give the turf what it needs and get away from the idea that 1 inch every Saturday
is the only way to irrigate.

[end exerpt]
--
Pat in Plymouth MI

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)