St. Augustinegrass: Drought injury

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St. Augustinegrass - Shade
St. Augustinegrass - Chinch bugs

One way is to shut off the irrigation.  It's often the dry season when we see how poorly our sprinklers have been working.
The photo shows a planned curtailment, to measure differences in drought survival among different St. Augustinegrasses.  The research was done at the University of Florida's Fort Lauderdale Center.  Some of the grasses were totally killed and others survived.
In other studies, the killing injury (damage %, below) was closely associated with the number of days wilt.  While FX-10 and Floratam were slower to wilt than Bitterblue and Seville, once a grass had been wilting off-and-on for a week, it was on a death course.

After the first week or so, St. Augustinegrass plots suffered 15% loss of canopy per day.  Plots which undewent two weeks of wilt were completely killed.  Any subsequent recovery was from stolons growing in from the sides.
Actual results which you might experience in a lawn will vary according to microenvironment, e.g., the presence of trees, exposure to the wind, the quality of your soil, and the condition of the turf.  Other organisms, such as nematodes, can compromise the root system and make the grass less able to stand up to lack of water.  Grass which has been fertilized recently with highly soluble fertilizer often wilts quickly.  The Fort Lauderdale experiment was done in a microenvironment of sandy soil under full sun exposure.

Technically, St. Augustinegrass uses only a little more water than other, drought avoidant grasses such as bermudagrass and bahiagrass.  What probably lends to the severity of drought damage in St. Augustinegrass is the exposure of the horizontal above-ground stems ("stolons") to desiccation.  In contrast, bahiagrass stolons are partially protected by the clasping leaf sheaths.   Bermudagrass and zoysiagrass have much of their stem material below the ground ("rhizomes"), and rhizomes are not only protected from desiccation by the soil, but they tend to be in a semi-dormant ("hardened") condition, thus are more resistant to desiccation.
Should the lawn be watered as soon as it wilts?  Not necessarily.  There's a chance it may rain within a few days from the first wilt, and generally it's safe to watch and wait.  Wilt is typically noticed at first in the mid-afternoon, 2 to 3 p.m. Daylight Savings Time, and the lawn becomes turgid again by the next morning.  The progression of afternoon wilt can continue for a week or so, expanding in area and occurring earlier in the afternoon.   When the lawn still remains wilted the following morning it is on its death course.  During this progression, traffic should be kept off the turf.
Any new growth in grasses must come from the stems.  Once the stems have dried excessively, the turfgrass plant can make no more leaves, roots, or stems.

Reference

Busey, P. 1996. Wilt avoidance in St. Augustinegrass germplasm. HortScience 31:1135-1138.