Grass needs water, but how much? The combined loss of water from plants and soil ("evapotranspiration") was measured in a classic turf study in Gainesville, Florida (Table 1).
Gainesville gets lots of rainfall. But typically, dry weather in July and August means that just about all the turf water must come from irrigation. And in other months, such March through June, rainfall is plentiful, on average, but sporadic.
The story is more complicated. Soils with good water holding capacity, such as organic soils, provide more water reserve for the roots than sand soils. Turf grown near heat sources, such as pavement, uses more water, while turf under trees uses less water.
The process of water loss by plants is a simple physical process of heat exchange. This is proven by the coolness of your bare skin when you walk out of the water, and the heat rising from the stove where you boil water. Plants generally differ little in how much water they use. Forests use a little more water than grasslands, hence forests generally occur in moist regions, and grasslands in arid regions. Except for cacti and other succulents, most plants use about the same amount of water. The main difference is that some plants need more irrigation, while other plants, woody plants especially, are better at tapping the underground reserves. With its long roots, bahiagrass can generally be grown in areas of Florida with no landscape irrigation.
January
2.0
February
2.5
March
3.4
April
4.2
May
5.2
June
4.3
July
4.8
August
4.8
September
3.9
October
3.4
November
2.5
December
1.9
Total
42.8
Means of five years' observations of evapotranspiration on St. Augustinegrass turf.
Back to the St. Augustine grass lawns. Typically, in Gainesville, FL soils, St. Augustine grass has about 3/4 inch total soil moisture reserve. That's how much water it loses before it wilts. The simplest way to know when the Gainesville lawn has lost this much water lawn is . . . watch it wilt. If you notice gray areas which "footprint," and individual leaf blades that are curled, it's time to irrigate, and damage is being done. The wilting is normally noticed in the late afternoon, and the turf should be watered the following morning, or certainly within a few days.
How much to water? About 3/4 of an inch. Since irrigation systems are not perfectly tuned to provide 3/4 inch to every corner of the lawn, one would normally water more than this amount. How much depends on how bad your Gainesville irrigation system is set up. The simplest way to find out is to place more-or-less straight-sided containers, such as coffee cans or frozen drink concentrate cans, etc., preferably 10 or 20 per lawn, and run the sprinklers for a timed test, say, one hour. If the sprinklers put out 3 inches in one hour, then you can figure on watering for 15 minutes to provide the "average" turf needs, until the next time it wilts.
Uniformity is Goal #1 of a good irrigation system. Bad irrigation wastes water. So how does one tune-up a bad irrigation system, or design a good system? The biggest corrupter of irrigation systems is poor sprinkler placement. Generally the sprinkler heads should be close enough so that the spray or stream from each head just barely touches the neighboring heads (in other words "head-to-head coverage). So if your sprinklers are place on a square grid, the arc from each head must touch four other heads. Except, along the edges and corners of the landscape, half- and quarter- circle matched precipitation heads would be used so as not to put water on the street or building. Accomplishing proper head placement, and uniform irrigation, will be the subject of another article. But for now, make sure you have an adequate water source, that you don't have too many heads per zone, and that pressure, pipe size, and layout make sense. Retail outlets that sell sprinkler parts normally have free pamphlets or inexpensive booklets that will tell you how but having a system that isn't installed by a trained professional is what typically leads to a poorly designed Gainesville irrigation system and a drought-stressed Gainesville landscape.
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. Augustine grasses. 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 underwent 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 Texas experiment was done in a microenvironment of sandy soil under full sun exposure.
Technically, St. Augustine grass uses only a little more water than other, drought avoidant grasses such as bermuda grass and bahia grass. What probably lends to the severity of drought damage in St. Augustine grass 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.
Mowing
Before Gainesville lawn mowing, be sure the mower blade is sharp. A dull blade will bruise and tear the grass tips, and they will eventually turn brown. Mow when the grass is dry to eliminate shredding and whipping of the grass blades.
Base the mowing on the growth rate of the grass rather than on a set time schedule. Never mow more than 1/3 of the grass blade. With this small amount of grass being cut, the clippings won't have to be bagged. Leaving the clippings on the grass will actually return the nutrients to the soil. Contrary to popular belief, clippings left on the lawn are not a significant contributor to thatch.
As the summer progresses and the temperature increases, raise the mower to 2 1/2 to 3 inches. The grass will be stronger and better able to survive drought when it is mowed at a higher cut. It will also discourage germination of weed seeds and insulate the soil against the drying heat.
Mow the lawn into the fall until the grass stops growing.
Thatch
Thatch is a major concern with some lawns. Some thatch is normal and even beneficial, as it acts as a mulch to retain moisture and keep soil temperatures cool. Thatch also discourages germination of weed seeds and as it decays it adds nutrients to the soil. But once thatch accumulates to more than 1/2 inch, it can choke your lawn and invite trouble from disease and insects. A healthy lawn in Central Texas will generally not have a thatch problem.
Thatch is a tightly intermingled layer of living and dead stems, leaves and roots, which develops between the layer of green vegetation and the soil surface. To keep the amount of thatch down, the lawn needs better conditions for micro-organisms and decomposition of organic matter.
Aeration can be done in the spring or fall of the year to allow for better water and air infiltration, and nutrient mobility. Aeration also improves the physical condition of the soil by relieving compaction and will help with all plant and soil processes, while keeping thatch at a desired level.