Water (part 1 of 2):  How much?
(Go to part 2)

Grass needs water, but how much?  The combined loss of water from plants and soil ("evapotranspiration") was measured in a classic turf study in south Florida (Table 1).
It should be easy to replace the total evapotranspiration loss of about 43 inches per year, because south Florida gets about 60 inches rainfall.   But typically, dry weather in April and May means that just about all the turf water must come from irrigation.  And in other months, such June through October, 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.  Most areas of south Florida are within 5 feet of the water table, and some turfgrasses live on this water.  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.   Sawgrass, Cladium jamaicense, the main vegetation of the Everglades, was reported to use 44 inches of water per year, about the same as the 43 inches used by St. Augustinegrass turf.  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 level areas of south Florida with no irrigation.

Month	Water use (inches)
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, Plantation, Florida (Stewart and Mills, 1967).
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Back to the St. Augustinegrass lawn.   Typically, in south Florida soils, St. Augustinegrass 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 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.  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 is the irrigation system.  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.  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.

References

Augustin, B. J. 1984. Developing landscape irrigation conservation projects. Journal of Agronomic Education 13:107-109.

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

Feldhake, C. M., R. E. Danielson, and J. D. Butler. 1983. Turfgrass evapotranspiration. I. Factors influencing rate in urban environments. Agron. J. 75:824-830.

Kneebone, W. R. and I. L. Pepper. 1982. Consumptive water use by sub-irrigated turfgrasses under desert conditions. Agron. J. 74:419-423.

Stewart, E. H. and W. C. Mills.  1967.  Effect of depth of water table and plant density on evapotranspiration rate in southern Florida.  Trans. ASAE 10:746-747.

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