When to irrigate after heavy rain

Check this index and wait several days

The Keetch-Byram drought index (KBDI) is used to track the moisture content of the upper soil and duff layers.
The Keetch-Byram drought index (KBDI) is used to track the moisture content of the upper soil and duff layers.

A viewer asked how many days I should wait to run my sprinkler so as to not oversaturate my St Augustine grass after a heavy rain.

The answer depends on the amount of rain and the time of year.

A good soaking is recommended twice a week in the summer for our area to maintain healthy deep roots. In heavier soils, as are often found in north Florida, less water may be adequate and you might only apply 1/2 to 3/4 inch.

Wait a few days to let the soil drain before irrigating to prevent excessive weed growth and fungus problems.

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Another approach is to look at the Keetch Byram Index (KBDI),  which is useful for knowing how much moisture is in the top 8 inches of soil.

The KBDI was developed at Florida State in 1968 for forest control management and fire danger assessment.

Although it was designed around Florida’s climate it is in use for other parts of the country.

The index determines long term drying in the upper part of the soil based on temperature and rainfall.

The index ranges from zero to 800 with zero being saturated and 800 the worst drought condition.  The index indicates deficit inches of available water in the soil. A KBDI of 250 means that there is a deficit of 2.5 inches of ground water available to the vegetation.

If the index is below 500 you can keep the irrigation timer off until the weather begins to dry out.

Current counties need no supplemental irrigation with wet soil conditions under a KBDI level of 100.

Soil dries out slower on cooler days compared to hotter afternoons.

Yards require a good soaking for the roots to absorb moisture where it is taken up into the grass blades and sent back out into the air through a process called evapotranspiration.

The risk for wildfires and browning of vegetation including sod increases when KBDI values decrease.

In north Florida if precipitation is less than the daily evapotranspiration rates grass will decline which could lead to a drought during a streak of dry days.

The index indicates deficit inches of available water in the soil. A KBDI of 250 means that there is a deficit of 2.5 inches of ground water available to the vegetation (Melton 1989).

Research carried out at the University of Florida presented direct measurement of evapotranspiration for north Florida St. Augustine and Bermudagrass yards  The highest water removal rates are in May and the least evaporation occurs in December and January.

Daily evapotranspiration rates for each month in north Florida:

Jan .06

Feb .09

Mar .11

Apr .15

May .16

Jun .14

Jul  .15

Aug .15

Sep .13

Oct .11

Nov .08

Dec .06


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