Monday, November 4, 2013

Effective Rain - Rain that soaked into the soil October 2013

Effective Rain - Austin received over 13" of  rain in Oct. 2013. This is more than all the rain we received in the first 9 months of 2011, but how much of this 13" is effective rain?  Rain that soaked into the soil and is available for grass, shrub, and tree use?

The problem is that rain falls so fast here in Central Texas, sometimes up to 5" per hour. The soil can only absorb 1/4" per hour, so the remaining 4.75 inches will "sheet" and runoff as evidenced by all the flash flooding we are seeing this time of year.

In october, we received a lot of rain, but we achieved "soil saturation" mid way through the month. Once we reached saturation, it was like trying to put 10 gallons of gas into your car when the gas tank is full.
Below is a chart showing the soil saturation levels in Austin Texas for the mid weeks of October 2013.

As you can see , on Oct. 12th, we reached soil saturation levels, after that point all the rain we received was runoff and not available for plant use.  The total "effective rainfall" for October , I calculated at around  1-2 " for clay soils which is typical in central Austin.  This is a far cry from the 13 inches we received.  The other reason only so little rainfall was effective, is because of the low evaporation rate this time of year. Had these deluges occurred mid summer, we could have absorbed about 7.3" of rain as the soil would have dried out in between these rain events.

So the real question becomes,  how can a homeowner program a simple time based controller properly and know how much and when to water? It is pretty much impossible, only a smart controller or controller with a soil moisture sensor would be able to apply the correct amount to water at the right time and the right amount. For most time based controllers, it would be best to just shut the controller off and conserve water during these heavy rain periods.

Matthew L . Stamm
Certified Landscape Irrigation Auditor
President, Cougar Irrigation, LLC

(Above calulations were made using  , clay soils, warm season grass, 5" root depth)

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