Wednesday, February 4, 2015

What is ETo ( Evapotranspiration ) and Why is Smart Control so important during this drought.

We have been sending out watering recommendations based on ET network data locally available in Austin, TX. There are 2 of these $5,000+ weather stations in Central Austin, one at LCRA Redbud office and one out on the Morris Williams Golf Course. Seems like it is a good time to explain what ETo means and how we use the data to program controllers.

is the loss or use of water through the combination of evaporation and transpiration.  The evapotranspiration rate in the area where the system is located is the first important factor to gather when creating an irrigation schedule.
Solar radiation and wind (1) cause water to evaporate from the soil surface.  Water is absorbed by the root structure (2) and is passed upward through the plant tissue (3).  Transpiration (4) occurs when water evaporates from the plant surfaces directly into the atmosphere, or into intercellular spaces and then by diffusion through the stomata to the atmosphere.
Reference evapotranspiration or ETo is a standard measurement of evapotranspiration.  ETo is expressed in inches per day, month or year.  ETo is an estimate of the evapotranspiration of a large field of four to seven inch tall, cool season turf that is well watered.  In most instances, it will not be necessary to replace 100 percent of the reference evapotranspiration rate.  The rate may be adjusted according to a plant factor or landscape coefficient.  Replacing too much may be a waste, while not enough will stress the plant material.

We typically use historical ETo and Rainfall data to seasonal program irrigation controllers since we have 30 year historical ETo data that is widely available and historical rainfall data. The data below was taken from and it covers the years 1981-2010.

I inserted our recommended water budgeting percentages by month. You can make monthly adjustments, 12x a year , when you perform you monthly system maintenance check. The problem is this is STATIC, 1x a week watering adjustment recommendations based on historical 30 year data.

For our clients on Monthly service plans , we are constantly monitoring past and future monthly weather conditions and checking soils moisture and making these monthly adjustments for them.

The problem with historical data is that it is an average over 30 years and some of these years may be super hot and dry and others may have been cooler and wetter.

 In 2011, with 90 days over 100 degrees and another 90 days over 90 degrees, we had an ETo of close to 100 that year and almost no rain during those 180 days, Supplemental irrigation required was almost 4x that of a normal or "average" year. This was what I would call a "panic year" for most landscapes. We saw millions of trees die and are still seeing the affects as this 7 year drought drags on.

With the data we send out weekly through this blog, you can make weekly adjustments ( 52 x a year if you like ) to runtimes and such, and this is pretty good since we are on 1x weekly watering restrictions and for most landscapes this is "close enough." But when we are entrusted with such a valuable life sustaining resource as water and a valuable asset like plants and trees that can out live people is "close enough" good enough anymore?

All automobiles today have a gas gauge and a warning light that comes on as you approach empty. Your HVAC has a thermostat that makes adjustment automatically to keep you home envirnment almost perfect. Mobile phones have a low battery indicator and even have a " LOW BATTERY plug me in" indicator that pops ups.

Unfortunately most legacy automatic irrigation systems don't have any ET automatic adjustment features and last I checked, my  plants and trees , don't send our text messages saying "water me i'm thirsty and about to die." Or even worse "don't water me, I'm drowning with all the rain we had this past week".

"Dumb" time based irrigation controllers are just going to put out the same amount of water regardless if the plant needs 1/4" or 1/2" or 1" of water this week or no watering at all.

Smart Controllers with onsite Weather Stations ( typically a Solar radiation sensor or a Tipping Rain bucket )  are the most economical and environmental solution,. They take data measurements approximately 50 times a day , 365 days a year and use that data to adjust watering as needed.

In December 2014 , Austin received close to normal rainfall at about 2".  But we received 5" of rain this past month for January 2015 ( 3" above average of 2") and some more rain yesterday.

I just checked the watering history on my Rainbird ESP-SMT smart controller on my personal residence and the last time it watered was Dec 8th,2014 and it is not predicted to irrigate again until the 8th of February for some grass zones and the 16th of February for my shrubs and bushes ( assuming we do not get rain between now and the 8th or  the 16th )

2 months of NO SUPPLEMENTAL irrigation which results in a huge water and energy savings, lower water bills and healthier and happier plants.


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