Topic: Which is better for doing irrigation: soil moisture or evapotranspiration?

[NeverDie]

I realize evapotranspiration is popular for deciding how much to irrigate, and I don't hear as much about soil moisture probes for that purpose.  If you had a mapping of how much irrigation is required to get from one moisture level to another, and you have a notion as to what the target moisture level should be, then wouldn't soil moisture probes at least as useful as evapotranspiration?  Because soil probes measure directly, somehow it seems like they would be more accurate as long as you have enough redundancy in your measurement probes that a probe which goes faulty doesn't ruin things.

I want to automatically water my landscape (mostly lawn, trees, and bushes) just what's needed and not waste water.

Anyone having success (or failure) in using soil moisture probes for that purpose?

[SLOweather]

I'd say ET is the most widely use of the 2. In fact, the California DWR created a Watering Index based on 7 Day total ET.

Basically, you total 7 days of calculated ET, divide it by the highest average historical 7 day ET for your are and multiply by 100 to get a percentage. Then, on most current irrigation controllers there is a percent adjustment. You set the controller for your highest required watering and then adjust the percentage control on your timer accordingly.

In fact, on my website I have a version of the Watering Index which I modified. It's basically the same function, except that my formula subtracts the last 7 days accumulated rain from the ET figure to more accurately (IMO) reflect what the watering needs really are. And I have the server calculate the Watering Index every day on the last 7 running days total ET and rain.

One thing... With either the regular Watering Index or my modified one, it's entirely possible to have more than 100% irrigation required. We just had a hot, dry, windy spell here and right now mine is 114%.

And, with my modified version, it's possible to have a less than zero percentage if we receive a lot of rain. It's rather nice because you can get an idea of when to start watering again as the negative index approaches zero.

FWIW, before I found the DWR's Watering Index, I'd created my own, which was purely based on ET and rain with no historic value. A simple version is just to sum 7 day's ET and that's how much water you need to put out in inches over the next week. Then you can subtract 7 day's rain from it to allow for rain if you want.

As far as soil moisture, I have a station over in my vineyard that I've been playing with. It's more for watching long term soil moisture, though. I only water the grapes 1x per week to get them established and then I'm going to try dry-farming them.

[Aardvark]

I'd go with soil moisture.  The trick is that the moisture/temperature sensors have to be at the root level of the crop you are growing.
http://www.desmoinesweather.org/weatherdata/wx3.html

[Skywatch]

I'd say both. Evapotranspiration is good for determining how much water is being lost Vs how much is received and watering accordingly. However soil moisture is better at helping determining how the water distributes throughout the soil medium. ET won't tell you how dry the soil is at Eg, 6" Vs 18". Crops have different root systems. Some deep some shallow. You can have a higher water table and the deep soil always remains wet while shallower soils dry out quicker. ET can tell you exactly how much water is being lost. I look at ET and rain as a budget and moisture as a distribution.

[Cutty Sark Sailor]

I have one of the CoCoRaHS evapotranspiration  devices, and you might find some info
at the manufacturer's link: http://www.etgage.com/
and at http://www.cocorahs.org/Content.aspx?page=et

Apparently a lot of producers monitor the ET since it approximates crop and soil water loss.
CoCoRaHS uses a 'grass' reference, but most commercial and private users appear to use
the 'alfalfa' reference...
Mike

[Cutty Sark Sailor]

Oh, by the way... CoCoRaHS provides graphs and 'budget' if you will covering the 'in' and 'out' of water over whatever time span selected, for that station....

[NeverDie]

I have one of the CoCoRaHS evapotranspiration  devices,...

How satisfied are you with its performance for controlling irrigation?

[Cutty Sark Sailor]

I have one of the CoCoRaHS evapotranspiration  devices,...

How satisfied are you with its performance for controlling irrigation?

Sorry... didn't mean to imply that I was an irrigator or farmer.  Only mentioned as a possible info source for your query. I can say, however, that it has saved me a few bucks on my water bill, -- the primary function here is observation and reporting.

[NeverDie]

I have one of the CoCoRaHS evapotranspiration  devices,...

How satisfied are you with its performance for controlling irrigation?

Sorry... didn't mean to imply that I was an irrigator or farmer.  Only mentioned as a possible info source for your query. I can say, however, that it has saved me a few bucks on my water bill, -- the primary function here is observation and reporting.

I only very recently heard about the etgage.  Yesterday, I inquired on a different thread (http://www.wxforum.net/index.php?topic=16959.msg194978#msg194978) some basic questions regarding the etgage accuracy, and I'm looking forward to the answers (which I hope will come).  Are you using the model A or E?  How accurate are you finding it?  If model E, how reliable?  I would encourage you to add your etgage experience to that thread as well.

Presently, the five options I'm investigating are: 1. ET from a weather station (possibly a hybrid one) that I would own and maintain, 2. a "virtual" weather station that's automatically cloned from the "best" nearby weatherstation out of a pool of nearby weatherstations on weatherunderground.com (or possibly synthesized by averaging the weather measurements from the pool), 3. soil moisture probes planted in the landscape, 4. soil moisture probes in a "box" that has a sample of my lawn growing in a sample of my soil (this becomes a kind of living ETgage that I can move around, and 5. the ETgage.  Out of these, #2 is so-far only a wish, because I haven't heard of any software that can do it.  #3 or #4 would be the least costly in hardware, but may require changing/replanting the probes more than I care to.  However, I'd like to hear from more users of soil probes before reaching that conclusion.  Pragmatically speaking, the choices are probably among #1, #3, and #5.

[SLOweather]

4. soil moisture probes in a "box" that has a sample of my lawn growing in a sample of my soil (this becomes a kind of living ETgage that I can move around.

IMO, this is probably not a viable solution. Watermark probes work on the concept of soil moisture tension, measured in centibars. Your box would constrain the moisture to a fixed volume, rather than letting capillary action pull it into the larger soil volume, and and it won't be subject to the same temperatures as probes buried in the ground. It might work if you did a lot of comparative studies to calibrate the box against probes buried soil. It also might work OK if the box was larger, but a size that would give comparable results might be too big to move.

[NeverDie]

4. soil moisture probes in a "box" that has a sample of my lawn growing in a sample of my soil (this becomes a kind of living ETgage that I can move around.

IMO, this is probably not a viable solution. Watermark probes work on the concept of soil moisture tension, measured in centibars. Your box would constrain the moisture to a fixed volume, rather than letting capillary action pull it into the larger soil volume, and and it won't be subject to the same temperatures as probes buried in the ground. It might work if you did a lot of comparative studies to calibrate the box against probes buried soil. It also might work OK if the box was larger, but a size that would give comparable results might be too big to move.

Excellent point.  I don't want this to turn into a science project.  On the other hand, the ETGage works without having any soil contact at all.  The notion I was toying with was for the lawn box to be a kind of poor man's ETGage, using living plant material to do true evapotranspiration instead of a ceramic node covered in green canvas doing a simulation of evapotranspiration.  However, for measuring ET, I foresee too many complicating factors.  I'm scrapping the idea. 

[NeverDie]

I'd say both. Evapotranspiration is good for determining how much water is being lost Vs how much is received and watering accordingly. However soil moisture is better at helping determining how the water distributes throughout the soil medium. ET won't tell you how dry the soil is at Eg, 6" Vs 18". Crops have different root systems. Some deep some shallow. You can have a higher water table and the deep soil always remains wet while shallower soils dry out quicker. ET can tell you exactly how much water is being lost. I look at ET and rain as a budget and moisture as a distribution.

It's a good point you make.  If the soil is very sandy, such that most of the water drains away, then a soil probe at root level could alert me to lower moisture than ET theory might predict.  In that case, I may have to replace more than the ET water.  On the other hand, a more clay-like soil should retain more of the water.  Also, come to think of it, depending on how the water distributes, I may not need to water the grass that's downhill as much as that which is uphill.  Maybe mapping it out with temporary soil probes that I move around is all I need do.  I wish I didn't have to drill/dig a bunch of holes though.  I wish it was more like a high-end metal detector, where I just wave a detecting wand at surface level and the detector tells me how far down it's buried.

Which soil moisture probes work best?  There's a confusing number of different types.  Can anyone share their experiences as to the relative efficacy?

[Skywatch]

I'd say both. Evapotranspiration is good for determining how much water is being lost Vs how much is received and watering accordingly. However soil moisture is better at helping determining how the water distributes throughout the soil medium. ET won't tell you how dry the soil is at Eg, 6" Vs 18". Crops have different root systems. Some deep some shallow. You can have a higher water table and the deep soil always remains wet while shallower soils dry out quicker. ET can tell you exactly how much water is being lost. I look at ET and rain as a budget and moisture as a distribution.

It's a good point you make.  If the soil is very sandy, such that most of the water drains away, then a soil probe at root level could alert me to lower moisture than ET theory might predict.  In that case, I may have to replace more than the ET water.  On the other hand, a more clay-like soil should retain more of the water.  Also, come to think of it, depending on how the water distributes, I may not need to water the grass that's downhill as much as that which is uphill.  Maybe mapping it out with temporary soil probes that I move around is all I need do.  I wish I didn't have to drill/dig a bunch of holes though.  I wish it was more like a high-end metal detector, where I just wave a detecting wand at surface level and the detector tells me how far down it's buried.

Which soil moisture probes work best?  There's a confusing number of different types.  Can anyone share their experiences as to the relative efficacy?

The type used on Davis are Watermark sensors made by Irrometer. Since Davis reads moisture in Centibars I don't believe other sensors would work. Another company based out of Illinois called Spectrum Technologies makes stations specifically for soil moisture monitoring and they use different moisture measurement methods. They use Watermarks as well as resistance based sensors which work similar to leaf wetness sensors. www.specmeters.com

A few years back I did an experiment by placing 2 moisture sensors at various depths in a loamy part of the yard and did the same in a sandy part of the yard and while the ET gave a general idea of the water being lost the moisture actually gave me a better picture of how water moves through the soil medium. ET will give you a general idea of soil moisture but the probes give a more accurate idea of the moisture

[NeverDie]

Here's an unusual soil moisture probe I ran across recently:

http://www.adafruit.com/products/1298#Technical_Details


It uses a STH10 temp/RH sensor in the probe.  I wonder how the accuracy would compare to the wired gypsum soil probes?  Maybe you wouldn't have to "replant" an RH sensor as one may have to do with sensors that rely on sensor-to-soil contact?  If so, that would be a big labor saver.   Are there any RH sensors that can survive being submerged in liquid water and still function normally afterward, or else modifications (like a Gore-Tex or Tyvek type shield) that would protect against such scenarios?

It turns out that for measuring moisture content in a concrete slab, the best of current methods involves putting a wireless RH sensor 4 inches down into a bore hole and then plugging the top of the hole.

[Trevor Dennis]

An interesting subject, and one that is HUGE in New Zealand.  I can remember a newspaper article a while back, which described water as the new gold.  Basically, the more water, the better the pasture, and the more stock it will support, but this has to be balanced against the high cost of irrigation.  I central Canterbury (around Christchurch in South Island) they have to pump from several hundred feet down in the lowest levels of the aquifer, which means several very powerful pumps is series to overcome gravity.  That leads to power bills in the tens of thousands of dollars per month for a decent sized dairy farm. 

Here in Marlborough it is almost totally laid to grapes which are drip fed, but we have a much more healthy aquifer than Canterbury, with good water from as little as two or three metres, so costs are way less.  So factors like soil types come into it, which means how much water will a given paddock (what we call fields) hold before the weight of it causes it to sink back into the aquifer. 

But for all the water they pump out of the ground, it never does half so much good as rain out of the sky.  I am told that rain brings nitrates and other chemicals down with it, and that aquifer water is too cold.  That may or may not be true, but there is no doubt that rain is king.