Proposed Evapotranspiration Algorithm in 3.1.4 Beta

[Mark Sanctuary]

Everyone,

 

I have been working on getting a basic set of wiki pages done for this module. I am running out of steam tonight so I will continue later. Please review them and let me know what I can improve. Much of it is just stuff I have copied over from this thread and trimmed and reworked a bit.

 

Wiki page: Evapotranspiration and Irrigation

Wiki page: Using the WeatherBug Irrigation Module

 

Please note: the Wiki server is slow today so it may take a bit to load.

 

Thanks,

[Z3phyr]

Freudian slip - "Using the WeatherBug Irritation Module"?

 

[Mark Sanctuary]

Oh wow that's a funny boo boo, especially after how much work and discussion it took to get to this point. I have fixed the incorrect name. Thanks!

[Michel Kohanim]

Hi Mark,

 

Thanks so very much! This is great ...

 

With kind regards,

Michel

[Mark Sanctuary]

Michel,

 

I was wondering if we might be able to add one small tweak by adding "Water Applied per Irrigation Cycle" to the wording of the irrigation cycle complete command?

 

Irrigation - Cycle Complete (subtract 0.5)

This way there is a double check of if the users if statement matches what the module is going to subtract.

 

Thanks,

Mark

[Michel Kohanim]

Hi Mark,

 

Currently there's no easy way of doing this but this is not to say that it's not doable.

 

Please keep a list of things you want done and we'll revisit them once we are comfortable with how the whole system is operating. I assume that eventually we would want to add multiple Zones each one of each could have different soil types and absorption factors.

 

With kind regards,

Michel

[Mark Sanctuary]

Finally got a chance to setup a javascript run time calculator for the new module.

 

Check it out and let me know what you think...

 

ISY Irrigation Run Time Calculator

 

Thanks,

[Nuttycomputer]

Hey All,

 

Please see http://www.forum.universal-devices.com/viewtopic.php?p=64975#p64975 for the entire post.

 

I'm having a fundemental problem with the Irrigation module as designed due to having clay soil. The problem is primarily with the Soil Absorption Factor. My understanding was this was added to properly account for how much irrigation is received via rain however it is also applied to the "Water Applied During Irrigation Cycle" number. I believe this is incorrect and should not be applied in this way. The result being my system is calling for water everyday in the spring.

 

Additionally this new number, which is the one subtracted from Irrigation Requirement when the cycle completes, is labeled Allowable Depletion. This term is being used incorrectly in it's application of the module.

 

Please read the post above and let me know if I'm way off base here. In the meantime I'm working around the issue by setting my soil absorption factor to 100%

 

I do appreciate all the work and headache that went into this module.

[Nuttycomputer]

Pulling this over to this thread to respond.

 

Absorption factor is the amount of water that's absorbed by the the soil (NOT the plant); i.e. the amount of water that's wasted. Unless I am not seeing things correctly, then, allowable depletion is the fraction of water that's ultimately applied to the plant. And, thus, this is the amount that can be depleted to get to empty. As far as I know, this value is a constant and not a rate as you suggest it would be.

 

Hey Michel,

 

This is part of the problem in understanding this complex system. Plants take in water from the soil. The soil is their gas tank of water. Water already absorbed into the soil is loss through two methods. Evaporation and Transpiration this module already uses the complex calculations to adjust for this so no further adjustment is necessary.

 

What I think the addition of the Soil Absorption Factor was trying to take into account was calculating what amount of water was not absorbed into the soil (gas tank) and instead was lost through runoff when it rained. The problem is it isn't a hard set number like 17% but instead it is a rate, ie. in/hr and when trying to calculate water lost to runoff soil absorption is only one of the factors and is highly house dependent. For example some landscaping designs have built in rain only gardens. These are designed to capture rain runoff so that almost none of it is lost, even if the soil absorption rate is very low. (Due to the high clay content of our soil cities in my state actually have these designs all over the place, they call them flood control zones that double as parks when dry) Water loss to runoff isn't worth trying to calculate in the module in my opinion so if this was the intent I think it should be removed.

[Michel Kohanim]

Hi Nuttycomputer,

 

Please see my other post ... I am not sure whether or not it's worth taking it into consideration. I have to leave that for you an others to decide. In the meantime, I went back to our original thoughts:

1. http://www.universal-devices.com/mwiki/ ... Irrigation

2. http://www.irrisoft.net/downloads/manua ... Manual.pdf

 

And, specifically on page 17:

Rocky conditions reduce the available water holding capacities by 70%

Organic Content increases available water holding capacities up to 10%

Compaction can reduce available water holding capacities by 20%

 

I think you are correct that allowable depletion might not be the correct terminology. But, there's certainly a relationship between how much water is wasted by different type of soil. And, I still think it's not a rate ... I think the names we used are misleading.

 

With kind regards,

Michel

[JMC]

I think I've figured out that Allowable Depletion represents the percentage of the Water Applied per Irrigation Cycle which is actually available to the grass or plants, with the remainder being lost to runoff, etc.

 

Following that assumption I set my absorption factor to 100% (Sand) and Water Applied per Irrigation Cycle to .5 inch. I'm using Coastal plains for my Irrigation Region even though I'm about 125 miles inland in the Sierra Nevada foothills.

 

Using the above settings, with daily temperatures in the mid 60's, relative humidity approx 70% and light winds (0-5mph) the EvapoTranspiration is still over .4" per day. That is way too high and would result in .5 inch being applied almost daily and the ET calling for more irrigation while the soil is still very moist from the previous session.

 

Incidentally Michel, I was able to work around the lack of rain data from WeatherBug with a rain sensor that can send an X10 On when it detects rain in excess of 1/4". I simply send an Irrigation Cycle Complete once each day that rain is detected. It's definitely a kludge but it is better than not having any rain data at all.

 

mike

[Nuttycomputer]

And, specifically on page 17:

Rocky conditions reduce the available water holding capacities by 70%

Organic Content increases available water holding capacities up to 10%

Compaction can reduce available water holding capacities by 20%

 

Agreed, soil conditions do affect Available Water Holding Capacity. Available water being the size of the gas tank available for plants/lawn. With that table showing correctly that Clay holds the most water.

 

I think you are correct that allowable depletion might not be the correct terminology. But, there's certainly a relationship between how much water is wasted by different type of soil. And, I still think it's not a rate ... I think the names we used are misleading.

 

With kind regards,

Michel

 

Can you please clarify Michel. Specifically you stated "But, there's certainly a relationship between how much water is wasted by different type of soil." What are you referring to here? It is water that is absorbed by the soil that is what the plants use. Wasted water is water lost to run-off (clay worse), drainage (sand worse), and evaporation. Everything else is used by the organic material present in the soil (read plants and organisms) The soil itself doesn't use the water simply stores it.

[Michel Kohanim]

Hi Mike, are you looking at the ET or Water Deficit Yesterday?

 

Hi Nuttycomputer, you are very much correct. I guess you and I have terminology mismatch issues (just look at the virtual node thing!!!) ...

 

What I meant was the condition of the soil. I really think that absorption factor is really meant to describe the condition of the soil and how much water is "absorbed" based on that condition (I used "absorbed" incorrectly in my last post).

 

All this said, how do you propose we take these conditions into consideration?

 

With kind regards,

Michel

[Nuttycomputer]

What I meant was the condition of the soil. I really think that absorption factor is really meant to describe the condition of the soil and how much water is "absorbed" based on that condition (I used "absorbed" incorrectly in my last post).

 

All this said, how do you propose we take these conditions into consideration?

 

With kind regards,

Michel

 

The condition of the soil is tricky as is the allowable water setting. It also depends on how items were implemented. Does the soil setting affect anything other than the Rain Received and Irrigation applied numbers? If not I would propose the following:

 

Remove Soil Absorption Factor/Rate (whichever term we want to apply here) calculating run-off (which I believe was it's original intent based on previous discussion) is too tricky at this stage. Runoff depends on rain rates, how moist the soil already is, slope of terrain, obstacles (see the flood parks my previous city uses exclusively to catch runoff due to clay content: http://g.co/maps/g9mh5) and I'm sure there are more. In fact just try googling Calculate Runoff and see Pages and Pages of documents all using different methods/formulas. Yikes. Some people may really want their sprinkling system to be that accurate but I don't think it's worth it for the majority of us.

 

Regarding the other variables needed to determine whether to water or not: Field Capacity, Maximum Available Water, Allowable Depletion. I need to review the module again. Now that I've read way too many papers on the subject I don't need them because I've calculated it myself. However if the intent is to make it easier for people who have no background it might be worth it. Here is the relationships:

 

Field Capacity (aka Soil Water Holding Capacity): How much water soil can hold. Expressed in english units this is in/ft. For every foot of soil I can hold x amounts of water. This is based on soil type composition. There are some tables out there but the gist is Clay holds the most and Sand holds little.

 

Maximum Available Water: This is a product of field capacity and plant root depth. This is plant dependent. Depending on how easy you want to make it for the user you can get root depths table or just have the user set the root depth in inches. The latter option is easier to implement immediately with little work and often times root depths are printed on the label of the seed. (atleast they are here)

 

Allowable Depletion (aka Management Allowable Depletion): This is a threshold of how much water (expressed in percentages) can be removed from the Maximum Available Water before plant water stress sets in. This is also plant dependent but a hard fast rule for most people is 50%. Once this level is reached irrigation is applied Note: In a way most users on this site already set their irrigation to run when this threshold is reached (they just call it irrigation requirement, with the assumption is they are starting the program already at Maximum Available Water)

 

Irrigation Requirement is how much water is needed to return to the Maximum Available Water number... if this goes beyond the Allowable Depletion number then water stress starts to set in for your plants.

 

In a way how the program is used now Allowable Depletion and Irrigation Requirement is redundant. However, I include it here and suggest we retain it to ease the programming burden. You already have complex calculations going into this and users have already designed programs around this variable or may be interested to know the value. However one bit of programming that might be needed is to cap off the irrigation requirement. It can't go beyond the Maximum Available Water number. If for whatever reason a user didn't water their lawn for a long period of time it's important to specify that you can only water so much. Any rain/irrigation beyond the Maximum Available Water amount is wasted as far as plants are concerned because their roots don't go down that low.

 

Water Applied per Irrigation Cycle should be retained with no reason to reduce the value by any amount when applied. We are making an assumption here that the user has properly configured their irrigation cycle to not cause run off itself.

 

Let me know if you have any questions on any of this.

[JMC]

HI Michel, I meant ET. If I'm understanding correctly ET could be seen as a snapshot of the rate the water deficit is accumulating whereas the Water Deficit Yesterday is the accumulation for the 24 hour period.

 

I've been watching the ET fairly closely for the past few days and its becoming apparent that wind speed is a very major factor in that calculation. In fact in the past few minutes I've seen that a change in wind speed from 3mph to 7mph (temp and humidity remained constant) resulted in an ET change from 0.3723 inches/day to 0.4481 inches/day.

 

I'm still nervous though that the daily Irrigation Requirement is going to cause more water to be applied than is actually needed.

[Michel Kohanim]

Hello JMC,

 

Wind speed is in fact quite instrumental in the calculation of ETo:

ea = actual vapor pressure @ dew point temp

es = mean daily saturation vapor pressure

wf (wind factor) = wind-speed x (es-ea)

a0 = (Gamma x 900/MeanTemp) x wf

ETo is a function of a0 ...

 

So, wind speed has direct impact on ETo. Not being an expert on irrigation, I do not know why this relationship exists but that's what the algorithm takes into account.

 

Yes, ETo IS the snapshot but what ultimately matters is Water Deficit Yesterday from which Irrigation Requirement is calculated:

a. At every sampling (poll from WB), ETo is calculated and then added to a tally

b. At 12:00 AM, the tally is divided by the number of samples

 

With kind regards,

Michel

[Michel Kohanim]

Hello Nuttycomputer,

 

Thanks so very much for the detailed explanation.

 

Assuming that we would eventually want to have an advanced section with all those parameters, it seems that all we have to currently do is to make sure everyone knows that Absorption factor of 100% would probably be a more optimized than using those from drop down list.

 

Would you be willing to come up with requirements for the advanced version? Obviously we would have to investigate the ROI but I think it would be valuable for many.

 

Thanks again and with kind regards,

Michel

[Nuttycomputer]

Would you be willing to come up with requirements for the advanced version? Obviously we would have to investigate the ROI but I think it would be valuable for many.

 

I don't think the requirements would be different than what is listed above. Are you looking for something more specific I could work on?

[Michel Kohanim]

Hello Nuttycomputer,

 

Not at the moment but when we get to implementation, I will have many questions as far as the algorithm.

 

Thanks and with kind regards,

Michel

[Nuttycomputer]

Michel,

 

I had an additional question on this that might explain some of the high daily ET numbers JMC and myself may be seeing. From what I think I understand the ISY uses latitude and longitude to approximate solar radiation for use in the ET calculation. Is this also by chance being used during night?

 

I've been just keeping track of the snapshots and weather information when my irrigation program runs and it seems like all things being equal (temperatures, windspeed, etc.) the snapshot varies little between day and nighttime.

[Michel Kohanim]

Hello Nuttycomputer,

 

That is correct (lat/long). We use Penman-Monteith daily (24-hour) algorithm.

 

With kind regards,

Michel

[Nuttycomputer]

Hello Nuttycomputer,

 

That is correct (lat/long). We use Penman-Monteith daily (24-hour) algorithm.

 

With kind regards,

Michel

 

Thanks for the information. I'm going to have to look into the actual algorithms I think a little bit further. Our state water department here in an effort to conserver water also puts out watering guides and ETo ETc numbers. Having done some more tracking the daily deficit being posted by the ISY is consistently about twice the amount as the local numbers. While they measure solar radiation directly I would not expect the error margin to be that large. My first thought was that the ISY was for some reason putting Solar Radiation in at night but you stated you are using the 24-Hour algorithm which I believe inputs hours in the day.

 

If you want to see what I see visit http://data.weatherreach.com/udwr, you can then set your ISY to one of the same locations.

 

For example today my ISY reported a water deficit yesterday of .3585 inches. The weather station however reported .17 for yesterday..

[Michel Kohanim]

Hello Nuttycomputer,

 

Please do a search on Penman-Monteith daily (24-hour) algorithm.

 

I am not entirely sure how you are comparing water deficit (which is ETo - water applied) to ETo reported from that site. Does that site take water applied into consideration? Or did you mean ETo is not the same?

 

With kind regards,

Michel

[Nuttycomputer]

Hello Nuttycomputer,

 

Please do a search on Penman-Monteith daily (24-hour) algorithm.

 

I am not entirely sure how you are comparing water deficit (which is ETo - water applied) to ETo reported from that site. Does that site take water applied into consideration? Or did you mean ETo is not the same?

 

With kind regards,

Michel

 

Michel - Found several papers on the algorithm I'm reading through and hope to be able to compare them shortly. Since the ETo listed is just a snapshot of current conditions I was using the Water Deficit on days that Water Applied = 0 (No rain or sprinklers) as the ETo number reported by the ISY and comparing.