The Factor needs to be "1". Otherwise, "Watering Time Today" will not equal "Watering Time on Longest Day" on the longest day of the year.

What about having two values: "Watering Time on Longest Day" and "Watering Time on Shortest Day". Then use linear interpolation to determine "Watering Time Today". If you do that, then I think this is your formula...
Wt=Watering Time Today
Ht=Hours of Sunlight Today
Wl=Watering Time on Longest Day of the Year
Hl-Hours of Sunlight on Longets Day of the Year
Ws=Watering Time on Shortest Day of the Year
Hs=Hours of Sunlight on Shortest Day

Wt = Ws + (Ht-Hs)*(Wl-Ws)/(Hl-Hs)

"How should I adjust the watering time based on the date? "

You can't adjust watering time simply by the date at least I can't see how it could be done and still be accurate.

The amount of daylight alone IS NOT a good indicator of ET and subsequent watering times.

The "Factor" you are searching for is calculated from information you are leaving out such as temperature .. humidity .. and WIND?? .. in addition to the days solar input that your are working with that by the way can be modified significantly by cloud cover.

.. if you want to develope an accurate software then the user must input the basic and current weather data each week. ( temperature .. humidity .. WIND ?? .. and solar radition as modified by cloud cover ).

The good news is the equations you are looking for are already in the library and have been developed by some very good scientist / engineers. Just plug them in to the computer and you can calculate a theoretical ET for a period of time and a given set of weather data. Then each user then needs to divide this theoretical ET by the actual ET for any given day or period to establish an error from the theoretical ET .. a cofficient sometimes refered to as a "crop" coefficient.

Once this coefficient is determined the software's calculated or theoretical ET determined for any set of weather conditions can be modified easily to give the actual ET for a given period of time ( a day week ect..)

ET ( Actaul ) = Kc x ET ( Theoretical )

This ofcourse can all be converted to run times given that you know your systems percipatation rate BUT keep in mind how much time you need to water is not directly related to actaul ET. The uniformity of the irrigation system as well as soil conditions and other factors will affect final run times.

Good Day ...

Mohave Kid,
Wow, you seem to know a lot about this...
OK, the system I'm working on will be plugged into the weather forecast, I was planning to use this to adjust water amounts taking into account expected precipitation rate, so the user will not have to enter the weather!
The number of daylight hours was my way of adjusting seasonality in a more formulistic way than the rough % that I'd read about elsewhere. Of course your method is much better.
Do you have a good reference for this? What does ET stand for?
I've uploaded the first pass at some of the program's screens here:
Screens

Could you take a look at them and critique them? Would you like o help in testing (or developing) the application?

Hello Roberto ...

We use such a system at work .. Rainbird's Maxicom system .. there are other systems as well ... the software is hooked to a weather station and recieves weather data to calculate ET.

ET stands for evapotranspiration. ET is the amount of water that evaporates and tranpires from an area in a given period of time. Transpiration is water leaving from the surface of plant materials.

Et values are often available for turf grass and it may be very possible that you could get daily ET values for a given locality downloaded into your software directly .. that is already calculated by someone else using local weather data. This would be good also since the models used for calculating ET vary a bit for different locations.

Check out the "Irrigation Association's " web site and I think you would be interested in taking or looking into their Irrigation Auditor Certification or at least read the manual the program is based on since it will give you all the basic theory irrigation schedules are based on.

I'll check into some references but search the term "evapotranspiration" and I'm sure you will find some info. Also search the term "Pennman equation" .. my brain is a bit rusty at the momement ...

I'll take a look at your pages too.

Good Day ..

I found a website that will give me ET data for California in a nice to parse XML file CIMIS Unfortunately that the data isn't available for all places, I'm not concerned because all I care is California (and just one zipcode of that!). Pretty cool!
I have questions:
- Should I use today's ET or monthly averages when doing the calculation?
- Does all of this stuff apply for low-volume systems (i.e. drip)?
- Any good resource for finding the crop coefficient for different ornamental plants? I'm particularly interested in warm-season, dry-area ornamental grasses.

Hello Roberto ..

The real value of ETo is to have a point of reference so that given a simalar set of weather data one can quickly calculate water requirements for their site in a consistent manner .. lets say from one summer to another.

"Should I use today's ET or monthly averages when doing the calculation? "

That depends I would guess as to how involved and accurate you want your system to be ... current systems use weather stations on site to calculate daily ET values from weather data .. the other extreme would be using historical averages and maybe a feature to dial up or down your system or program to compensate for under or above average conditions ( maybe like a sliding percent key ).Getting something like a weekly predicted average would work well for watering ones home.

Another system being used now are moisture sensors to measure real time conditions in the soil. No system is really "automatic" this is why crop coeffients are used and one must still check conditions in the field.

"Does all of this stuff apply for low-volume systems (i.e. drip)? "

It is much easier to model turf conditions then various plants or desert landscapes and in general most of what you see in the irrigation industry is ET for turf not plants.

There is info out there I'm sure .. ecologist no doubt use ET models in there studies so you would have to look into that at the library. Crude guestimates are available at irrigation sites also but still it is the irrigation operators job to calibrate these models to actual field conditions.

The real world is more complex then models used to calculate ET values.

Good Day ...