Using the HAM Weather Irrigation Module
The Basic Idea of this Module
For more in depth discussion of Evapotranspiration, please check out Evapotranspiration and Irrigation.
This module is designed to calculate the amount of water that is lost from the soil and plants and replace it with a close approximation of what was lost. Today in a world of "Green is good" every consumable resource such as water can be better conserved with such a tool.
The module takes the local WeatherBug data, the user settings, and calculates each day the average water that has been lost. The user then can have programs that respond to these calculations and apply the appropriate amount of water to replace that loss. When those programs complete they trigger a command that lets the module know that the water has been applied and can be subtracted from the module's tally. Each day this is done completing the cycle of tracking the data and applying the water. During cooler times of the year the watering days frequency will slow down, and during the warmer parts of the year the watering frequency will ramp up to daily.
Irrigation Module's Settings
There are several variables that are adjustable by the user.
- Calculation Method - The method used to calculate ETo
- Penman-Monteith: Accurate, but relies on averages of Temperature, Wind speed, and Dewpoint throughout the day. Some users had reported problems with communication to the HAM Weather servers which can cause errors in the averages.
- Hargreaves-Samani: Less accurate, but relies on Min/Max temperatures alone. These can be acquired during one snapshot at the end of the day. As a result, the calculation is far more robust in the presence of HAM Weather access issues.
- Irrigation Region - adjusts for the differing ET in inland vs coastal areas.
- coastal region, interior plains region
- Absorption Factor/Soil Type - adjusts for the relative efficiency of soil absorption.
- 80% absorption is recommended for level ground. This is a generally accepted industry % absorption for rainfall. Users with steep slopes can apply lower percentages to compensate for runoff.
- Absorption is applied to both rainfall and irrigation:
- 1) Rainfall .5 inches: Effective rain = 0.8 * .5 = .4 inches
- 2) Irrigation 0.5 inches: Effective irrifation = 0.8 * 0.5 = .4 inches
- Crop Coefficient/Plant Type - Plant needs are species dependent and defined as ETc = ETo * Kc (Kc = crop coefficient).
- Cool season grasses (Bluegrass, Rye, Fescue) use Kc of 0.8
- Warm season greasses (Bermuda, Saint Augustine, Zoisya, and Centipede) use a Kc of 0.6.
- Additional values are provided to encompass a range of plants/trees/gardens.
- Note: the current Evapotranspiration variable is actually ETc (=ETo * Kc)
- Water Applied per Irrigation Cycle - amount of water the user's irrigation program applies to the soil during each run.
- Allowable Depletion - how much water is allowed to leave (be depleted from) the soil before the next irrigation cycle.
Irrigation Module's Status Output
These variables are calculated from the WeatherBug data. They can be used in the program module for customizing the irrigation system.
- Evapotranspiration - approximation of the total amount of water leaving via a combination of soil evaporation and plant transpiration.
- Irrigation Requirement - amount of water to be applied based on the accumulation of previous day(s) water deficits.
- Yesterday's Water Deficit - total amount of water that has evapotranspired during a twenty four hour window from the day before.
Irrigation Module's Commands
These are commands that can be used in ISY programs.
- Irrigation - Cycle Complete - triggers the module's calculations for after water has been applied.
- Irrigation - Reset Variables - triggers a reset of the module's Irrigation Requirement variable.
Program Example: Irrigate If Needed
The IF condition decides whether to water by comparing the Irrigation Requirement balance to a constant. The user is responsible for setting this constant equal to Water Applied per Irrigation Cycle, and updating it if they change the value of water that they apply per irrigation in their own programs.
If Time is 11:00:00PM And Module 'Climate' Irrigation Requirement >= 0.50 Then Set 'Front Yard' On Wait 10 minutes Set 'Back Yard' On Wait 5 minutes Set 'Back Yard' Off Irrigation - Cycle Complete
Please note: If you are using an EZFlora device you only have to turn off the last station. When you turn on the next station the EZFlora first turns off the current station then turns on the next one. If you use an EZio device you will have to turn off each station before turning on the next one.
Calculating the Station Run Time
In order to replace the water that is lost from the soil your station run times will need to be properly calculated. Each type of emitter/sprinkler has a different amount of water that is being delivered per minute/hour. The calculator linked below can be used to calculate the appropriate run time for each station.
Initialize the Module
After you have installed the Irrigation Module the Irrigation Requirement variable will most likely display zero. You will have to trigger a reset of the module variables to get things going correctly. Below is a walk-thru of the steps to perform this reset. Please note: If its the rainy season it may be difficult to tell if Irrigation Requirement is having an issue, so its recommended this be performed anyways just after the first install of the module.
- In the ISY Admin Console go to the Programs tab.
- Create the temporary program shown in the example below.
- Right click on the program in the folder tree and select "Run Then" to reset the module.
- Go back to the Weatherbug tab.
- Check again Irrigation Requirement and you should now see a number instead of a zero.
- Once you have done this initialization process you can delete the temporary program.
If - No Conditions - (To add one, press 'Schedule' or 'Condition') Then Irrigation - Reset Variables
Forum thread: Proposed Evapotranspiration Algorithm in 3.1.4 Beta