Detailed Irrigation Specification ... Pack'em up and Take'em with you.
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Irrigation considerations begin with ET Data (Evapotranspiration Data). Though it may seem academic, it is a place to begin. It is estimated that irrigation sites which are adjusted by intuition are often over watered by 100%. All of the following considerations are for a well established landscape, not a new landscape installation.
Evapotranspiration is the process of evaporation and transpiration of water from a hydrozone. This includes evaporation from the surface of the ground and transpiration from the leaves of plants. It is an average over a period of time, generally over several days that indicates the need for hydration by the rate of water loss. This was originally done by placing evaporative pans in various locations through the hydrozone. By monitoring the rate of evaporation from the pans, measuring the humidity and temperature, day and night it is possible to determine an ET graph for the rate at which moisture is leaving a hydrozone. This then indicates the probable requirement of artificial inflow to maintain an efficient hydration for the plant type under consideration. The PET or potential Evapotranspiration is estimated for crops and turf and is provided by several weather stations throughout the state.
The complex consideration above is for research purposes. Often done by Universities or Agricultural Institutions, the studies provide information as a base point. By knowing the current ET Data it is possible to predict the makeup requirements of large concerns and also know when to best economize and withhold watering programs. Over watering can be a detriment to a healthy landscape. Dry cycle planning can also deter mold, fungus, & insect damage. For more information on ET you can visit the ET web site a Texas A&M University at http://agen.tamu.edu/wgit/petnet/pet.html
In large concerns such as golf courses, computerized irrigation systems can be instrumental in efficient watering and economy. Manual soil moisture probing throughout the site and entering the data into the system, allows the ET calculation to be updated. The system then adjusts the watering cycle accordingly. It will automatically reduce, increase or eliminate watering as needed. The operator merely enters the moisture readings per zone. The savings in water costs is substantial.
Primarily the ET Data is used to determine how much makeup precipitation will be required for a prospective site. Projected annual makeup as well a peak demand determine the source. That is, what size meter or quantity of meters or mainline inflow will be required. This may also indicate a holding pond and pumps or may even indicate an improvement in the municipal water supply or irrigation district canals. Knowing the rate of required makeup of water gives us the starting point around which the entire system may be designed.
First consideration - Water Source
Along with the ET Data and the annual and peak makeup water quantity specified, we now must consider the required time for the application of this water. In general day time watering, unless you are establishing an new landscape, is frowned on. In periods of water conservation, daytime watering loses much to evaporation. The watering window is generally considered to be from 3 hours before sunset to 4 or 5 hours after sunrise. During summer this will be from 6 P.M. to 10 A.M. or 16 hours in which to apply the required water. Depending on the size of the site and amount of gallons to be delivered, this can be a difficult task, especially if you require soak cycles to allow the ground to absorb the water, so it won't run off into the gutter.
At this point we look for a delivery mechanism such as spray heads or rotor heads as well as bubblers and drippers. We look for the most economical and efficient means to deliver the water. The heads are laid out on a plan, the total flow of all devices is determined. The peak demand for the system determines the amount of time the system must run to achieve the required inflow for the makeup precipitation. The system can then be divided up into sections. The required runtime determines how many sections must run at the same time to meet the watering window. The flow of the individual section then determines how large the water source must be, as well as the size of pipes and pressures required to operate the devices properly. If all sections can run one at a time and complete their delivery in the watering window, the source only needs to be as large as the requirement of one section.
IN a sense we do a backward design to determine the source. The computer aids us in this process. We are able to quickly and automatically do a head layout, extract the total flow of all devices and project possible sectioning and source requirements. If for example the required makeup is one quarter inch precipitation over the entire hydrozone, the calculation will be as follows...
One Quarter inch of water is .02 of one foot. The total area of the proposed site is 140,000 sqft. or 3.2 acres this becomes 2800 cubic feet of water to be installed during the watering window. There is 7.48 gallons of water to a cubic foot, therefore we require 20,944 gallons to be delivered in 16 hours. We projected a layout of 217 heads each running 3 GPM for a total flow of 651 GPM. This gives us 32.2 minutes if we could run all heads at once. Of course this is not possible so we begin to down size. Lets try 12 sections for 54 GPM. This gives us 20,944gal./54GPM or 387.9 minutes divide by 60 and you see you can water the entire site in 6.46 hours. 54 gallons per minute is easily supplied by a 2 inch water meter. (Max for a 2 inch meter is 75 GPM for a loss of only 4 psi). by running each section 27 minutes with a dual start time allows a 3.2 hour soak cycle for each section. This is well inside our watering window and can be done in the early morning hours between 2 and 9 A.M.
Second consideration - The Performance layout
Now that we have a good starting point, we can get serious about the design. There may be elevation changes, hardscapes, bedding areas, Xeriscape locations and water features in our proposed site. ET data will be different for each location. Run time, overspray, underspray, microspray and drip are all methods and devices to help you makeup the proper precip. for each hydrozone. It is highly unlikely that anyone is going to run an ET test. Most of us are going to use our rainfall report from the weather service, and with a good general sense of the types of plants in the hydrazones, and with some moisture checking either with a probe or by digging in a little, we will adjust the run times to increase or decrease the moisture level. Conceptually we want to use the proper device for the area to be watered. Choosing devices is often a matter of economy. i.e. Rotor heads cover a large area with fewer heads and less pipe as opposed to spray heads or drip pipe. As long as the device can deliver the required precip. in the required watering window, you are free to choose whatever device fits your desire.
We are not going to attempt to explain all the things involved in head placement in this detail. Suffice it to say that the considerations are many. It is advisable that you involve a licensed irrigator with years of field experience to aid you in this area. In general we use a head to head spacing and depending on the specifications of the manufacturer we will widen the pattern to compensate for odd precip. curves of a given device, or we will widen or shorten a spacing to fit an area that can not be spaced evenly. i.e. a 30 ft. spacing does not fit evenly in 100 ft. The following drawing indicates the problem. The area we are trying to fit with a 30 ft. rotor must be stretched in some dimensions and shortened in others to fit the area. This can cause too much water in some areas and not enough in others. Unless you divide the hydrozone in to two sections, this is a condition you must live with. Running the section long on one day and short on the other will help compensate. In general, unless we have standing water or dry spots we are unconcerned.
Another consideration is regulated pressure. Devices are designated to deliver a specified precipitation at a specific pressure. Over or under this pressure your precip. increases or decreases. For an added expense you can pressure regulate the individual head. Regulating the system at the main is not effective, because of pipe losses and elevation changes. The pressure at the beginning of a section and the pressure at the end of a section are considerably different. Pressure compensation at the head allows you to have sufficient pressure at the end of the section as well as the beginning. Each head has a regulator and delivers the proper precipitation. Here is a situation where economy in the initial installation causes high water costs and improper watering. Opting to go without pressure compensation for a cheaper initial cost, hurts you in the long run. With a beginning pressure of 50 psi. and an ending pressure of 25, the delivery of a specified 3 GPM nozzle will be 4.8 GPM at the front of the section and 2.7 GPM at the end. You will undoubtedly increase the run time to eliminate dry spots and green things up. As a result you are wasting water on the front end that you never realize until you get your water bill.
In the example below we are using drip line in the beds, spray heads in the small grass areas and rotor heads in large grass areas. Each device must be on a separate section because the run time are different. You can not combine or mix devices together. The run time on the rotors may be 45 minutes while the spray heads may complete in 15 minutes, and the drip line may take one or two hours. This all depends on the delivery rate of the device and the required precipitation makeup.
Third - Some things to consider ...
Courtesy of Earth Irrigation & Landscaping Inc.
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