The short answer is: You want to be able to replace the water in the plant root system with what has been lost due to what the plant has used (transporation) and what is lost from the soil (evaporation), together these are known as evaportransporation or ET.
The long answer is: Not enough water will cause a plant to wilt or die. Too much water can also be damaging to plants due to a reduction in oxygen around roots or development of a shallow root zone. You will need to calculate various factors in order to come up with an accurate assessment of your watering needs, or ET, such as: Type of plants, soil type and conditions, efficiency of sprinkler system, weather and the amount and quality of water available to the plant.
Let’s start with:
ET (evapostransporation): This will tell us irrigation requirements that are needed for a specific plant in a specific region at a specific time of year. This is expressed as depth of water per unit of time, such as inches per day, week, month or year. ET data can be obtained from local sources such as Cooperative Extension Services, water agencies or meteorological networks. ET data is based on alfalfa crops or grass and can be based on historical reference for the region or real time measurements from an on-site weather station.
Species Factor or Crop Coefficient: These are values calculated for the variable rates of ET among different plant species and are divided into groups of High, Medium and Low depending on their water requirements. These values are available through local Agricultural Extension services.
Example of Irrigation Requirement (IR): If the ET data for a month is 6” and the crop coefficient for a cool season grass with high water use is 0.80. The formula would be as follows: 6”x0.80=4.8ir”. Even though the ET rate is 6”, the grass at this site requires only 4.8” of water for the month.
Distribution Uniformity(DU): A sprinkler system with good uniformity will cover an irrigated area with a very even distribution of water. This is especially important for turf areas since every square inch is covered with a dense root system. If there are small areas within the irrigated zone that do not get as much water as other areas, this could cause a dry spot and force you to run the sprinklers longer on that zone to cover the dry spot while over-watering the rest of the zone. This would result in a very inefficient use of water. A perfectly installed sprinkler system would have a DU of 100%, but the average sprinkler system has a DU of 55% to 75% with some much lower. Sprinkler systems would need to be field tested or audited by a qualified Irrigation Auditor in order to determine the level of Distribution Uniformity (DU). The DU can then be used to determine an even more accurate irrigation requirement.
Example of Irrigation Requirement: From the example above we determined our ET of 6” times the crop coefficient of 0.80 gave us our irrigation requirement of 4.8”. Now we would divide our irrigation requirement by the DU (we’ll say 75% in this example). So 4.8” divided by .75 = 6.4” of irrigation required.
Precipitation Rate(PR):This is a measure of the rate at which water is distributed in an irrigated area over time, usually measured in inches per hour and calculated as an average within the irrigated area. Factors in determining PR include type of sprinkler and nozzle size, water pressure at the sprinkler head, spacing of sprinkler heads and run time on the sprinkler controller. In other words the total flow of water (gpm) through all the sprinkler heads on a specific area or PR = flow (gpm) x 96.3 (a constant that will convert the equation to inches/hour) divided by area(square feet). PR is an important factor because precipitation rates can exceed some soils ability to absorb the water resulting in excess run-off.
Example: If we have a flow rate of 8gpm over an area of 900sf we would say 8gpm x 96.3 = 770.4 divided by 900sf = .856 inches per hour in that area. Therefore, if your run time on that zone is 35 minutes, .856 divided by 60 minutes = .014in./min. x 35min. = .49 inches per watering cycle for that area.
Run Time (RT): To calculate a basic schedule for a sprinkler zone you will need the irrigation requirement (IR) and the precipitation rate (PR) of the sprinkler zone.
Example: From our examples above, we calculated an irrigation requirement of 6.4” and a PR of .856. Our formula will be 60min. x (6.4ir divided by .856pr) = rounding off to nearest minute gives us 51 minutes of run time.
Maximum Run Time (RT): This will factor in the amount of water that can be absorbed by certain soil types over time (in./hr.) called the Basic Intake Rate (BIR). These intake rates have been established over time and range from .10in/hr for a clay soil to .60in./hr. for a sand soil. In order to calculate maximum RT we will need the Basic Intake Rate (BIR) of your soil and the Precipitation Rate (PR).
Example: For this example we use the BIR of a sand soil (.60) and divide it by the PR of .856. Therefore .60bir divided by .856pr = .70 x 60min. = 42 maximum minutes of run time the soil will allow before being saturated creating unnecessary run-off. With our run time of 51min. putting us past the point of soil saturation we will want to split the run time for that zone into two separate cycles of 25 minutes each allowing the first cycle to soak into the soil and root system before the next cycle starts.
These calculations do not cover all of the factors that can be used to calculate how long you need to water your lawn. There are more calculations that can fine tune this even more, but I can say that these would only become helpful if you were watering a very large commercial site or golf course using hundreds of thousands of gallons of water each month. For the average homeowner, the calculations shown here would be about as far as you would need to go to get the most out of your sprinkler system.
Midwest Lawn Sprinklers recommends a weather-based controller that can do these calculations automatically (such as http://www.rainbird.com/landscape/products/controllers/ESP-SMT.htm). This controller gets information from an on-site weather station attached to the house. After entering some information, the system collects and tracks rainfall amounts in order to adjust watering for the yard. If you prefer not to have an on-site weather station there are subscription services that perform this same function.”