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How long or how often should I water? Who, or what, is ET? Anyone responsible for keeping landscape healthy and green may want answers to these questions.

Today’s irrigation systems uses a controller, which is an automatic timer that regulates when and for how long your sprinklers run, depending on the watering schedule you enter. An automatic controller, set properly, is much more efficient than operating sprinklers manually.

A program is a set of instructions that tells the controller exactly when to run the sprinklers. It consists of Watering Days, Start Times, and Run Times. If your lawn needs one inch of water per week to stay healthy, how do you set the program? What do I base my Run Time on? Yes, irrigation scheduling can be quite a challenge.

High-tech, computerized Central Control systems provide updated and adjusted schedules on a daily basis to the field. But this level of scheduling precision demands software, special equipment, and expert dedication to design and maintenance. How then, can the average end-user achieve this degree of sophistication?

Irrigation professionals, landscape architects and homeowners alike, want a quick and easy solution to scheduling, good enough to get in the "ballpark". True, the method described in this article may not reflect the accuracy necessary for sports turf management, but it can save water. It is derived from proven turf science, it beats guessing, and it helps establish a usable benchmark schedule for most residential and light commercial applications.

This benchmark is the key to what is known as ET Scheduling. "ET" stands for evapotranspiration. Water in the soil evaporates and plants transpire, or use water, daily. Hence the term, "ET". Let’s say that you live in a hot, dry climate, and it is now the middle of July. Your turf grass needs ¼ inch of water per day to stay green. Therefore, your evapotranspiration rate is 0.25 inches per day. This is the amount or depth of water that needs to be replaced by the irrigation system.

ET-based scheduling works well with solid-state controllers that have a feature called water budget. This lets you change you usual station run times without resetting each station. One hundred percent is your peak programmed run time. You can adjust this percentage up or down when you want more or less run time, according to the season and weather.

Once calculated, the end user can reprogram an entire system in less than one minute! There is no need to change each individual station run time, start time, or day schedule during spring or summer. That’s because water budget adjusts all run times, for every station.

The idea is to establish a Base Schedule Index (BSI) for each zone. This is the maximum amount of run time per station necessary to achieve proper watering during the hottest month in summer. Next, apply a monthly water budget for off-peak scheduling, and you’re finished.

We will use factors such as Evapotranspiration (ET), Precipitation Rate (PR), Water Budget (WB), and Adjusted Run Time (ART) to calculate our schedules. Together, they reveal the amount of water to be replenished each irrigation cycle, adjusted for seasonal differences.

The Evapotranspiration (ET) rate equals the total loss of water by evaporation from the soil surface, plus the transpiration from turf grass or ornamental plants, over a given area, in 24 hours. ET = inches/day.

The Precipitation Rate (PR) of the sprinklers is similar to rainfall. It expresses the rate of applying water over a given area in one hour. Measured in inches per hour (in/hr), the PR tells us how fast we are filling the soil back up within the sprinkler zone.
PR = inches/hour

How to find the Precipitation Rate (PR) for each zone in your landscape:
1.Find the approximate area covered by each zone in square feet.
2.Calculate the gallons per minute (gpm) used by each zone. To accomplish this, add up the gpm output of each sprinkler head. The data for each sprinkler nozzle can be obtained from the manufacturer or supplier, or from the irrigation plan. Many nozzles have the gpm figure stamped or molded right on them. Generally, spray heads use about 0.75 gpm for ¼-circle, 1.5 gpm for ½ circle, and 3.0 gpm for full-circle heads. "Rotor" style heads use 2 to 5 gpm each, for most residential applications.

PR (in/hr) = 96.3 x gpm
Area (ft2)

Example:
There are 4 sprinklers on this zone. Each uses 3 gpm for a total of 12 gpm. They irrigate an area that measures 30’ x 30’, or 900 ft2.
PR = 96.3 x 12 gpm = 1.28 inches per hour
900 ft2

The Water Budget (WB) factor is a coefficient that factors periodical changes in ET. Represented as a percentage, WB fine-tunes the scheduled run times to help reflect current weather conditions. WB need only be calculated once per given geographical area.

Water managers may correctly argue that this equation ignores soil infiltration rates, field capacity, crop coefficients, rooting depths, distribution uniformity, microclimates, and wilting coefficients. The criteria for BSI are simplicity and ballpark run times, calculated on the fly. Oversimplified? Yes, but it is based on a twist of established principles and data.

Current turf grass studies indicate over-watering is a problem. The BSI equation assumes 100% watering efficiency, with no losses due to runoff or lack of distribution uniformity. In fact, this is not entirely true. Normal watering run times set only to the current ET and discounting all other factors would result in over-watering. The BSI equation, however, uses irrigation inefficiencies to its advantage. The actual water applied to the soil using the BSI equation will be 20% to 30% of adjusted ET by the mere fact that it never accounted for inefficiencies to begin with. This winds up being very close to optimum settings, without the hassle.

Okay, so how do we find ET? Historical ET data exists for every part of the country, every month of the year. Contact Rain Bird Corporation at 1-800-RAIN-BIRD or www.rainbird.com if you need help. You may also check with your County Agricultural Extension or local water authority.

Here is the Base Schedule Index (BSI) equation to satisfy the maximum daily water requirements for turf grass:
BSI = Peak ET/PR x 60

Where: BSI = maximum summer run time in minutes

Peak ET = peak summer evapotranspiration rate in inches/day

PR = GPM x 96.3 / sprinkler zone area in square feet

Calculate and enter the Base Schedule Index run time per station into the controller.
Next, use the water budget (WB) equation to calculate the percentage to enter into the water budget feature of your program. You only need to do this once for any given geographical area, based on your historical ET data. BSI requires peak ET and assumes 100% water budget.

WB = Non-peak ET/ Peak ET

Where: WB = Water budget percentage to be entered seasonally

Non-peak ET = Historical ET data for remaining 11 non-peak months

Peak ET= Historical ET data for peak month

Calculate, document, and enter the water budget for the appropriate month into the controller.
Use the Adjusted Run Time equation if your controller does not have a water budget feature, or if the site has certain water restrictions, such as odd or even days only. In this case, find the cumulative run time necessary for a 2, 3, or 5-day schedule and program the run times accordingly. Be aware of long run times exceeding soil infiltration rates. Split the run times into several starts over the course of the day.

ART = BSI x WB

Where: ART = Adjusted Run Time in minutes, monthly or quarterly

BSI = Base Schedule Index in minutes

WB = Seasonal Adjustment percentage

Example: Calculate BSI, WB, and ART for Palm Springs, California:
Given: Max ET = July @ 0.29 in/day historical ET data
Rain Bird 5000 series rotor zone @ 0.75 in/hr (Precipitation Rate)
BSI = 0.29 in/day = 0.39 hours x 60 minutes/hour = 23.2 minutes
0.75 in/hr

Next, calculate a yearly schedule adjusting run times using Water Budget (WB).

WB = Non peak ET / Peak ET

If your controller does not have Water Budget by program, or you cannot water daily, use the ART equation.

Remember that the Water Budget calculations only need to be performed once for your area. You may apply them to all future projects. BSI however, must be calculated for each station or zone with similar precipitation rate characteristics.