One of the first things you have to determine is how much water your soil can absorb before run-off occurs (INFILTRATION RATE). This can be achieve, by simple trial and error, timing the interval between watering commencement to the point where no more water can be absorbed (i.e run-off).
There are basically two groups of grasses, WARM SEASON which will not tolerate temperatures below 3° C and includes couch, Kikuyu, Buffalo and Queensland blue couch, and COOL SEASON which will tolerate temperatures below 0°C including species such as Kentucky Blue, Rye grasses, Tall and Fine fescue and the Bent grasses.
The cool season grasses have a higher water requirement than warm season types. This has been determined to be between 60% - 65% of nett evaporation which can be calculated as follows:
EVAPORATION (measured from a standard open pan - this figure is published daily with weather statistics in the newspapers) minus any RAINFALL for that period equals NETT EVAPORATION.
A simple example follows:
Weekly evaporation in mid-summer = 75mm
Rainfall for the same week = 20mm
Nett Evaporation therefore = 55mm
The cool season grass requires 65% of 55mrn so you would have to replace 36mm.
Warm season grasses only require 45% of 55rnrn = 25mrn.
The type of sprinkler you have distributes water at a certain rate, dependent upon the pressure at the nozzle.
These figures are readily available from the manufacturer in pamphlets, brochures etc.
For example, a Toro 300 series pop-up sprinkler with an 02 nozzle has a PRECIPITATION RATE of 13.7mm per hour at 344 kpa or 50 psi.
From this we can determine, using the previous nett evaporation figures, just how long this particular type of sprinkler has to run for on a weekly basis to replace 36mm of water (we are assuming a cool season grass) and i is simply 36mm 7 13.7mm equals 2 hours 40 minutes approximately .
The next step is to work out just how you will apply this water. Having determined the rate at which your soil will absorb this water , as outlined in the opening statement, you have to divide the irrigation into watering days. It has been proven horticulturally that it is better to water infrequently but deeply.For argument sake, let's decide to irrigate twice a week. This automatically dictates that each watering cycle will take 1 hour and 20 minutes.
You have however already established that after forty-five minutes, water begins to puddle or run-off. In order to prevent this from happening you would further divide your irrigation cycle so that you have two watering’s say 4 hours apart and for 40 minutes each. This exercise would have to be repeated for each zone or station and the time per area would vary depending upon the type of sprinkler used.