There are many factors contributing to the success or otherwise of a lawn. A main cause leading to failure is a compacted soil, which lacks oxygen in the root zone, and does not allow for the adequate infiltration of water. Other vital factors include a correct mowing regime, a suitable feeding programme, and periodic de-thatching. Adequate moisture though is the single most important question as to whether a grass will succeed or not.
Firstly, how much does your grass need? This depends on where you live and the lawn type you have. The perennial grasses in general use in hot, dry climates, such as varieties of Bermuda, Zoisia, Kikuyu, or Paspalon, “consume” moisture at some 50% of the daily evaporation rate for a particular area. In central Israel where I live, this average figure during the summer months is around 8mm per day, which means that lawns need some 4 liters per square meter per day. Contact your local meteorological station, to find out the figure for your locality.
The second question is how often you should water the grass. The grass types previously mentioned are deep rooting and actually do better on less frequent, but longer watering. Such a regime encourages the roots to grow deeper into the soil, making the grass hardier to drought, pests, and disease. Assuming that the depth of soil is over 50 cm, then an established lawn can be watered every 7-10 days in heavy, clay soils, 5-7 days in medium soil, and perhaps 3-5 days in light, sandy soils. If one has been irrigating frequently, but with much smaller amounts, then it makes sense to break into this new regime carefully, monitoring the results as you go.
To provide a personal example, I watered new lawns I put down this summer 3 times a day during the first week, once a day from the second week, and so on, so that by the end of the summer they will be irrigated once a week, but with proportionately greater amounts in accordance with the changing frequency. The lawns are every bit as green as those which are watered every day. Here is an example for calculating the amount of water to be applied in a particular locality, where the daily evaporation rate is 8mm, the area of lawn 30 square meters, and the soil of a medium/heavy type.
Quantity (liters) = Average daily evaporation rate * 50% * Area of lawn (square meters) * Interval between watering
Quantity (liters) = 8(mm) * 50% * 30 (square meters) * 7 (days) = 840 liters
How long should you water for? This is quite easily calculated by dividing the quantity that is required by the flow rate. (the amount of water emitted per hour) The flow rate can be discovered by registering how much water the system emits in say 10 minutes, and then multiplying that figure by 6 in order to arrive at the amount per hour. For example, if the quantity required is 840 liters, and the flow rate is 500 liters per hour, then watering time = 1 hour, 40 minutes.
One word of caution though. This method only works satisfactorily if the irrigation rate (the quantity of water emitted per hour relative to the area ) is low enough for the soil to absorb it. If there’s a lot of run-off from your lawn, then you should change the sprinkler nozzles to ones that emit less water per unit of time. Failing that, more water has to be added to the calculation, in order to compensate for the run-off.