There are many factors which contribute to how much water a lawn needs at each watering, and also how often a lawn needs to be watered. These factors include temperature, wind, the grass type, how healthy the lawn is, the soil type and the amount of thatch in lawn. So while we cannot give an exact magical number for the amount of water a lawn needs or how often to water, we can get very close to some general usage guidelines, and then hone these guidelines to achieve the greatest benefit from our watering regime.
The greatest aim of watering our lawns is to apply enough water so that it reaches deep into the root zone, this practice encourages the lawn to set down deep roots into the soil which in turn are less prone to drying out, and will keep the lawn greener and healthier with less water. By not encouraging a deep root system from deep watering, the roots of the lawn will stay very close to the top of the soil, this shallow soil will dry out very quickly on hot or windy days, and when this soil dries out, so do the roots of the lawn, with the end result being a dried out, damaged or even dead lawn in extreme circumstances.
Watering deeply also aids in the next important factor in watering, which is to water less often. The aim of watering less often is so that there is less water around the top of the soil, if there is an abundant and regular supply of water at the top of the soil from very frequent watering, then the lawn will not set down deep a root system. The importance of this factor was explained in the previous paragraph. When we water less often, the lawn will naturally send out its roots to a deeper level where soils naturally hold onto more water.
In theory, a lawn should have around 2 1/2 inches of water at each watering so that a watering depth of around 6 inches is achieved, these figures are the basis we must work from.
For sandy soils, the water will more easily drain through the soil profile to reach the desired depth of 6 inches, so we don't need to apply the full 2 1/2 inches of watering to achieve this, so we can reduce the amount of time we water for.
The next factor when watering sandy soils is to understand that the sandy profile will drain water away much quicker than a clay soil would, so the soil will dry out more quickly, this means is we have a sandy soil we must water more frequently.
So for sandy soils, we apply less water, but apply it a little more often.
Clay and Loam Soils
Clay and loam soils hold onto water far better, for this reason we will need to apply more water so the water can reach down deep enough to get to the desired depth of 6 inches. Otherwise the top of the soil will simply hold onto the available water, and the end result will be a shallow root system.
This capability of clay and loam soils to hold onto water for longer means that we will not have to water a lawn with a clay / loam profile as often.
So for clay or loam soils, our aim is too apply more water, but to water less often.
Seasons and Weather
The other factors which determine how often and how much to water a lawn is based on seasons and weather.
As the weather heats up in Summer, this will obviously require more water to be applied more often, the same is true for when the lawn is subject to a high amount of wind, as this wind will evaporate the water from the top of the soil, and dry out the lawns moisture much quicker.
In Spring and Fall, the lawn's water requirement will also decrease, and the amount and frequency of watering can be dropped accordingly.
Winter lawn care and watering varies considerably around the country, and while many lawns may be under snow, others won't be and won't require any watering at all. But in the warmer regions, we can be easily fooled into thinking that just because its Winter, the lawn doesn't need any water at all, and this is not always the case!
In temperate climates, always monitor the health of the lawn in Winter, even though the weather is colder, the soil can still evaporate its water supply, even if it's at a far reduced rate compared to other seasons, any wind will also aid in this evaporation, and sandy soils will be more susceptible again. In monitoring the soil wetness in Winter, check for any signs of stress to the lawn, and if necessary dig a sample of the lawn and soil profile out with a spade and check for uniform wetness throughout the soil profile, if necessary begin a very light watering regimen for the Winter.