Saving water by knowing your soil type

When it comes to irrigation, many people like to rant on about ‘deep watering’ and how this is the best way to get healthy plants and lawn. Unfortunately what isn’t explained is that throwing a lot of water at your garden in one cycle doesn’t always work. Deep watering can’t always be achieved in one watering period each week. There is one simple reason for this that is overlooked, Soil Type.

Soil type plays a big part in how long and how often you can water your garden. How can this help me save water?

There are several types of soil from sandy right through to clay. If you live around Victoria, you are unfortunately in a moderately clay based soil region.

How does soil affect irrigation?

Soil affects irrigation in two ways. Firstly if you are in a clay based soil area then water takes along time to get through the ground to be used by plants, if you’re in a sandy area then water runs through and its hard to get it to stay there.

Secondly with a clay soil, once the water is in the soil, then clay soil will retain it for longer, as opposed to sand that has very little water retaining qualities to it at all.

Now having taken soil into account and established what type of soil you have. Now comes the technical part.

If I have clay soil what can I do?

Don’t despair, just because you have clay soil. Clay soil just means you have to have a slightly different irrigation plan of attack.

Firstly with any irrigation it is best to look at the whole picture. Yes, you just want to get water to your plants and lawn, but no you don’t want to waste money or water doing it. This is what a lot of landscaping / irrigation companies don’t take into account. If you want to keep plants and turf healthy, you can just set your irrigation to water for an hour once a week, because it just wont work……why? Evapotranspiration.

What is Evapotranspiration?

Evapotranspiration is the process of getting water into the ground and what factors affect it.

When you run your irrigation there are several factors that need to be taken into account.

  • Soil – what type of soil do you have and how well does it take water?
  • Time – when are you watering.
  • Slope – how much slope is affecting water run off?
  • Weather – sun, wind, temperature

Simply put ‘evapotranspiration’ is the rate of water going into the ground to the rate of water evaporating, and the amount running off the ground.

So imagine you have a clay soil lawn which is 10ft x 10ft (100sqftf) with a slight slope, irrigation heads have been installed perfectly with the correct head spacing (say each corner at 10ft apart). Now its time to set the clock for watering, your irrigation company has said 1 hour once a week . That’s good if you want to pay for water that will run down the drain after probably 15 minutes of watering.  What irrigation heads do you have in that lawn? Rotors…that means that each is pushing out 1+ gallons per minute. That’s 4 gpm or 240 gallons per hour.

Rate of absorption for clay soil; 0.25 – 1.25cm per hour

The irrigation companies like rainbird, Toro etc have calculated that to get 1” of water to your lawn take 0.62 of a gallon per sqft.

1”(2.5cm) of water for 100sqft is 62 gallons

With this calculation, we can quickly see that a clay lawn will only take approximately 30gallons per hour.

Having figured this out, now we can start to look at alternative options.

The best option is to look at reducing zone running time. If the zone is putting out 4 gpm, then it is best to run the zone for 7 minutes and then give the lawn time to take the water in before running for additional 7 minutes. I would maybe have one watering time in the morning and the second in the evening. This will give the lawn time to utilize the water from the first running time before adding additional water.

With this method you can monitor your lawn and adjust zone watering times, through carefully visually monitoring your lawn to see if you can increase or decrease your zone running time.

However with this example we have now just made a saving of  178 gallons in one watering time, that’s over 9000 gallons in a year!  Now thats saving water!