So what is managed allowed depletion?

It is the way plants and lawns are meant to be irrigated.

It is the technique of waiting until the moisture level in the soil is depleted to a predetermined level before applying more water. By watering this way, your plants and grass will be healthier because it will encourage deeper root growth and allow oxygen into the root zone.

To be able to water this way, we need to know where the moisture level is currently, and at what moisture level do we need to water again.

Where is the moisture level in the soil?

Water is lost from the soil in two ways, evaporation, and transpiration. It is called evapotranspiration (ET).

Evapotranspiration is affected by soil moisture, temperature, solar radiation, humidity, and the wind. By measuring and using this information we can calculate how much water the soil has lost and as a result when we need to irrigate.

Water is added via precipitation either naturally or via irrigation.

We can keep track of the water entering the soil by measuring rainfall and knowing how much water we apply when we irrigate.

How is managed allowed depletion used?

We know how much water is lost each day via evapotranspiration and we know how much water enters the soil via precipitation and irrigation. All we have to do is manage the water depletion.

Based on the type of soil and the depth of the roots there is a fixed amount of water available to the plant. Water that is deeper than the depth of the roots is unavailable to the plant and different soils can hold different amounts of water. Using soil type and root-depth we calculate the maximum amount of water available to the plant, this is called the plant’s available water (PAW).

Depending on the plant type, we manage the amount of plant-available-water (PAW) we allow to be depleted from the soil before we have the irrigation system turn on and replenish the soil.

Typically the managed allowed depletion point is around 50% of the plant-available-water.

How does it compare to timer-based irrigation?

Let’s take a look at two systems, System A and System B. The two systems are identical in every respect except for how the controller operates.

A conventional controller operates on a fixed schedule.

Most clients set their controllers to apply a minimum of 1″ of water per week. That is normally accomplished by applying 0.4″ of water every other day. Most conventional controllers also use rain sensors, so let’s assume this system has a rain sensor that will suspend irrigation if it measures 0.25″ of rain.

An ET controller uses the managed-allowed-depletion technique to plan its own schedule.

It will calculate daily the water requirements of the plant and water accordingly. We assume the grass has roots that are 3″ deep and the managed-allowed-depletion is 50% the PAW.

System A with the smart controller allows the top 50% of the soil to dry before watering

System A with the smart controller used 62% less water and increased the health of the lawn