Allowed Depletion

Lately I have been having a number of discussions with contractors about Allowed Depletion.  It's been getting a lot of buzz in the industry. But what does it really mean and how do we apply it?

Often, when we stand in front of a sprinkler controller, we are trying to decide how often to water and how long to run the valves. We also know it may be 30 days or more before we can come back to change it again. I read a great article by Bruce Carlton several years ago titled, “Wednesday is NOT a Good Reason to Irrigate.” The article teaches a simple principle; don’t water until the soil has had a chance to dry out. This is Allowed Depletion; allow soil moisture to deplete to a managed level before watering again.

Bruce is with a soil moisture sensor manufacturer. Soil moisture sensors prevent watering until the soil needs water. ET controllers, also called weather-based irrigation controllers can do the same thing; only cheaper, easier and more reliably. But wait, not all ET controllers follow the Allowed Depletion concept. Many just replace the water that evaporated since the last watering.

So why is allowed depletion so important? You have heard me echo the plea from horticulturist to water deep and less frequent. It is true, plants are healthier and more stress tolerant when watered deeper and less often. Roots need air; when we water too often, roots struggle to get air so roots stay close to the surface. The concept of deep, less frequent watering is tied directly to Allowed Depletion. As water evaporates from the soil, moisture is “Depleted” from the soil. Irrigation should replace depleted soil moisture once the soil has dried out to an allowed level.

I have read several studies that conclude hose draggers are the most efficient water users. Why, they wait to water until the plant shows a little stress. They also water deep, because they don’t want to water again tomorrow. These same studies conclude that those with manual sprinklers take second place in efficiency. Once again, they wait to water until the lawn needs water. When automatic sprinkler controllers came along they were set and forgotten and as an industry we began using more water than the landscapes need. It also became almost standard procedure to water every day.

To save money and improve irrigation efficiency our industry is now moving towards smart controllers. Smarter irrigation controllers not only react to weather conditions, they follow the allowed depletion concept and know when to water. Smarter controllers also know how much it rained and know how long to leave the sprinklers off after it rains. Yes, rain is very much a part of the allowed depletion concept; moisture from rain needs to evaporate before watering again. You see, some “smart” controllers, say they follow allowed depletion but base it only on daily ET. Smarter controllers are more efficient and run hourly, real-time moisture balances based on ET and measured rainfall.

There is one other aspect of allowed depletion I would like to share. It has to do with root depth. When we talk about moisture depleting from the soil, we are talking about water in the plant’s root zone. Deeper roots means more water storage and less frequent watering.

Let’s take an example of a turf area with roots about 5” deep in a silty loam soil. The soil can hold about 1” of water at Field Capacity. The goal is to wait until about half of that water evaporates until watering again. In other words, once 0.50”of water has evaporated, irrigate 0.50”. Spray heads apply water at about 1.5” per hour, so to apply 0.50” of water will take about 20 to 25 minutes; depending on system efficiency. In the heat of the summer, it not uncommon for 0.25” of water to evaporate in a day. You can see where the idea of watering spray heads every other day came from.

The above example looked at turf; shrubs have much deeper roots and should be watered less often; the soil holds more water so it takes longer to deplete soil moisture.

Here is the problem, the weather changes, 0.25” of water doesn’t evaporate every day, plus we get rain. Below are three, two-week, soil moisture balance simulations that show how changing weather conditions affect how often watering is needed:

 

 

 

 

Let me invite you to our website to run your own irrigation simulation. http://www.weatherreach.com/mad/ Here you can enter your plant type, soil conditions and root depth. Then select a time of year and weather station near you. The website will display a simulation of smart watering.

 

You have many properties and hundreds of sprinkler controllers to program. Technology can become your best friend. Smart controllers know how fast the landscape dries out. Smarter controllers use accurate high-quality data to manage watering schedules.