How To Determine When and How Much to Irrigate (the long version)

Q. I read your article this morning in the RJ and found there to be a significant difference in the watering recommendation  you gave versus that from Springs Preserve. Currently, we have set our drip system to three days a week for one hour each day. We're using one and two gallon emitters on shrubs and multiple four gallon emitters on palms and trees. We have read our water meter and my wife is aghast. For the one hour we are using approximately 975 gallons without using any other water in the house. We have no grass. If we cut back to two days a week, what should we look for in terms of adverse effects?

A. Sorry, I am not aware of what they recommend so I don’t follow the Springs Preserve recommendations. It is difficult to give one recommendation for everyone who lives in the Las Vegas Valley because there are so many different soils, microclimates and irrigation systems. I base my recommendations on a few things that I will cover here as a result of your question. And yours is a good one.

One major piece of research I follow for the area is the plant water demand (called PET; Potential Evapo-Transpiration) that we developed in the Las Vegas valley about ten years ago. Water District. I attached the PET table for all 12 months for you to see.

PET table for the Las Vegas Valley. This table tells you how much water (in a range of low to high and an average) tall fescue turfgrass will use each month. So in one year tall fescue will use anywhere from about 84 to 96 inches of water with an average around 90 inches. It also gives you monthly average water use in the same table, again in inches. I know, I know.....but my  controller is in MINUTES!

 

This is the same PET information but in graphical form. Notice how PET, or plant water use, changes through the calendar years. Water use in June and July are about 8 times more than in January and December.

 

When we irrigate there are three questions we have to answer to set a controller;

1. How many minutes should I set the controller (volume of water applied)?

2. Which day or how many days of the week should it come on (frequency of application)?

3. What time of day should I start watering?

These questions don’t relate at all to what the plant needs and how it should be watered. Here is where the confusion lies. The questions are only directed at the setting an irrigation controller. By answering these questions we hope to try and match the plant water needs. We hope we enter enough minutes and water frequently enough that the plants get enough water.

 

A major problem with irrigating plants in any landscape with an irrigation system of pipes and valves is that the plant receiving the LEAST amount of water drives the setting of the irrigation controller. If we see a plant not getting enough water (and it might be only one plant in the entire irrigation circuit) we rightfully increase the amount of water it gets. Unfortunately, ALL the plants on the same irrigation circuit are increased as well...whether they need it or not. If one plant is not getting enough water, it is possible then that we might over irrigate everything else just to give one underwatered plant enough water. This is why it is critical that the number and placement of emitters for plants on the same irrigation circuit be determined all at the same time, not independently of each other. I will explain how to do that in another blog entry.

 

I will make a separate posting of how to choose drip emmitters and how many to use for each plant. If I forget PLEASE remind me to post it!

A plant should be watered at each irrigation so that the applied water wets the soil under the canopy of the plant to a depth just past the majority of its root system. We are filling a water reservoir in the soil for the plant to draw upon, like a gas tank for a car. The water should come on again when about half of the water in the soil is gone (gas tank is half empty). We then fill this tank again to the brim and start over again.


Bigger plants typically have deeper roots and a bigger size. This means that "big plants need more water than little plants".
I borrowed this picture from an Arizona publication on scheduling irrigations but I don't remember the title. A very nice publication.

 

With a normal gas tank for a car we might fill it when it is below 1/4 full. We can't do that with plants. At 1/4 tank there is not enough water in the soil to keep the plant from wilting or dying... with the exception of true desert plants like cacti. With true desert plants you can go longer between irrigations because many are drought tolerant (they can survive in drier soils).

 

Add emitters as the plant gets larger. The gas tank is made larger in diameter (not deeper) as the plant gets larger. The way we make the gas tank larger (make it wider in diameter but the same depth) is by adding emitters under the canopy. More later.

 

The amount of water we apply to the plant is basically the same each time we water, all year long. This is why I tell people to keep the number of minutes on the irrigation controller the same all year long. If you drive your car less you don’t stop and fill it up the same day each week. As you drive less (less water is used by the plant) you fill your gas tank less often. You change the frequency of application (days each week). But you want to fill the tank or reservoir with the same amount of water each time as long as the reservoir is half empty each time.

The water is used up because of 1. evaporation from the soil, 2. water used by the plant (transpiration), 3. drainage beyond the roots and can’t be recovered (lost). This does not translate into number of minutes, days of the week and time of day at all! So somehow we have to bridge the gap from what the plant needs and how we set the controller.

1. How Many Minutes Should I set My Clock and How Far Apart Should I Put My Emitters?

The number of minutes tells us (with some work on our part) how much water is applied. Let me explain. With drip emitters, the emitters are designated in gallons or liters per hour. One emitter only drips water in one spot.

Once the water enters the soil it moves both down and sideways (laterally). Gravity moves the water down. Water is moved laterally by two forces; the size of the spaces between soil particles (how sandy the soil is and how much organic matter it contains) and how dry the soil is surrounding the soil water reservoir. The dryness of the soil “pulls” the water laterally but the distance the water is pulled is controlled by the spaces between soil particles (how close together or far apart they are).

Sandy soils have particles that are very far apart and so water is not allowed to go very far. As we increase the organic matter in the soil, it increases the distance between soil particles. In sandy soils you can expect the water to move laterally in about a two foot diameter (one foot from the emitter) in all directions. This is why we normally recommend that emitters are spaced about two feet apart if we don’t know what the soil is (worst case scenario, sandy soil). In a clay soil, the water can move as far as six feet laterally! In a sandy soil a five foot tall shrub might need two or three emitters. In a clay soil the same shrub might need only one emitter. (However it is usually never a good idea to have only one emitter for a plant in case it plugs).

The real world. So how many minutes should you set it? Run it long enough so that the water from the emitter waters the entire root zone. The root zone on small plants would be one foot. The root zone of large trees would be two feet or more but two feet is usually adequate.

1. Run the circuit for 15 minutes.

2. Use a 3/8 rebar (long screwdriver if you have one, long pointy thing if you have one of these) and push it into the soil  a foot from the emitter and measure how far it moves into the soil with ease (the depth of wetness). It will move with difficulty when it hits dry soil or a rock. If you hit a rock, move over a few inches and try again. If it is not deep enough, immediately run it for another 15 minutes. Repeat until you have the water moving to the proper depth for the plant. The total number of minutes your ran it will determine the run time or how many minutes to fill the “gas tank”. This should rarely change through all seasons.

3.  To see how far laterally the water is moving, push the rebar into the soil further and further from the plant until you reach dry soil (pushes into the soil with some difficulty). Do this after 30 minutes so that the water has plenty of time to move laterally. Once you hit dry soil you can see how far the water moves from the emitter. This distance is the maximum distance emitters should be placed from small to medium-sized plants. Now double the distance from the emitter to dry soil. This is the maximum distance emitters should be placed if you want the soil wet between emitters and the maximum distance apart you can place multiple emitters around plants.

 

2. Which and How Many Days of the Week Should I Water?

This is another way of saying “frequency of application”. This is how many days we should wait before we fill the soil reservoir again. Remember, we want the plants to use half of the water in the reservoir before we fill it again. This is true of all nondesert plants including most ornamentals and fruit trees.

True desert plants are a different story. Generally speaking, these plants can handle drought or a lack of water more easily with less damage than nondesert plants. So we can actually let the reservoir become drier before we irrigate. This is the reason that we should put true desert plants on a separate circuit (valve) from normal ornamentals and fruit trees. You cannot adjust how often to fill the soil reservoir by adjusting the minutes or putting different emitters on the circuit. This can ONLY be done by selecting which days of the week or month to water. And this requires a separate circuit or different valve.

In order to answer the question, “Which days of the week” requires that you know how much water plants are using and how fast they are using it (inches of water per day, week or month). The PET of a plant or landscape answers this question. The graph and table above tells you the monthly average water use for a tall fescue lawn (PET of tall fescue). The PET graph contains adjusted values (adjusted for tall fescue) of the actual ET of plants. We took the actual ET measurement we determined from research and adjusted it for tall fescue because that is how it is conventionally used. You can convert it back to the actual ET by increasing the daily values during the cool months by 15% and decreasing the monthly values during the hot months by 5%. These values are so close to the PET values that I don’t even bother. I use the PET values as they are to determine plant or landscape water use.

Since the type of soil dictates how much water is in the gas tank and how deep the plants roots are determines the size of the gas tank it is impossible to come up with a generalization that fits everyone. For this reason I have taken the leap of blind faith used our watering schedule at the orchard as a typical landscape site for the valley. Therefore I arbitrarily tell people that for large trees and shrubs

1. Around February 1 we set watering for one day a week (fill the gas tank).
2. May 1 (the PET table makes a significant jump here) for twice a week
3. When it gets above 110F then I suggest three times a week

Disclaimers.

• Turfgrass, flower beds and vegetable gardens are different and are watered more often than this because they are more shallow rooted than trees and shrubs.
• These are for plants that are mulched, not growing in bare ground. Watering must be more often since they use water faster.
• Very sandy or soils with large rocks have smaller gas tanks and must be watered more often.
• Plants in poor health cannot handle stress as well and need water more often.

 

3. What Time of Day Should I Water?

With drip irrigation it makes no difference except you must follow local guidelines and ordinances.

Container plants should be watered before the heat of the day to put water into the soil and cause the soil to heat up more slowly, and, hopefully, lower soil temperatures.

Turfgrass should be watered in the summer just before day break, some time after 2 AM if possible and finish by the time the sun rises. This gives turfgrass a chance to dry out and have fewer disease problems. It is less windy at this time. Wind will blow the water from sprinklers off course and can cause the irrigation system to put water down less uniformly. Less uniform applications of water means more water has to be applied to make up for shortcomings.

I hope this explains how I came to my recommendations.