As a landscape contractor, I bought into the myth that an automatic irrigation system was a necessity. Now I have worked through the math and science behind plants and soil, I’m not so sure.
Let’s do the math on our recent project at the San Marcos Historical Society to find out.
How much water do plants need to grow where you are?
Plants need water to live. They lose water through both evaporation and transpiration – plant sweat. Together this plant water loss is called Evapotranspiration and it is abbreviated to ET. When scientists measure this, they use cool season turf grass as the baseline. So, when we discuss how much water plants need, it’s shown as a percentage of this baseline. You can look up the Evapotranspiration rate for your location on the CIMIS website. Here in San Marcos ETo is 49.7”/year – let’s call that 50” to make it easier to do the math.
What are you trying to grow?
Clearly we are not trying to grow cool season turf grass! We are trying to grow California Native plants with moderate, low and very low water needs. The Water Use Classification Of Landscape Species (WUCOLS) is a list of plants and their water needs. The water needs are expressed as a plant factor (a percentage of ETo). For moderate water plants the plant factor is 50%. These plants need only half as much water as cool season turf grass to grow well. If we are growing moderate water plants in a place with an ETo of 50” per year, the plants need 25” per year.
Likewise, low water plants have a plant factor of 25% so 12.5” per year. Very low water plants have a plant factor of 10%, so 5” of water per year.
How much water is in different types of soil?
Different soil types hold onto different amounts of water. When we go to the beach and pour water into a hole, we see how quickly the water just drains away. Sandy soil has very little ability to hold onto water. On the other hand, when we dig a hole in clay soil and put water in that, it can take hours or days to drain. Clay soil clearly demonstrates its ability to hold onto a lot of water.
Studies have shown that mixing in 3” of organic matter into sandy soil increase that soil’s water holding capacity by 2.5 times. Other reports show for one part of organic matter we add to the soil we can increase its holding capacity by 4 times. This figure may even be larger, but let’s be conservative and go with the 2.5 times figure.
How much water is in the soil at our project?
At our project in San Marcos the soil type is silty clay or clay, so the chart used by soil scientists:
We can assume that our soil is likely to hold onto somewhere between 1.20 – 1.70 inches of water per foot of soil. Let’s go somewhere in the middle and say 1.50” of water per foot of soil. Let’s then add in the additional water holding capacity of the organic matter, bringing our water holding capacity to 3.75” of water per foot of soil.
Since many of our plants are likely to be small, with roots that are less than a foot long, we will probably want to talk about how much water there is in each inch of soil. Let’s divide 3.75” of water per foot by the 12 inches in the foot, to give us 0.3125” of water per inch of soil.
Just take a moment to picture that: an inch of soil that contains about a third water. Remember this is a conservative figure, it could be more like half water depending on the study you believe.
How big a bucket does that plant have?
OK, so now let’s think about how our plants get to that water – through their roots. But different plants have different root lengths, so they can only access the water that’s available to their roots. I like to think of this as a bucket: bigger/longer roots = bigger bucket. Young plants only have a little bucket, and that’s why we need to irrigate more often as they get established. More mature plants will have bigger buckets, they can probably get by without our help.
How much water is available to our one gallon plant in the ground? The amount of water available to the plant (water holding capacity of the soil) at our project is 0.3125” of water per inch of soil. Our baby plants have about 10” of roots so the amount of water available to the plant (Plant Available Water or PAW) is the water holding capacity of the soil (0.3125”) times the root depth of the plants (10”), or 3.125” of water.
How much water does that plant need per day?
The next question is, of that 3.125” of water available to our new plant, how much gets used each day? Let’s assume that the plant in question is a low water plant meaning it needs about 25% of ETo, and because we know we are trying to grow it in San Marcos and the ETo there is 50” per year, the amount of water per year is 12.5”. Let’s further divide that by 365 to get the amount for each day – 0.034” of water per day. Literally, a fraction of an inch per day.
Bringing plants back from permanent wilting point
If there is just over 3” of water available to our plant, and it only needs 0.034” of water per day, that’s quite a few days of water right there. But there is a bit of a catch here: while soil can hold onto water, it’s not very keen on giving it up when it gets to be dry. We know when plants can’t get to the water they need they start to droop or wilt. They go into this state called the permanent wilting point, and it’s hard to bring them back from that. We choose an arbitrary point and say that once 50% of the water has gone from soil, the plant will go into its permanent wilting point. So how often will we need to top the soil up with water?
How often does the water bank need topped up?
We know that the plant available water is about 3” and we are choosing not to let this go below 50%, so the amount of water that we are going to allow to deplete (Allowable Depletion), before we top up the water in the soil, is 3.125” x 50% = 1.5625”
The number of days between each top up (called the Irrigation Interval) is the allowable depletion (1.5625”) divided by the daily plant water requirement (0.034”), or 45.88 days. Golly gosh! Over a month between irrigation intervals. That is only if we stick to the very safe figure of 50%. Maybe our native plants are happy for the water to get to 30% remaining in the bank, giving us even more time between irrigation.
Now we do want to be sure that when we do irrigate, we add enough to top the water bank up. We know the plant has been using 0.034” of water per day, and it’s going to be 45 days between irrigation cycles, so we will need to add 45 x 0.034 = 1.53” of water.
Phew. An inch and a half every month or so.
But wait… what about rain?
We do get rain around here. Last year (2015) Lindberg Field got close to 12”, but Ramona (the closest weather station to San Marcos) only got about 8”. Our average is about 10”.
Our plants only need about 12.5” in a year, so we are only really missing 2.5” per year. We just said we only need to add about an inch and a half each month, so with the rain, we are looking at needing to irrigate for about 2 months per year, while our plants are small. When our plants are bigger, with bigger roots, they will have a bigger bucket to pull from, and will be able to go for a longer period between water top ups. Let’s be totally pessimistic and say we are only going to get another 8” of rain this year. We will need to add an additional 4.5” over 3 months.
How does this translate into sprinkler runtimes?
Time to irrigate 1-1.5”Break into0.25” per hour4-6 hours3 x 2 hr runs
Handheld hose end 5” per hour18 minutes2 or 3 days of watering, holding the sprayer in one place for at least 6 minutes each time.
|Regular sprinklers||2” per hour||45 minutes||3 x 15 minute runs||MP Rotators||1” per hour||1hr 30 minutes||3 x 30 minute runs||Rotators||0.5” per hour||3 hours||3 x 1 hr runs||Oscillating sprinkler (plus spigot timer)|
What time of year are we talking about here?
California native plants need their water when we traditionally get it – during the winter – October to May. If we have a dry October – less than 1.5” of rain, we will need to irrigate. If we have a dry November, we will need to add 1.5” of water, and so on. But let’s then assume we have a bumper December with 3”-4” of rain, we can skip January and February’s irrigation. During that time, we might get even more rain, pushing the time to start our irrigation out further.
What about that rainwater capture the San Marcos garden does?
Yep, that’s right the garden harvest rainwater from two adjacent roofs. When added up, the roofs are a total of 1700 square foot. In a one inch rain, we can capture 1,000 gallons of water. The 1700 square foot of roof is more than double the size of the landscaped area (700 square foot) so if we do get 10” of rainfall, because we are collecting from the roof and directing into the garden, it would be the equivalent of 30” of rain. More than enough for our plants that only need 12.5”.
During the early days after planting, the plants will need some irrigation. The winter and spring are when we usually get our rain. If we have a particularly hot, dry summer next year, they may need some more. After that the plants will be on their own, taking care of themselves. A light spray in mid-summer might make them look better, but it’s not necessary.
You do the math
The question then becomes is it worth the time, expense and management costs of an irrigation system for just a few months of use? The San Marcos project is run by volunteers so automating irrigation is a good thing. The trouble with automation is that people tend to ‘set it and forget it’, and things like over- or under-irrigation go missed until something goes wrong. It is easy to miss an irrigation leak for several months until a large water bill arrives. Automatic irrigation systems are horribly inefficient. Traditional spray systems deliver only about half the water they put out into the right spot. Drip systems are better but still not 100% efficient. Plants that need 50 inches of water irrigated with a spray system will need close to 8 feet of water.
The other thing about this project is that the landscape area is relatively small – 700 square foot. You might say a larger area it would be too much for someone to manage with a hose. Most suburban front lawns are that size, and ‘hose draggers’ are the most efficient users of water.
Thirdly, the project collects water from 1700 square foot of roof. Suburban roofs are typically 1200 to 1500 square foot. It’s often not possible to collect the water in one place. The project collects more than enough water, so a smaller roof area will still provide enough.
There you have it. Pay for a professionally installed irrigation system that is used a handful of times. Or buy a new hose, an oscillating sprinkler, a timer and pay yourself to go out and water. Your choice.
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