Case studies: Three water conservation projects
The following stories depict situations you may encounter and provide possible solutions and recommendations.
Case Study #1 - Removing a front lawn lined with street trees
Jennifer has decided to reduce the watering in her front yard and will be replacing her lawn with a low-water use ground cover. Two Southern Magnolias (Magnolia grandiflora) street trees will be affected by this change. They are along the street – half of the tree canopy is over her yard, and half over the street. Jennifer wants to keep using her spray irrigation system, but is not sure how often or when to run it. She knows that the trees are her responsibility, and wants to retain the value of her property since she plans to sell her house next year.
Among the two solutions below, solution B is strongly preferred, as it is a much more efficient use of water.
Solution A: Keep the same irrigation system, but run it less frequently
Jennifer can save water and preserve her trees by running her irrigation system less frequently, but for a longer duration. She will want to irrigate the trees to promote deeper rooting, and thus water to a depth of 18 to 24”. She looks up Magnolia in the Canopy Tree Library; they fall into the Moderate Water-use category. From the Canopy Watering Chart she sees that they call for 2x a month watering during the dry months.
Since Jennifer has new plantings, her landscape will need more frequent watering for the roots to establish. If she runs the irrigation for 45 minutes weekly the first summer, she will help establish the ground cover and water the trees. Because she is using spray irrigation, she will keep her mulch layer over the soil to a depth of only 1”, so that the water can penetrate.
If Jennifer leaves on her existing spray nozzles, it is best to split the watering into two sessions, so that it does not run off the clay soil. For example, run for 22 minutes, turn off for 1 hour, then on for another 23 minutes. Most irrigation controllers can be programmed to do two start times automatically. A better choice would be to replace each nozzle with water-efficient nozzles, which have a much lower water application rate. She will not have to split the water run time into two segments, because the water will not run off the soil. The next summer Jennifer will cut back watering to twice a month, assuming that the new groundcover has rooted in well.
Solution B - Preferred: Install drip irrigation
To use water most efficiently, Jennifer converts her system from spray to drip method. She screws off the pop-up spray heads and caps off all of them but one. On the remaining riser, she screws on a sprinkler body replacement that comes with a filter and pressure regulator. (Consult with your irrigation contractor to look at all conversion possibilities. Another great source for do-it-your-selfers is the Sunset Publishing book “Sprinklers and Drip Systems”.) From the new sprinkler body she attaches a PVC swivel Tee, and 1/2" drip tubing. This “backbone” tube of the system is laid on top of the ground around the perimeter of her watering area. Off the 1/2" tubing, she attaches 1/4” tubing which extend 1 Gallon Per Hour (GPH) drippers to each of her groundcover plants. She also replaces her existing on/off valve with a low flow valve, because the drip system will be drawing water much more slowly. She mulches the entire area with a 4” layer of bark chips.
From the Canopy Watering Chart she sees that drip irrigation should be run for about 60 minutes each watering. As in Solution A, she runs the irrigation once a week during the dry months, next year reducing it to 2x a month after the ground cover has established.
Case Study #2 - Two tree species at the back of the lot
David has three 40 year-old Monterey Pines (Pinus radiata) at the back of his lot which have never been on an irrigation system. Everyone tells him the trees add character to the neighborhood. His wife planted two small Japanese Maples (Acer palmatum) underneath them 5 years ago, as it is one of the only places on his property that has some shade. The maples have never looked good, and someone told him his Pines probably have beetles. He does not want to lose any of the trees. He tried to dig in the soil and it was hard as a rock.
Solution: Drip line, mulch and transplant
To help his trees, David must periodically deep water the Pines in the dry months. The Japanese Maples like a rich, well drained soil with an ample water supply. Monterey Pines have a Canopy Tree Library rating of Moderate Water Use, and Japanese maples, a rating of High Water Use. First, David loosens up the top layer of the soil under the trees with a pick ax or spade. This makes a huge difference in allowing water and air to penetrate the soil. Next, he decides to keep it simple and run a 1/2" drip line on top of the soil to the trees from a hose bib he rarely uses on the side of the house. He will need a vacuum breaker, filter and pressure regulator at the faucet connection. He runs the 1/2" line along the edge of the property to the trees and along the back fence. Off of the 1/2" line, he adds 1/4" tubing that has 1/2" GPH internal emitters spaced every 18", covering the area in a grid pattern. After consulting the Canopy Watering Chart, he decides to run the irrigation 2x a month for 90 minutes during the dry months. After installing the irrigation, David lays down a 4" layer of bark chips to keep the soil moisture in.
David will need to inspect the drip system every spring since he cannot see it working under the mulch. He is going to hire a Certified Arborist to check the Pines' health, especially checking the large limbs for likelihood of breakage. Finally, because the Maples are not compatible with the clay soil or the watering plan, he and his wife transplant the Maples to the one high-water-use planting bed they have decided to keep in front of the house where his prized azaleas and camellias grow well.
The following year, David automates the irrigation system by adding a low pressure valve and automatic controller, and he delivers the water to the back part of the lawn through buried PVC pipe instead of the 1/2" tubing. These changes make his system easier to maintain.
Case Study #3 - Native oak
Maria has a very old Coast Live Oak (Quercus agrifolia) in the corner of her small lot. It shades the house keeping it cooler in the summer. There is ivy underneath the tree, and spray heads water the area twice a week. She think this might be an area she can reduce water usage, but is not sure how to proceed.
Maria has a tremendous asset that she should absolutely preserve! From the Canopy Watering Chart she sees that Coast Live Oaks should not receive supplemental water in the summer. First she removes all the ivy under the tree, so it does not compete for water with her oak tree. This loosens the soil, so that rainwater can easily percolate to the roots. She applies a 6” layer of mulch under the dripline of the tree, leaving the trunk undisturbed, with no mulch against it. Then she turns off the irrigation system completely. Her native oak will use just the water nature provides during the winter rains.