01.22.13Alan Harris

5 Excellent Perspectives: How to Irrigate a Slope

Montage Resort & Spa slope irrigationThe Montage Resort and Spa understands how to irrigate a slope.

Irrigation on a Slope: What is your experience?

This was the question posed on LinkedIn’s group for the American Society of Landscape Architects. I was intrigued by the question about how to irrigate a slope. As I was gathering my thoughts to share my uniquely professional and sage advice, I started reading the great responses already posted. Disappointingly,  I soon realized I had nothing new to add and even read a few points I might not have suggested. Since the LinkedIn group is private and the information posted was so good, I decided to edit the responses and share here.

 Slope Specifics

  • 4 ac site in Oregon (Willamette Valley)
  • Slope varies 25-35%
  • New construction with many retaining walls
  • Soil profile is approx. 18″ deep loam
  • Plant bed widths vary between 4’ and 35′ with mixed shrub, groundcover and tree plantings
  • Pump on site will assure 55 PSI min. at the top of the slope

Tim May, PLA, LEED AP is a Landscape Architect at Teague Nall and Perkins in Fort Worth, TX offers several good points for consideration on how to irrigate a slope. In order to minimize erosion, design the zones to be parallel with the contours and schedule the zones in a staggered or skipped fashion to allow percolation. (i.e. 5 parallel zones from top to bottom and order the operation in a 1, 3, 5, 2, 4 sequence. The irrigation zones at the bottom of the slope should require less water than the zone at the top of slope due to runoff and gravity moving water down the slope (surface and/or subsurface).

Drift from spray irrigation will be compounded with slopes so use a low angle or flat trajectory spray nozzle and specify a controller with multiple start times and “cycle-n-soak” options. Several quick syringe-type irrigation cycles will apply the proper amount of water for the plant material while minimizing runoff.

William Valdez, President and Founder of FreeFlow in Oceanside, California and a landscape contractor for over 30 years, agrees with Tim’s recommendation of specialty spray heads and suggests using Hunter’s MP Rotators for best percolation. William also points out the cycle and soak will help fertilizers migrate deeper into the soil. When this is not done plants at the bottom of the slope will be verdant while the top of the slope will be chlorotic as the fertilizer leaches from high ground to low ground.

Michael Cook, Vice President, Planning & Landscape Architecture at Firma Design Group in the San Francisco Bay Area writes, “Remember to check with the local water supplier for their regulations. In some areas overhead spray on steep slopes is not allowed and drip irrigation is required.” If overhead irrigation is allowed use a very low precipitation rate sprinkler such as the Hunter MP Rotator or Toro Precision Nozzle. Keep the precipitation rate under ¾” or 1″/hour.

John Woods, Principal with Revival by Design in Salt Lake City, UT points out the importance of the angle of installation for spray heads. If the heads (particularly rotors and impact heads) are installed vertically instead of perpendicular to the soil surface/slope there will be a wash-out condition on the uphill side of the head where a concentrated spray is aimed directly at the soil uphill. This also creates a reduced precipitation rate of coverage of in downhill areas where the spray is projected upward away from the soil.

Geometry has to be considered in the design when designing the irrigation for a slope. Head spacing on a slope might appear to be adequate in plan, but when the additional surface area created by the angle of slope may result in greater than optimum spacing and result in dry spots. Gravity also comes into play as spray distribution pattern down slope. Since the spray pattern is not vertically oriented like it would be on flat terrain, the distribution point shifts toward the downhill side. The pattern will be circular relative to a plan view of the site and elliptical in relation to the actual soil surface, but the center point of the spray pattern is usually a little downhill from the physical location of the sprinkler head.

Separate from LinkedIn, I asked Eric Santos, Division Leader, Irrigation and Water Management at ValleyCrest Companies for his suggestions on how to irrigate a slope. He has started seeing a lot of regulation being mandated where shrubs can no longer be watered by overhead irrigation, regardless of slope. This limits the design options to bubblers, point source or line source drip. Some cities are also limiting precipitation rates on slopes to below 1 inch per hour, which means drip must be designed with low gph emitters spaced far enough apart so 1 inch per hour is not exceeded.

Eric personally prefers bubblers over point or line source. GPH Irrigation makes an excellent bubbler with a built in check valve that works well for shrub plantings on slopes. Zone trees separately from shrubs so the trees can be scheduled for deep root watering. The irrigation design for trees should account for future root expansion as the canopy and root zone expands.

What has been your experience designing, installing or maintaining irrigation systems on slopes?

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Alan Harris

Alan Harris is a water management pioneer. With roots in landscape architecture, Alan has worked with irrigation throughout his career experimenting with hydrozones and a variety of high efficiency irrigation systems. Now, over thirty years in the landscape industry, Alan continues to stay apprised of the latest technology even in a sales leadership capacity as our Director of Sales Operations for our landscape maintenance division. In addition to his contributions to this blog, Alan keeps his hand in water management as a regular contributor to Lawn & Landscape Magazine and speaker at WaterSmart Innovations Conference.


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  1. Tuesday, 6:03 WilletHoward

    RT @ValleyCrest: 5 Excellent Perspectives: How to Irrigate a Slope: Irrigation on… #watermanagement

  2. Tuesday, 1:22 LiteracyandTech

    Ah, the CAPTCHA Code has returned to IE!
    Thanks for writing such technical information in a way that a landscaper wannabe can have some understanding of it.
    Actually I’ll try to keep my day job so that I can hire an excellent landscaper to improve my piece of the planet.
    Thanks for writing, Alan

    • Tuesday, 1:29 Alan Harris

      > The CAPTCHA does a pretty good job of keeping the spammers out and then has the ability to keep everyone out when the enter box takes a vacation!
      Thanks for being one of our kind, consistent and loyal readers. I am glad you were able to follow. I wanted the post to be technically correct for the irrigators, but understandable (and at least somewhat interesting) to the layperson.

  3. Thursday, 9:36 feri asnofi

    Increased knowledge about irrigation, the need for case studies and research for planning, ..

  4. Thursday, 1:25 Joshua Willis

    In my short and narrow experience, watering a slope (as well as other areas) has only short-term value in a landscape. The question I ask is not how to water, but “why water?”
    A vegetative flush and bloom cycle can be regulated with a well-designed irrigation system, but this is costly in the long run. Water is cheap, and install isn’t too expensive, but over the course of decades the maintenance and recurring water cost adds up.
    While irrigation is necessary for certain plants, and certain customers will always need it due to their plant choice, it isn’t a necessity in every landscape. In the Willamette Valley, it’s probably not necessary unless marginally drought intolerant plants are used. But then, I don’t know the area perfectly.
    The best course of action, which would create the most value for the customer, would be an initial watering schedule aimed at plant establishment, and then a tapering off of all watering, excepting extraordinary drought conditions. More important, however, is choosing plants which can thrive in the area by themselves.
    But, our profit is needed. Perhaps the creation of irrigation-intensive landscapes is the best way to make money on irrigation maintenance. It’s not the best practice, though.

    • Friday, 1:21 Alan G. Harris

      > Joshua – thanks for your comments. Due to the word limitations some of the background was omitted. From the Landscape Architect designing the project, “…in the valley the summers are dry and the winters are wet. Once the plant material is established it most likely will only need supplemental water when the summer days get windy and wick all the moisture out of the soil. The maintenance will be required to be at a minimum.”
      There was also discussion on the use of an ET/Smart controller to only apply the amount of water to replace what is lost through evapo-transpiration.
      I hope this addresses some of your questions and concerns.

      • Sunday, 12:07 Joshua Willis

        I see, that makes more sense. Thanks for explaining!>

  5. [...] prime agricultural land into housing we were forced to start carving up hillsides for new tracts.  Irrigating hillside landscaping is problematic in that it is both difficult and ill advised to trench for pipe installation on [...]

  6. Tuesday, 2:28 John Cox

    I find that plant material does like to be washed and why, other than parking islands and such where hardscape need protecting, i avoid it. Additionally, a subsurface cut of the line is virtually undetectable. if, plant obstruction isn’t , or isn’t in the future, going to be an issue, than risers with aformentioned MP rotators and such can be used similar to past stream nozzles. i am of the opinion that the plant will require X amount of water regardless, and weather you provide that over a long period of time in low gallonage instruments, getting the job done right away also has it’s appeal for me. As Mr. Woods implies, yes the angle of the heads (especially on the lower half of the slope) would be proper to angle back the heads/risers to accomodate slope geometry, while on the top of slope the heads can reside vertically to offer more clearance and arc. What i do often is to only shoot rotor heads in one direction top of slope allowing for rapid retraction and return to maximize water retention to the top and not waste water on the down throw. The center system, if there is one should perform operation of an upward half to two thirds angle, never rotating down. Ultimately, these functions make questionable the use of the toe of slope system, and, if necessary, the lower system would operate at a 30 degree angle straght up and angled back to eliminate all ill effects of side to side irrigation at the toe of slope, thereby eliminating water waste into the gutters and balancing the water provided to an uneven plane.

  7. [...] 5 Excellent Perspectives: How to Irrigate a Slope  [...]

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