Monitoring Irrigation for Green Roofs

• Intro/TOC
• Soil Structure & Green Roofs
• Baseline Soil Moisture Sensors
• Summary of Webinar & Resources
• Questions

0:00 – 2:15: Intro/TOC

2:16 – 16:44: Soil Structure & Green Roofs

• Soil particles are identified by their size.
• The larger the particles, the more space between them.
• Soil is classified based on the relative proportions of types of particles in a sample.

 

Basic soil types:

• Gravel
• Sand
• Silt
• Clay

 

Keep in mind (4:20):

• Water is held between the particles – not by the particles.
• Soils on a given site will rarely fit into the textural triangle presented in this webinar.
• It’s easier and more repeatable to measure than to guess.

 

Plant water use characteristics (5:00):

Plants are as variable as soils:

• The same plant planted in three different soils will have three different water use characteristics.
• The same plant planted in three different in the same soil with three different fertility levels will likely have different water use characteristics.
• A healthy, disease-free plant will have different water use characteristics from those of a sick plant in the same soil conditions.

 

Understanding soil moisture holding capacity (5:40)

• Saturation
• Field capacity
• Maximum allowed depletion
• Permanent wilting point
• Oven dry

 

Field Capacity: How much water can be held in media in the face of gravity (7:48)

 

Allowed Depletion: How much media dry-down can occur to maintain plant health or aesthetic value (8:50)

• Dependent on many environmental factors
• Can change dramatically in a short period
• Depends on the opinion of the landscape manager

 

 

Field Capacity vs. Allowed Depletion (9:34)

Coarse media will have a lower field capacity and, therefore, a tighter allowed depletion than a tighter soil (such as a heavy clay).

 

Do the same principles apply? (10:12)

• Green roofs and walls use specified, engineered media.
• How are field capacity and allowed depletion found for the blend?
• Will water be available for the plant material when needed?

 

Don’t call it dirt! (Chuck Friedrich, RLA, ASLA) (11:00)

“Generally, if you are mixing mixing your own substrate, a good guideline is approximately 75–80% inorganic (i.e., expanded slate or crushed clay) to 20–25% organic (humus and some clean topsoil). This will provide essential drainage and soil air capacity, and sufficient organic nutrients for the shallow-rooted plants.”

 

Good media should possess six properties (11:50):

• Good drainage and aeration
• Water folding capacity (without getting too saturated or heavy)
• Nutrient holding capacity (cation exchange capacity, or CEC)
• Permanent
• Lightweight but sturdy (can’t shrink or blow away)
• Stable (must support the plants)

 

 

As a result, engineered soils are the only option, whether the project is extensive or intensive (12:25).

Components for intensive projects:

• Lightweight aggregate 35 to 75%
• Sand 10 to 50%
• Organics 5 to 20%
• Clay and silt 0 to 2%

 

Components for Extensive projects:

• Lightweight aggregate 50 to 100%
• Sand 0 to 30%
• Organics 0 to 40%
• Clay and silt 0%

 

 

Example green roof application: Facebook Campus in Menlo Park, CA (9-acre green roof) (13:20)

 

Note: If you don’t measure (for example, using moisture sensors), you can’t manage to meet expectations or objectives. (15:53)

16:45 – 27:59: Baseline Soil Moisture Sensors (16:45)

How Baseline soil moisture sensors work (18:00):

They send a high-frequency pulse of electricity down an embedded wire path

The high frequency of the pulse causes the sphere of influence of the pulse to move outside the sensor blade and into the soil around it.

When the pulse travels through moisture, it slows down.

The sensor measures the speed and then converts this measurement to a moisture content reading.

 

Light travels through air faster than through water, which allows the sensors to provide an accurate measurement of the amount of moisture being held between the soil particles. (19:10)

 

Moisture movement in the soil profile (19:59)

 

You’ll set a soil moisture threshold in the Baseline irrigation controller (as with a thermostat). (22:40)

• The sensor monitors the soil moisture.
• When the threshold is met, the system can be set to either turn on the irrigation at the next scheduled start time or shut off irrigation.

 

 

Soil moisture sensor watering strategy (23:30)

Lower threshold (also known as lower limit)

• Best for conserving water
• Maximizes the plant’s rooting potential
• Default setting for BL3200s
• Most often, this strategy is used to irrigate deeply and infrequently (often referred to as the BMP).

(Lower threshold tells the system to turn on based on soil moisture, and then turn off based on time.)

 

 

Maintain a desired moisture content in the soil by watering more frequently for a shorter duration. (26:20)

 

For example: Program three start times within the allowable water window and break up the total zone on time into three equal durations. Let the sensor determine how many cycles are needed.

28:00 – 32:53: Green Walls (28:00)

Air movement around the structure creates many microclimates, varying the irrigation need of each panel. (28:20)

 

How does a moisture sensor monitor this? (29:07)

 

Vertical irrigation zones (i.e., top, middle, and bottom) with a sensor in each zone, programmed for less-frequent watering at the bottom of the green wall (29:55)

32:54 – 35:49: Summary of Webinar & Resources

• Soils vary greatly in composition, which determines how much water can be held and made available to the plant material.
• The same field capacity and allowed depletion principles apply to engineered media.
• The right sensor technology measures moisture accurately in real time to allow for intelligent irrigation decisions.
• Maintain a desired moisture content to promote plant health and aesthetic value.

 

Online resources for specification (34:40)

• BaselineSystems.com
• email Chris Wright at This email address is being protected from spambots. You need JavaScript enabled to view it.

35:50 – end: Questions

Question: Soil-based and fabric-based green walls will have different moisture profiles. Will the sensor account for these differences? (35:50)

Answer: As long as the moisture sensor has good material contact to the medium (soil or fabric), it will produce an accurate reading of the moisture content.

 

 

Question: How many sensors are on the Facebook roof? Are they operating valves or just used for measurement? If they’re operating valves, do they need to be calibrated to their locations? (37:45)

Answer: The roof has 25 soil moisture sensors. The sensors are planted throughout the green roof based on hydrozones, which in turn are based primarily on plant material, or on sprinkler type. The sensors are set up to control the frequency of irrigation of various zones within each hydrozone. Similar zones are linked to the same sensor within a given hydrozone, allowing like zones within a hydrozone to be controlled by the same sensor.

 

 

Question: Is there a general depth at which moisture sensors need to be buried on green roofs? (39:56)

Answer: The Baseline specification is to bury a sensor within the top 1/3 of the active root zone of the plant material it monitors.

 

 

Question: Is mulching recommended for preventing moisture loss? If so, what type of mulching is recommended, and how often should it be replenished? (42:40)

Answer: Regardless of the type, mulch will help by preventing evaporation. The recommended mulch type and rate of replenishment will vary depending on the region and climate.

 

 

Question: If a site does have different hydrozones with varying root depths, at what depth should the sensors be placed? Will they measure moisture at the different depths? (44:00)

Answer: The typical Baseline sensor has an 18-inch blade.  The moisture reading is averaged across the blade profile. Sensors are often installed at an angle for a more indicative moisture reading across the deeper profile. For turf grass, the sensor should be installed horizontal to grade in order to be buried at the recommended 3 to 4 inches. For trees or shrubs, it should be buried at 30 or 45 degrees for a better indication of moisture.

 

 

Question: When used on green walls: the sensor often only reads a small part of the wall, and the moisture profile can be sporadic in a fabric wall. Is it possible to link multiple sensors and adjust the readings to an average so the watering isn’t based only on one spot? (45:40)

Answer: No, this isn’t possible. The sensors act independently in conjunction with the zones to which they’re assigned. A lot of it would depend on the design of the irrigation zones in the panel. More zones equals more sensors and a more accurate reading.