Irrigation Design 

Pipe Sizing

BASED ON VELOCITY OF WATER THAT WILL FLOW THROUGH IT IN FT/SEC

PIPE DIAMETER AND MATERIAL AFFECT VELOCITY

Irrigation Design

Soil Properties

• Infiltration rate (inches/hour): depth of water infiltrated after 1 hour
• Field capacity: volume of water soil will hold after gravity drains excess.
• Available moisture: % moisture in soil available to plants

Irrigation Design

Infiltration Rate

Affected by

• soil type: coarse grained soils, high; cohesive soils low (sand >loam>clay)
• slope (steeper slope, less infiltration; easier for water to move along surface)
• antecedent moisture: dry soil, high initial rate; decreases as soil approaches saturation

Irrigation Design

Field Capacity

• Water held in soil by capillary action (attraction to soil particles)
• Fine particle soils (silts, clays) have greater surface area/volume and therefore higher field capacities
• High field capacity: hold larger volume, deeper and less frequent watering

Irrigation Design

Available Moisture

• Differs from field capacity.
• Plants must be able to suck up water.
• High field capacity soils (e.g., clay) may hold water tightly, therefore less available
• Expressed as % of field capacity

Irrigation Design

Plant Characteristics

• Depth of root zone
• Evapotranspiration rate in peak growing periods
• Water requirement for good appearance

Irrigation Design

Plant Characteristics: Root Zone Depth

Determines depth of soil profile for water calculations

• Ornamental turf grass ~9-18 in
• Mown turf ~6 in
• Shrubs assumed ~  24 in
• Trees ~36 - 48 in

Irrigation Design

Components

• Water supply
• Delivery system
• Distribution devices

Irrigation Design

Water Supply: Municipal

• Usually requires only tapping existing line behind a water meter or connecting to main and installing new meter
• Volume of water available depends on supply line size and water meter size
• Commercial/institutional usually 1 inch service or larger

Irrigation Design

Water Supply: Municipal

• Pressure available at meter varies and depends on design of water distribution system.
• Field testing necessary.
• Test pressure more than once and at different times of day. Nighttime usually higher because lower water use period

Irrigation Design

Water Supply: Wells

• Best to have preliminary idea of volume of water expected as well as quality
• If found at reasonable depth (500' or less), can be very cost effective
• Requires pump and optional pressure system

Irrigation Design

Water Supply: Wells

Pumps:

• self-priming centrifugal (shallow wells, low volume & pressure requirements)
• submersible: fit entirely into well casing; for deeper wells with higher volume & pressure req's.
• submersible turbine: high vol & pressure demands

Irrigation Design

Water Supply: Wells

System Pressurization

• pumps (simple system)
• gravity (elevated water tank)
• pressurization device

Irrigation Design

Water Distribution System

• Irrigation Pipe & Fittings
• Valves
• Sprinkler Irrigation Heads & Drip Emitters

Irrigation Design

Irrigation Pipe & Fittings

Thermoplastics

• PVC (good all-purpose), Polyethylene (low pressure only), ABS (sprinkler bodies, nozzles, drip emitters, valve bodies)

Metals

• Cast Iron: most durable; protective rust layer
• Galvanized steel: deteriorates, not good for ground contact
• Copper: flexible, strong, chemical resistant but expensive 

Irrigation Design

Pipe Fittings

• join lengths of pipe
• threaded: galvanized steel, thick-wall PVC
• bell & spigot: cast iron, large diameter PVC
• mechanical joint pipe (w/flange): cast iron, large diam. PVC
• slip fittings: copper pipe (soldering, sweating), small dia. PVC (solvent welding/chemical bond)

Irrigation Design

Valves

• used to control flow of water in system from supply to individual heads and to drain or maintain water in pipe
• manual valves: allow parts of system to be isolated for maintenance or repair while rest of system can still operate
• remote control valves: turn water on/off at specific points in system

Irrigation Design

Manual Valves

• Gate Valve: flow straight through; have machined seat that relies on tight fit to seal valve
• Globe valve: preferred for dirty water since washer and seat can be replace; less efficient hydraulically than gate valve

Irrigation Design

Remote Control Valves

• operated hydraulically or electrically
• available with plastic or brass bodies, with or without pressure reg devices.
• plastic good for valves up to 2 in. and water pressure up to 100 psi

Irrigation Design

Special Valves

• Check valve: allows flow only in one direction
• Drain valve: used to purge water from lines and pumps (often when subject to freezing temp)
• Pressure regulating valve: reduce pressure in selected parts of system (e.g., different zones, pump systems); needed to protect drip emitters from high pressure
• Master valve: remotely controlled, downstream of main water supply; turns supply off when irrigation system is off; also helps protect potable water supply from contamination

Irrigation Design

Backflow Prevention

• any irrigation system supplied by potable water source must be protected from contamination in the event of system pressure drop leading to backflow

Irrigation Design

Controllers

• consists of two systems: timer and valve operator
• most irrigation clocks based on even 14-day watering period and 24-hour clock graduated in minutes

Irrigation Design

Sprinkler (Spray) Heads

• distribute water in uniform pattern, usually fixed as full or part of a circle
• commonly used for small turf areas, planting beds in residential and small commercial projects
• useful in low-pressure situations

Irrigation Design

Types of Spray Heads

Pop-up:

• installed flush with grade
• rises above body when water pressure applied
• bodies available with riser hts. of 2 - 18 in.
• turf applications: 2 - 4 in
• ground cover & shrubs: >4 in
• usually plastic
• facilitate maintenance
• no safety/tripping hazard when retracted

Irrigation Design

Types of Spray Heads

Shrub Spray Head:

• spray nozzle mounted on permanent riser
• nozzle usually fitted to normal pipe thread adapter
• mounted high enough to prevent foliage from interfering with spray pattern
• High rise pop up sprays recommended near walks

Irrigation Design

Types of Spray Heads

Rotary sprinklers

• distribute water from one or more rotating nozzles
• used for large turf areas

Irrigation Design

Drip Emitters

Single port: for single plants that will not increase in size

• Multiport: for trees/other plants that will get larger or for watering several plants
• made for installation on PVC or flexible polyethylene tubing
• Drip system must be actuated by valves that open/close at very low flows
• Zones must be protected against sand/small particles in water supply and high pressure

Irrigation Design

Hydraulics

Pressure

• expressed as head measured in feet (ft) or
• pressure measured in pounds per square in (psi)
• column of water 1 foot high exerts uniform pressure of .433 psi
• change in elevation results in loss/gain of pressure equal to change in elevation expressed in feet of head

Irrigation Design

Hydraulics: Operating Pressure

• pressure selected for operation of sprinklers
• all sprinklers have range of operating pressures with upper and lower limit

Irrigation Design

Hydraulics: Radius

• distance from sprinkler head to the outer rim of water distribution pattern
• as pressure goes up within operating pressure range, radius of throw will increase

Irrigation Design

Hydraulics: Flow Rate

• amount of water discharged from nozzle in gallons per minute (gpm)
• as pressure increases, flow rate increases

Irrigation Design

Hydraulics: Velocity

• rate of low measured in feet per second (fps)
• critical in determining pipe size
• as velocity increases, friction inside pipe increases, and pressure drops
• at high velocity, water scours pipes, erodes the seats in valves, and can break pipes due to surge pressure (water column meeting resistance in line due to change in direction/obstacle/end of line)

Irrigation Design

Pipe Sizing

Irrigation Mains

• carry water from supply source to operating zone valves
• in large systems remain under pressure at all times

Irrigation Laterals

• runs of pipe that carry water between zone valves and heads/emitters
• usually smaller in size than mains
• only under pressure when zone is being operated

Irrigation Design

Layout

• Heads: uniform coverage over areas to be watered with minimal overspray onto streets & walks
• Head-to-Head Spacing: 50% of diameter of full circle head; ensures adequate overlapping to account for loss of depth with increasing distance from head
• Square spacing: slightly less efficient than triangular spacing with respect to uniformity of distribution but easier to control edges and overspray
• Triangular spacing: fewer heads needed to get uniform coverage, but harder to get uniform coverage at ends of rectangular areas. Most practical use is for large turf areas

Irrigation Design

Precipitation Rate

• how fast irrigation head applies water to surface
• goal is to match precip rate of head to infiltration rate of soil
• measured in inches/hour

• Velocity = feet/second (fps)
• Flow rate (volume from nozzle) = gallons/min (gpm)
• Pressure = pounds/sq. in (psi) or feet of head
• Infiltration rate (of soil) = inches/hour
• Precipitation rate (rate at which irrig. head applies water to soil) = inches/hour