Introduction to Irrigation Design
Sprinklers – uniform application over entire
area – lawns
Micro sprays – Water uniformly over small
areas
Drip – Trickle – specific area – shrubs
Plants need water for
• Cooling – evaporation and
transpiration use energy – results ins
cooling plants
• Nutrient transport
• Dispersion of plant expelled waste
Site Information needed
• Soil texture, profile and infiltration rate
• Water source – quantity and pressure,
quality, cost
• Layout of site – areas to be irrigated,
elevations
• Plants to be irrigated
• Rainfall and ET rates
• Budget
Water Conservation
Potable to effluent
Irrigation scheduling
Steps to develop plan
1. Obtaining site information
2. Determining the irrigation requirement
3. Determining water and power supply
4. Selecting sprinklers and other equipment
5. Lateral layout (or “circuiting” sprinklers),
locating valves and main lines
6. Sizing pipe and valves and calculating total
system pressure loss
7. Locating controllers and sizing wire
8. Preparing the final irrigation plan
Site Plan
• A site plan is a scaled drawing of the areas that
are impacted by the irrigation system.
• Before going through the effort of creating a site
plan yourself, check to see if a plan already
exists.
• If the site being designed falls within city
boundary, there may be a site plan or survey on
record at the city or county planning/zoning
department.
Site plan
• Locate all buildings, walkways, driveways,
parking areas, light or utility poles, retaining
walls
• Indicate where there are slopes and in which
direction and how steeply the ground slopes.
• Locate all trees and shrub areas and pinpoint
the plant material on the drawing.
• soil type
• Location of any new planting areas and the
types of vegetation that these areas will
contain.
Available Soil Water
• Soil absorbs and holds water in much the same
way as a sponge.
• A given texture and volume of soil will hold a
given amount of moisture.
• The intake rate of the soil will influence the rate
at which water can be applied.
• The ability of soil to hold moisture, and amount
of moisture it can hold, will greatly affect the
irrigation operational schedule
Soil moisture
• Hygroscopic water is moisture that is
held too tightly in the soil to be used by
plants.
• Capillary water is moisture that is held in
the pore spaces of the soil and can be
used by plants.
• Gravitational water drains rapidly from
the soil and is not readily available to be
used by plants.
Source: USGS – Basic Ground-Water Hydrology - Heath
Source: USGS – Basic Ground-Water Hydrology - Heath
Source: USGS – Basic Ground-Water Hydrology - Heath
Source: USGS – Basic Ground-Water Hydrology - Heath
Source: USGS – Basic Ground-Water Hydrology - Heath
Soil moisture
• The permanent wilting point represents the
boundary between capillary water and hygroscopic
water.
• Because hygroscopic water is not usable by plants,
continuous soil moisture levels below the
permanent wilting point will result in the damage or
death of the plants.
• Field capacity represents the boundary between
gravitational water and capillary water. It is the
upper limit for soil moisture that is usable by plants.
Available Water
Soil Texture
Range
Average
in./in.
in./in.
0.04-0.08
0.06
0.06-0.10
0.08
0.10-0.15
0.13
0.13-0.19
0.16
0.15-0.21
0.18
0.13-0.21
0.17
Very coarse-textured sands and
fine sands
Coarse-textured loamy sands
and loamy fine sands
Moderately coarse-textured
sandy loams and fine sandy loams
Medium textured very fine sandy loams,
loam and silt loams
Moderately fine-textured sandy clay loams,
clay loams, and silty clay loams
Fine-textured sandy clays, silty clays,
and clay
Reference: USDA, NRCS, Engineering Field Manual
Available for each soil group by soil horizon from
NRCS Soil Surveys
Source: NRCS
Water needs
• Soil AW (inches/foot or inches/inch) = Field
Capacity (FC) – Wilting Point (WP)
• For specific plant AW (in) = (soil AW) X
Rooting Depth
• Readily available water = plant AW X Allowable
Depletion (use 50 % if not provided)
• Irrigation interval = AW X AD/ usage per day
• Efficiency = net water to root zone/gross water
applied to system
Reference Evapotranspiration
• ETo or potential evapotanspiration represents a
well watered, fully developed plant such as
grass
• Reference evapotranspiration is multiplied by a
crop coefficient to obtain the ET rate for a
specific crop
• The crop coefficient varies throughout the
growing season
• For Example Corn at 20% = 0.67 at 50%=1
Estimates of ET
•
•
•
•
•
•
Max and min temperatures
Relative humidly
Wind
Net radiation
Modified Penman to estimate ETo
U of Wisconsin web site
• http://www.soils.wisc.edu/wimnext/et/miet.html
Precipitation Rates
P = q/A
Units MUST be consistent. For:
P = precipitation rate (in/hr)
q = flow onto area (gal/min)
A = area (ft²)
P = 96.3 q/A
Precipitation Rate Importance
• Do not want to apply water faster than
the soil can absorb it
• We need to know the time required to
apply the correct amount of water
Sprinkler Irrigation
Uniform application by overlapping
non-uniform wetting paterns
• Sprinkler spray heads – no moving
parts - small area – pop up
• Microsprays – overhead – small area
• Rotary Sprinklers – Impact or gear
driven
Rotary Sprinklers – uniform application
over entire area – lawns
Micro sprays – Water
uniformly over small areas
Drip – Trickle – specific area – shrubs
Technical Information
• Nozzle - removable –
controls velocity and
flow
• Operating Pressure –
inc pressure – inc
water flow and
change wetting
pattern
• Radius of Throw –
furthest point
• Water Distribution
Patterns
• Trajectory – angle
Spray Heads
Technical Information – operating
pressure, flow, radius of throw, and
nozzle options
• Fixed head
• Rotary Motor-driven
• Rotary Impact
• Bubbles