Micro Design

BASIC SYSTEM DATA

(Refer to NRCS Standard 441- Irrigation System, Micro Irrigation, for design requirements)

Total area irrigated ……..…………….... (acres)

Available water supply flow rate …….... (gpm) Available flow per acre: (gpm/acre)

System design flow rate …………….... (gpm) @ operating pressure of (psi)

# Zones Planned …………….... Area irrigated by each zone: (acres)

# Zones irrigated concurrently……….... Application rate per zone: (inches/hr)

Lateral line material: _________________________________ Inside diameter: (inches)

Drip tape/line material: ________________________________

Inside diameter: (inches)

Drip tape/line spacing: ………………….. (inches); Drip tape/line depth:

(inches)

Flushing velocity: ………………………. (ft/s); Flushing end pressure: (psi)

Flushing flow rate: (gpm)

Describe Emitter (make, model, etc.):

Type (circle one): laminar laminar/turbulent turbulent Emitter spacing: (inches)

Emitter path width:……………………..... (inches); Emitter path height: (inches)

Describe Filter system (type, model, capacity in gpm):

Pressure loss across filter: ….………. (psi); Head required at filter: psi

Time required for backwash: …………. (hours); Backwash flow rate: gpm

Describe Sand Separator (type, model, capacity in gpm):

Describe Chemigation Valve (type, model, location):

Describe Check Valve (type, model, location):

Zone Number: Type of drip tape/line:

Emitter data (model, type,

etc.)

Spacing: (inches)

Design manifold inlet pressure downstream of zone control valve: (psi)

Emitter discharge = q = Kd Hx (gal/hr) Kd = x =

Manufacturer’s Coefficient of Variation, (Cv):

Average emitter design discharge, qave: (gal/hr) @ line pressure of (psi)

Maximum emitter discharge, qmax: (gal/hr) @ line pressure of (psi)

Location of maximum discharge emitter:

Minimum emitter discharge, qmin: (gal/hr) @ line pressure of (psi)

Location of maximum discharge emitter:

Flow Variation = _________ % Emission Uniformity, (EU), = __________ %

System Capacity

**100

*(1 )r

gR

ET TGross Application F

EU L

dayplantgalf

SSFF rpg

dgp //**

623.0)/(

100( )11.6 ( / )

( )ft tapeQ gpm

Application Rate in hrLateral spacing in

( ) ( )

( )

453 ( )in ac

hr

D ASystem Q gpm

T

Crop Water Needs ExampleCalculate capacity required for a

proposed 1 ac. Micro irrigation system on Vegetables. Using drip tape with a flow of 0.45 gpm/100’ and 12” emitter spacing, 200 ft rows, 5 ft row spacing, and 10 fields of 0.1 ac each.

Q = 453*DA TD = .2” / system

efficiencyA = AreaT = 22 hrs

Crop Water Needs Example Answer

Q = 453*DA T

= 453 x (.2”/.9) x 1 ac22 hrs

= 4.5 gpm

Each field will have a capacity of 4.5 gpm.◦ 200 ft rows with a 0.45 gpm/100’ drip tape

flow will give you 0.9 gpm per row.◦ At 5 ft row spacing, 1 ac will have

approximately 10 fields, each of these fields will have 5 rows, 200 ft long.

◦ 5 rows times 0.9 gpm/row is 4.5 gpm per field.

Minimum water requirement is 4.5 gpm for 1 ac, we only need to run 1 field at a time to meet the crop water demand.

Hours of irrigation per day to apply .2”(1 field of 0.1 ac each)

T = 453*DA Q

= 453 x (.2”/.9) x 0.1 ac4.5 gpm

= 2 hrs and 15 minutes

WELL

8:00 a.m. field 1

2 hrs and 15 min @ 4.5 gpm

10:15 a.m. field 2

2 hrs and 15 min @ 4.5 gpm

12:30 p.m. field 3

2 hrs and 15 min @ 4.5 gpm

2:45 p.m. field 4

2 hrs and 15 min @ 4.5 gpm

5:15 p.m. field 5

2 hrs and 15 min @ 4.5 gpm

7:30 p.m. field 6

2 hrs and 15 min @ 4.5 gpm

9:45 p.m. field 7

2 hrs and 15 min @ 4.5 gpm

12:00 a.m. field 8

2 hrs and 15 min @ 4.5 gpm

2:15 a.m. field 9

2 hrs and 15 min @ 4.5 gpm

4:30 a.m. field 10

2 hrs and 15 min @ 4.5 gpm

When you get to the field you discover that the well only produces 2.7 gpm. So @ .9 gpm/row water 3 rows and have15 sets

Hours of irrigation per day to apply .2”(given a well capacity of 2.7 gpm and fields of 3 rows each)

T = 453*DA Q

= 453 x (.2”/.9) x 0.07 ac2.7 gpm

= 2 hrs and 40 minutes

WELL

8:00 a.m. Field 1

2 hrs and 40 min @ 2.7 gpm

At 6:00 a.m. the pump has been running for 22 hrs and at 8:00 a.m. we need to go back to Field 1 but we haven’t irrigated all the fields!!!!

10:40 a.m. Field 2

2 hrs and 40 min @ 2.7 gpm

1:20 p.m. Field 3

2 hrs and 40 min @ 2.7 gpm

4:00 p.m. Field 4

2 hrs and 40 min @ 2.7 gpm

6:40 p.m. Field 5

2 hrs and 40 min @ 2.7 gpm

9:20 p.m. Field 6

2 hrs and 40 min @ 2.7 gpm

12:00 a.m. Field 7

2 hrs and 40 min @ 2.7 gpm

2:40 a.m. Field 8

2 hrs and 40 min @ 2.7 gpm

5:20 a.m. Field 9

2 hrs and 40 min @ 2.7 gpm

Watering strategies

Select emitter based on water required

Calculate set time

adgp

a qe

FT )/(

Adjust flow rate or set time

If Ta is greater than 22 hr/day (even for a single-station system), increase the emitter discharge

If the increased discharge exceeds the recommended range or requires too much pressure, either larger emitters or more emitters per plant are required.

Select the number of stations If Ta ≈ 22 h/d, use a one-station system (N =

l), select Ta < 22 hr/day, and adjust qa accordingly.

If Ta <11 h/d, use N = 2, select a Ta <11, and adjust qa accordingly.

If 12 < Ta < 18, it may be desirable to use another emitter or a different number of emitters per plant to enable operating closer to 90 percent of the time and thereby reduce investment costs.

Pressure flow relationship (Pa)

1

xa

a

qP

K

Where:

qa= average emitter flow rate (gph)

Pa = average pressure (psi)

x = emitter exponentK = flow constant

Start with average lateral

Standard requiresPipe sizes for mains, submains, and laterals

shall maintain subunit (zone) emission uniformity (EU) within recommended limits

Systems shall be designed to provide discharge to any applicator in an irrigation subunit or zone operated simultaneously such that they will not exceed a total variation of 20 percent of the design discharge rate.

Design objectiveLimit the pressure differential to

maintain the desired EU and flow variation

What effects the pressure differential◦Lateral length and diameter◦Manifold location◦slope

Four Cases

Allowable pressure loss (subunit)

This applies to both the lateral and subunit. Most of the friction loss occurs in the first 40% of the lateral or manifold

nas PPP 5.2

DPs =allowable pressure loss for subunit

Pa = average emitter pressure

Pn = minimum emitter pressure

Emission Uniformity

1 1.27 100n

a

qCVEU

qn

Q is related to Pressure

1

1 1.27 100

xx

n n n n

a a a a

x

n

a

q P P q

q P P q

So

PCV

Pn

HydraulicsLimited lateral losses to 0.5DPs

Equation for estimating◦Darcy-Weisbach (best)◦Hazen-Williams◦Watters-Keller (easiest, used in NRCS

manuals)

LDC

QFhf 87.4

852.1

5.10

C factor Pipe diameter (in)

130 ≤ 1

140 < 3

150 ≥ 3

130 Lay flat

Hazen-Williams equation

hf =friction loss (ft)

F = multiple outlet factor

Q = flow rate (gpm)

C = friction coefficient

D = inside diameter of the pipe (in)

L = pipe length (ft)

Watters-Keller equation

75.4

75.1

D

QKFLh f

hf = friction loss (ft)

K = constant (.00133 for pipe < 5” .00100 for > 5”)

F = multiple outlet factor

L = pipe length (ft)

Q = flow rate (gpm)

D = inside pipe diameter (in)

Multiple outlet factors

Number of outlets

FNumber of

outlets

F

1.851 1.752 1.851 1.752

12345678

1.000.640.540.490.460.440.430.42

1.000.650.550.500.470.450.440.43

910-1112-1516-2021-3031-70>70

0.410.400.390.380.370.360.36

0.420.410.400.390.380.370.36

Adjust length for barb and other minor losses

Adjusted length

e

ee

S

fSLL

L’ = adjusted lateral length (ft)

L = lateral length (ft)

Se = emitter spacing (ft)

fe = barb loss (ft)

Start with average lateral

Pressure Tanks

Practice problem